. _ ^ ^ „ OCT 14 1975

175118 '-cy NSRDS-NBS 49

Transition Metal Oxides

U.S. DEPARTMENT OF COMMERCE/National Bureau of Standards

NSRDS

Ice dat*

Crystal Chemistry, Phase

Transition and Related Aspects
NATIONAL BUREAU OF STANDARDS

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Mil Bureau of Standards

)V 1 1974 p

5aSo

&

transition Metal Oxides

U573

Iq-i

1 Crystal Chemistry, Phase Transition

and Related Aspects

C. N. R. Rao and G. V. Subba Rao

Department of Chemistry

Indian Institute of Technology

Kanpur-16, India

t> N$RD6-KB5 »o.

^

NSRDS

U.S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

W.*5. NATIONAL BUREAU OF STANDARDS, Richard W. Roberts, Director

» * %

.OVAJTIO/u

Issued June 1974
Library of Congress Catalog Number: 73-600267

NSRDS-NBS 49

Nat. Stand. Ref. Data Ser., Nat. Bur. Stand. (U.S.), 49, 138 pages (June 1974)

CODEN: NSRDAP

© 1974 by the Secretary of Commerce on Behalf of the United States Government

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1974

For sale by the Superintendent of Documents, U.S. Government Printing Office

Washington, D.C. 20402 (Order by SD Catalog No. C13.48:49) Price SI. 70

Stock Number 0303-01190
Foreword

The National Standard Reference Data System provides access to the quantitative data of physical science, critically evaluated and compiled for convenience and readily accessible through a

variety of distribution channels. The System was established in 1963 by action of the President’s

Office of Science and Technology and the Federal Council for Science and Technology, and

responsibility to administer it was assigned to the National Bureau of Standards.

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Research Council of the National Academy of Sciences-National Academy of Engineering. A number of Advisory Panels, each concerned with a single technical area, meet regularly to examine

major portions of the program, assign relative priorities, and identify specific key problems in need of further attention. For selected specific topics, the Advisory Panels sponsor subpanels

which make detailed studies of users’ needs, the present state of knowledge, and existing data re- sources as a basis for recommending one or more data compilation activities. This assembly of advisory services contributes greatly to the guidance of NSRDS activities. The System now includes a complex of data centers and other activities in academic insti- tutions and other laboratories. Components of the NSRDS produce compilations of critically

evaluated data, reviews of the state of quantitative knowledge in specialized areas, and computations of useful functions derived from standard reference data. The centers and projects also establish criteria for evaluation and compilation of data and recommend improvements in ex- perimental techniques. They are normally associated with research in the relevant field. The technical scope of NSRDS is indicated by the categories of projects active or being

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Reliable data on the properties of matter and materials is a major foundation of scientific and technical progress. Such important activities as basic scientific research, industrial quality con-'

trol, development of new materials for building and other technologies, measuring and correcting

environmental pollution depend on quality reference data. In NSRDS, the Bureau’s responsibility

to support American science, industry, and commerce is vitally fulfilled.

Richard W. Roberts, Director

iii
Preface

As part of our program in presenting critical reviews of phase transitions of inorganic solids,*

we have surveyed the phase equilibria, crystal chemistry, and phase transitions of transition metal

oxides in detail. Transition metal oxides form an interesting series of materials for the study of a variety of solid state phenomena, many of which arise from the presence of different d-electron

configurations. Crystal structure transformations in several of the transition metal oxides are ac- companied by significant changes in electrical, magnetic, and other properties and we have, therefore, briefly presented the recent data on the various properties of these oxides. We trust that this

monograph will not only serve as a source of valuable information, but also provide a fascinating case history of an important class of solid state materials.

In this paper, we have covered the binary oxides of 3d-, 4d-, and 5d-transition metals with

important references to the literature up to 1973.

The authors acknowledge the support by the National Bureau of Standards through their

Special International Programs. The authors’ thanks are due to G. Rama Rao for his assistance in

the preparation of the manuscript.

* The first monograph of this series is on binary halides by C. N. R. Rao and M. Natarajan, NSRDS-NBS-41, 1972. (Supported by NBS Project G-77).

IV
Contents

Page

Foreword iii Preface iv

Symbols and abbreviations 1 Introduction 1

I. Oxides of 3d transition elements

1. Scandium oxide 7

2. Titanium oxides 9

3. Vanadium oxides 31

4. Chromium oxides 50

5. Manganese oxides 54

6. Iron oxides 60

7. Cobalt oxides 69

8. Nickel oxides 75

9. Copper oxides 79

10.

Zinc oxides 81

II. Oxides of 4d transition elements

1. Yttrium oxides 83

2. Zirconium oxides 85

3. Niobium oxides 92

4. Molybdenum oxides 99

5. Technitium oxides 102

6. Ruthenium oxides 103

7. Rhodium oxides 104

8. Palladium oxide 105

9. Silver oxides 106

10.

Cadmium oxides 107

III. Oxides of 5d transition elements

1. Lanthanum oxides 109

2. Hafnium oxides Ill

3. Tantalum oxides 113

4. Tungsten oxides 117

5. Rhenium oxides 123

6. Osmium oxides 125

7. Iridium oxides 127

8. Platinum oxides 127

9. Gold oxides 128

10.

Mercury oxides 129

IV. Some recent studies 130

v
... .
Transition Metal Oxides

Crystal Chemistry, Phase Transitions and Related Aspects

C. N. R. Rao and G. V. Subba Rao

A survey is made of the data describing the thermodynamics of phase equilibria, crystal

chemistry and phase transformations of binary oxides of 3d, id, and 5d transition metals. Changes

in electrical, magnetic, and other properties which accompany phase transitions are discussed- Nearly complete coverage of the literature is provided up to 1973. Key words: Crystal structure transformations; critical data, transition metal oxides; electronic properties; phase equilibria; phase transitions; magnetic properties.

Symbols and Abbreviations

AFMR—Anti -Ferromagnetic Resonance

APW—Augmented Plane Wave

B—Crystal field Racah parameter

bp—boiling point

Cp—Heat capacity at constant pressure

eal—Thermoehemical calorie = 4.1840 Joules

Cv—Heat capacity at constant volume

CS—Crystallographic shear

DTA—Differential Thermal Analvsis

d—Density

E ft—Energy of activation

Ef—

ermi level

Eg—Band gap

e—electronic charge

EMF—Electro Motive Force

ESCA—Electron Spectroscopy for Chemical

Analysis

ESR—Electron Spin Resonance

AG—Gibbs free energy change

H—Enthalpy

AH—Enthalpy change

h—hours

k—Knight shift

k—Boltzmann constant

LCAO—Linear Combination of Atomic Orbitals

M0—Spontaneous magnetization

Ms—Saturation magnetization

min—minutes

m—free electron mass m*—Effective electron mass mp—melting point

mp *—Effective polaron mass NMR—Nuclear Magnetic Resonance n—Refractive index nc—number of charge carriers

P—Pressure: 1 atm = 101325 Nm~2 =

1013250 dyn cm-2 P02—Partial pressure of oxygen

Rh—Hall constant

AS—Entropy change

T—Temperature in degrees Kelvin

Tc—Curie temperature

Tn—Neel temperature

Tsc—Superconducting transition temperature

Tm—

ransition temperature (Morin

transition)

Tt —Transition temperature

t—Temperature in °C (degree Celsius)

TEC—Thermal Expansion Coefficient

TGA—Thermo Gravimetric Analysis

A V—Volume change

cl—

ongitudinal sound-wave velocity

vs—Shear sound-wave velocity

UHF—Ultra High Frequency W—Watt

a—Seebeck coefficient; angle

of*—

rohlich polaron coupling constant a td—Thermal diffusivitv (x/Cpd)

/3 —Angle

7—Angle

7gGriineisen constant

e—Dielectric constant

do—Debye temperature

k—Thermal conductivity

Mb—

ohr magneton

Md—

rift mobility

Mh—

all mobility

p—Resistivity

Ap—Jump in resistivity a—Conductivity

X—Magnetic susceptibility

Xm—Molar magnetic susceptibility

Z—Number of molecules per unit cell Introduction

Transition metal oxides probably form one of the

most interesting class of solids, exhibiting a variety

of novel properties [1-5]. 1 These properties un- doubtedly arise from the outer d electrons of the

transition metal ions. These d electrons can neither

be described by a collective electron model (as in

the case of s and p electrons) nor by a localized

electron model (as in the case of / electrons which

1 Figures in brackets indicate the literature references on page that appear

at the end of each section.

1
are tightly bound to the nuclei). Outer d electrons are not screened from the neighboring atoms by the

outer core electrons, and the intermediate character

of these electrons shows itself in terms of localized

electron behavior in some oxides and collective elec- tron behavior in some others; in a few instances,

both kinds of d electrons can exist simultaneously.

In recent years, the field of transition metal oxides

has been a subject of intensive study by chemists

and others interested in solid state materials. A

study of the properties of these oxides not only

serves as one of the best introductions to solid state chemistry and materials science, but also provides a wealthy source of fascinating research problems for

experimental and theoretical investigations.

Transition metal oxides of the general formulae

MO, M2O3, M02 , M2O5, MO3, M„02 n-1, MrAn+i are known to exist. The nature of bonding in these

oxides varies from purely ionic (e.g., NiO, CoO) to purely covalent (e.g., 0s04 , Ru04 ); metallic bonding

is seen in oxides like TiO, NbO, and Re03. The

crystal structure of transition metal oxides varies

from cubic to triclinic symmetry. Simple binary

oxides of the composition, MO, generally possess

the rock-salt structure, while the dioxides, M02 , possess fluorite, rutile, distorted rutile or even more

complex structures; many sesquioxides, M2O3, possess the corundum structure. Transition metals form

important ternary oxides like perovskites, spinels,

bronzes, and garnets. Many of these oxides show

interesting phase transitions from one crystal struc- ture to another accompanied by changes in magnetic,

electrical, and other properties.

Many transition metal oxides show a wide range

of non-stoichiometry as in the case of oxides of

titanium, vanadium, iron, and niobium. Deviations

from stoichiometry can be due to cation deficiency

(as in Fes04 ) or anion deficiency (as in ZnO). Large

deviations from stoichiometry cause marked changes

in unit cell dimensions or in crystal structure. In

many oxides, the defects are so highly ordered

that we cannot really consider them as ordinary

defect solids (e.g., TiO, NbO). Similarly, many

oxide phases which may appear as nonstoichiometric

could truly be stoichiometric compositions (with narrow ranges of homogeneity) resulting from dif- ferent modes of sharing of the metal-oxygen polyhedra (e.g., Tin02 „-i and similar Magneli phases)

[ 6, 7]. In oxides where there are wide ranges of

homogeneity lattice constants vary with composition; a typical case is that of TiO* ( 0.8 ^ x ^ 1 . 2). In such oxides a comparison of the pyknometric

density with the x-ray density will be useful; generally, when compounds deviate from stoichiometry, an increase in density indicates interstitials and a decrease denotes vacancies. Electrical conductivity

of oxides is markedly affected by deviations from

stoichiometry. Similarly, foreign metal ions, present

as impurities or doped intentionally, would also cause marked changes in the properties of metal

oxides. There are many instances where foreign impurities stabilize a particular phase. Thus, sulfate

ion stabilizes the anatase phase of Ti02 while Fe+3 stabilizes the high temperature pseudobrookite phase

of Ti306 . Transition metal oxides exhibit a wide range of

magnetic properties: TiCh, ZrC>2, V2O6, and M0O3

are diamagnetic; metallic TiO and Re03 are Pauliparamagnetic; VO2, Nb02, and Ti2 03 are paramagnetic; MnO, CoO, NiO, V2O3 and O2O3 are antiferromagnetic with well-defined Neel temperatures;

Mns04 and Fes04 are ferrimagnetic; OO2 is ferromagnetic. The magnetic properties of binary

oxides have been reviewed by Goodenough and

others [ 1 , 3 , 4, 8]. Transition metal oxides show a spectacular range

of values of electrical conductivity. At one extreme

we have the insulator behavior as typified by MnO

(<r« 10-15

£2_1cm-1

), and at the other extreme we have

oxides like Re03, Cr02 , Ru02, TiO, and NbO with

metallic conductivities (fig. 1 ). In between these

extremes we have a large number of oxide semiconductors. The mechanism of conduction in such semiconducting oxides may involve the hopping of charge

carriers (as in Nb205) or the excitation from the

valence band to the conduction band (as in SrTiOs

and Sn02). Of special interest are the transition

metal oxides which exhibit insulator (or semiconFigure 1 . Resistivities of highly conducting (metallic) oxides.

2
Counts,

x

IQ 3

Cp.cal/

deg.

mole

Endo—

AT

<*,/uV/

K

it,a

cm

H.(x10

emu/g.)

Latt.

Parameters

Figure 2. Phase transitions in metal oxides investigated by various techniques.

3
ductor)-to-metal transitions; typical of these oxides

are V2 03 , V02 , and Ti2 03 (fig. 2). Conduction in

oxides can be electronic or ionic; for example, lith- ium-doped NiO is a pure electronic conductor while

Ca-doped Zr02 is a pure ionic conductor.

A number of authors have recently reviewed elec- tron transport and related properties of transition

metal oxides [3-5, 9-15]. Of the various attempts to explain these properties, the qualitative approach

of Goodenough [5, 16-18] based on principles of

chemical bonding has been eminently successful.

With empirically derived criteria for the overlap of

cation-cation and cation-anion-cation orbitals as determined by the crystal structures, Goodenough has

attempted to provide a unified understanding of

the magnetic and electrical properties of variety of

inorganic materials including simple transition metal

oxides, perovskites, spinels, bronzes, sulfides etc. Goodenough distinguishes two classes of oxides, those

having large interactions between d electrons on

neighboring atoms or ions because of large cationcation interactions via a small cation-cation separation (Class I) and those having large interactions

between d electrons of neighboring atoms because

of large cation-anion-cation interactions via a large

covalent mixing of anionic p orbitals into the cationic

d orbitals (Class II); there are also oxides belonging

to Class I-II where the two mechanisms coexist.

Our knowledge of the band structures of transition

metal oxides is far from satisfactory. A detailed discussion of the theoretical models would be outside

the scope of this work and the reader is referred to the various reviews [1-5] and other articles listed at the end of this section. The intuitive approach

based on molecular orbital theory provides crude

but reasonable approximations. In figure 3 is shown

a summary of the over-simplified one-electron energy

diagrams for a few crystal structures with the predicted Fermi levels for the various d electron configurations. This figure predicts either metallic or semiconducting behavior for any crystal structure

depending on the number of d electrons in the cation.

Such models will undoubtedly provide a basis for

further studies which may eventually turn out to be

technologically important. We may note here that

the properties of highly conducting oxides such as TiO can be satisfactorily explained on the basis of

such elementary band structure models. The status

of our understanding of the electrical properties of

the metal oxides which exhibit electrical transitions

is, however, in a constant state of flux; there are many theories, none of which is entirely satisfactory

and the field is rapidly developing [4, 10-15]. In

figure 2 we have summarized the recent data on the

RHENIUM

TRIOXIDE ROCKSALT CORUNDUM RUTILE (PEROVSKITE) MO M2 0 3 M02 M03

d'°

Figure 3. Schematic one-electron energy level diagrams for

different cation electronic configurations in various crystal struc- tures (after Vest and Honig). Dotted lines show predicted Fermi levels.

electrical transitions of three transition metal oxides

(V203 , V02 , and Ti2 02 ) in order to illustrate the

wealth of information available in the literature.

There is considerable doubt concerning the mechanism responsible for the observed changes in V2 03 and V02 . Both these transitions are related to the

changes in crystal structure that occur simultaneously. The situation is not straight-forward since

the structural changes could themselves be determined by the electronic properties of the oxides.

Thus, it is possible that distortions occur at the

critical temperature in such a way as to create a gap at the Fermi level. This may lower the energy

of the occupied electron states or raise the energy

of the unoccupied states. If initially the band is half-filled and then split into two (accompanying

the crystalline distortion) there will be a transition

from metallic behavior of the symmetric phase

(rhombohedral in V203 and rutile in V02 ) to semiconducting properties for the less symmetric phase

(monoclinic in V203 and V02 ). Several other mechanisms have been proposed to account for the transition in V2 03 . (i) One of the

mechanisms invokes the onset of antiferromagnetism

(whose existence has been verified by polarized

neutron scattering). Here, the potential of the elec- trons differs for the spin-up and spin-down alignments of vanadium ions of the antiferromagnetic

phase and there is a doubling of the unit cell in the

magnetic superlattice. This is accompanied by a

4
splitting of the band, (ii) A combination of the two

preceding mechanisms has also been invoked. The

band gap opening up under the combined action

of both these effects would be sufficiently large to cause a decrease in the carrier density and account

for the increase in resistivity. This would mean that

the mobility remains nonactivated and essentially

unchanged. On the other hand, if either of the two

mechanisms (distortion or antiferromagnetism) were

alone to operate, there would be a decrease in carrier

density of only two orders of magnitude. In such a case a change in mobility of five orders of magnitude

would be necessary to account for the observations.

The mobility in the low-temperature monoclinic

phase would then be low enough to correspond to an

activated process, (iii) The possibility that band

overlap may occur in V2 O3 has been proposed, based on transport measurements. In this event, the transition could be thought of as being driven by the

coulomb interaction between the holes and electrons

(simultaneously present in the overlapping bands).

This would result in a distortion which lifts the

band overlap. The insulating phase, according to this

mechanism, would be an 'excitonic insulator.’

Accordingly, the transition can be entirely suppressed by application of hydrostatic pressure. Resistivity studies of the metallic phase under compression down to liquid helium temperatures indicate

the presence of overlapping bands, (iv) It has been

suggested that V2O3 may be an example of a ’Mott

insulator.’ This implies that small changes in interatomic distances in the lattice causes sufficient alterations in the band overlap to change the system from

itinerant to localized behavior, (v) The effect of

electron correlations has also been considered important, since the bands in V203 are relatively nar- row. According to this theory, if the bandwidth is within a certain narrow range and is further reduced

(by temperature changes or so) then a splitting into

two sub -bands takes place. It should be possible to

devise a single, comprehensive theory, in which the

possibilities discussed above become special cases. To decide among the various alternatives it is neces- sary to obtain more detailed experimental data. The

case of the VO2 transition is similar to that of V2O3,

with the exception that VO2 does not exhibit magnetic order.

Ti203 differs from the cases discussed earlier in

that, this oxide undergoes a transition over a rather

wide temperature range. It does not exhibit either

magnetic order or undergo a crystallographic transition. A band broadening model has been suggested

to account for the transition of Ti203. At low temperatures, the completely filled band is separated by

a gap of ~0.06 eV from the next higher one. The

pure material is, therefore, an intrinsic semiconductor at liquid helium temperatures. With increase in

temperature, the distension of the unit cell is in such

a direction as to widen the bands and eliminate the

gap. The material thus gradually is converted into an overlap material. All these theories of the semiconductor-metal transitions have been discussed in

the various references listed at this introduction and

they are again referred to in the respective sections

in the text. It was our purpose initially to review phase transitions of transition metal oxides in this monograph.

We soon came to the conclusion that such a review

would be incomplete unless the relevant information on the crystal chemistry, phase equilibria as well as

electrical, magnetic and other important properties

are also included. This is because, phase transitions

in many of these oxides are associated with interesting changes in various properties (as in fig. 2).

In the phase transitions of some of the oxides like

Ti02 and Zr02 electronic factors may not be very

important, but the transitions themselves are of

academic and technological interest. The phase

equilibria and crystal chemistry of many of the

transition metal-oxygen systems are not fully in- vestigated. As new phases of these oxides get established, there will undoubtedly be further scope for

the study of phase transitions and other properties.

The Magneli phases of titanium and vanadium serve to illustrate this point. We have tried to keep the

future possibilities in mind in presenting the available information on transition metal oxides.

We have presented the information on the binary

oxides of the 3d, 4d, and 5d transition metals by

first summarizing the general features of the phase

equilibria, phase transitions and physical properties

in each metal-oxygen system followed by a detailed

tabulation of the data. This tabular presentation of

the data is expected to give a brief summary of the

present status of knowledge on each oxide system.

The material on each oxide system forms a self- contained section by itself and includes the relevant

information on all the important properties. Such

a review of transition metal oxides is not presently

available in the literature and we hope that this

present effort will be well-received by chemists, solid state scientists and others.

The various binary oxides discussed in the paper

are shown in table 1. We have listed most of the

important references to the literature (up to 1973) on each oxide system and we apologize for any

omissions due to oversight or errors in judgement.

Crystallographic data only from the recent literature

5
TABLE

1.1.

Magnetic

and

electrical

properties

of

stable

oxides

of

transition

metals'

O! Os

o:

•a: u; o-3u otc S3

Oi3 1 Oj

hoi Oi

to:* o3 o3 Ui <\ <3 < ow> S3

i!Zi

°!0Ui oz

ql

ol Ui O

nsCU

O'

oi

«l

UhI «

oi

£je

o!

«l

fel 0 o3

PP

ft, a

CP O' q| q| oi o-. >

Cl s|

Cl SI c i Si

C; Si o 0 0 0 H o«» Ph oMAoCD o

Ol

Hi uH v

Ph oV

pp

CP

i—i >

O'CN | U I y ooS oo> OS

O'

S! O O Oi $ $ ?!

o.oO.o o; o0£ d0 o0- 0 o0 ol

o-.

Oi q sOo hu uu U:73 1 aa S s s £

CP> oo <NVHV

<No

oi

K*' >1 O!

>! >1B. I o> o> o

<F‘

O,H o>0.

0,

<5j oj

>• » s0 o>0

/d! O

>10 1

qi

>il

C3

31 Ol

>1a d>0

CD o

-Qzaa

<N| O'

.a! Z!a q vi i e ^ VI z

o> o

jaZ

to d

-Qz

o'

-a; Z:73 oCO H

qj

«i d 1

o. CV.

L? ojto* o o d o oj

CP

N Sia S

CO s S s fr! > u V HH CD H

_*i

V r-l OSA or~© q

oj

Si

O;

Sj o:

.^Ti HI q:S i oa ol<-M 1 >—1 1 A r< e. i a f a ; a i N Sh l qoX 73 i os& 6 a

Magnetic

and

electrical

properties

are

shown

with

the

following

symbols:

d,

diamagnetic;

/,

ferromagnetic;

af,

antiferromagnetic;

fe,

ferrimagnetic;

p,

paramagnetic;

pp,

Pauli

paramagnetic;

s.c.,

superconductor; ,

metal;,

semiconductor; ,

exhibits

semiconductor-metal

transition;

degenerate

semiconductor.

(?),

not

investigated

in

detail;

(??),

not

known

or

doubtful

existence.
have been quoted and well-established data on standard substances found in the papers have not been

repeated. Studies on solid solutions of oxides, liquid

oxides, oxide hydrates and thin films have not been

included and the review pertains only to the bulk

properties of oxides in solid state. Effects of impurities have been discussed wherever results are rele- vant. We must add that the information on the

phase transitions of ternary oxides and other systems

is very vast, but does not fall within the scope of

this monograph. We have not included any general

discussion of the theory of phase transitions of in- organic materials as this subject has been adequately

reviewed elsewhere [19-21].

References

[1] Goodenough, J. B., Magnetism and the Chemical Bond

(John Wiley-Interscience, New York, 1963).

[2] Howe, A. T., and Fensham, P. J., Quart. Rev. (Chem.

Soc. London), 21, 507 (1967).

[3] Adler, D., Solid State Phys., Eds. F. Seitz, D. Turnbull,

and H. Ehrenreich 21, 1 (1968).

[4] Rao, C. N. R., and Subba Rao, G. V., phys. stat. solidi

(a) 1, 597 (1970).

[5] Goodenough, J. B., in Progress in Solid State Chemistry,

Vol. 5, Ed. H. Reiss (Pergamon Press, Oxford, 1971).

[6] Anderson, J. S., in Modern Aspects of Solid State Chemistry, Ed. C. N. R. Rao (Plenum Press, New York,

1970).

[7] Chemistry of Extended Defects in Nan-Metallic Solids,

Eds. L. Eyring and M. O’Keeffe (North Holland Publ.

Co., Amsterdam, 1970).

[8] Menyuk, N., in Modern Aspects of Solid State Chemistry, Ed. C. N. R. Rao (Plenum Press, New York,

1970).

[9] Bosman, A. J., and van Daal, H. J., Adv. Phys. 19, 1 (1970).

[10] Adler, D., Rev. Mod. Phys. 40, 714 (1968).

[11] (a) Mott, N. F., Phil. Mag. 20, 1 (1969); Rev. Mod.

Phys. 40, 677 (1968).

(b) Zinamon, Z., and Mott, N. F., Phil. Mag. 21, 881

(1970).

[12] (a) Honig, J. M., IBM J. Res. Dev. 14, 232 (1970).

(b) Honig, J. M., in Modern Aspects of Solid State Chemistry, Ed. C. N. R. Rao (Plenum Press, New York,

1970).

[13] Rice, T. M., and McWhan, D. B., IBM J. Res. Dev.

14, 251 (1970).

[14] Doniach, S., Adv. Phys. 18, 819 (1969).

[15] Hyland, G. J., J. Solid State Chem. 2, 318 (1970).

[16] Goodenough, J. B., Bull. Soc. Chim. France 4, 1200

(1965).

[17] Goodenough, J. B., Czech. J. Phys. 17B, 304 (1967).

[18] Goodenough, J. B., J. Appl. Phys. 37, 1415 (1966); 39

403 (1968).

[19] Rao, C. N. R., and Rao, K. J., in Progress in Solid State Chemistry, Yol. 4, Ed. H. Reiss (Pergamon Press, Oxford, 1967).

[20] Rao, C. N. R., in Modern Aspects of Solid State Chemistry, Ed. C. N. R. Rao (Plenum Press, New York,

1970).

[21] Rao, C. N. R., and Natarajan, M., Crystal Structure Transformations in Binary Halides, IN at. Stand. Ref. Data Ser., Nat. Bur. Stand. (U.S.), 41, 53 pages

(July 1972).

I. Oxides of 3d Transition Elements

1.1. Scandium Oxide

Scandium apparently forms only one oxide, SC2O3.

Petru and Dufek [1] reported a grey-black oxide,

which they believed to be a monoxide, ScO, obtained

by the hydrogen reduction of SC2O3 at 2170 K; this

monoxide has, however, not been characterized. SC2O3

is cubic [2-5] and has the C-type rare-earth oxide

structure in which the metal atoms are distributed

in the ratio 1 : 3 into two crystallographically distinct

sites each with six-fold coordination. The structure

is relatively open and can be described as a defect

fluorite form in which one fourth of the anions are absent. No magnetic ordering is known in SC2O3

and heat capacity data [6, 7] do not indicate any

phase transformations in the range 53 to 1800 K.

Reid and Ringwood [8] found that SC2O3 transforms

to the monoclinic B-type rare-earth oxide structure

at 130 kbar pressure and 1273 K. The unit cell volume decreases by 7.8 percent (density increases

by 8.3 percent) accompanied by an increase in co- ordination number from six- to seven-fold for two

thirds of the metal atoms. No change in the crystal structure is noticed at 30 or 80 kbar pressure at 1273 K (or at 89 kbar at 1573 K) and at 110 kbar

at 1273 K, poorly crystallized high pressure phase

of SC2O3 is obtained [8, 9]. SC2O3 is a semiconductor at all temperatures [10

13] and considerable ionic contribution to the total

conductivity is noticed at and below 1100 K [11,

12] . SC2O3 forms a complete series of solid solutions

with ln203 [9, 14], Y2O3 [15, 16], Yb203 [15], and

Cr2 C>3 [17] and binary systems of the type M02

- Sc203 where M = Ti [18], Zr [18-21], Hf [22, 23], Th

[24], and U [24] and CaO-Sc2C>3 [25] have been in- vestigated in detail in the literature. Generally, solid

solution formation occurs up to certain composition

and the binary systems exhibit considerable ionic

conductivity.

7
Scandium oxide

Oxide and description of the study

Data Remarks and inferences References

SC2O3

Crystal structure and melting point. Ordinary form:

Cubic; space group, Ia3; Z = 16; a =9.844 ±0.002 A. High-pressure form: Monoclinic; space group, C2/m; Z = 6; a = 13.173 ±0.01 A; 6=3.194±0.005 A;

c = 7.976±0.01 A; 0 = 1OO.4O°±

0.05°. Melting point =2740 K.

This relatively open C-type

structure is adopted by many

rare-earth oxides and is closely related to the fluorite structure. The high pressure form is denser

and is akin to the B-type struc- ture adopted by the rare-earth

oxides. The transformation does

not involve the intermediate

corundum structure.

[4, 8, 26].

Electrical properties. Semiconductor behavior in the range 370-1870 K; reliable data on pure and single crystal material are not available.

[10-13].

Optical properties. Optical energy gap = 5.4 eV; in- frared stretching frequency =

635 cm-1.

[27, 28]

References

[1] Petru, F., and Dufek, V., Z. Chem. 6, 345 (1966).

[2] Milligan, W. O., Vernon, L. W., Levy, H. A., and

Peterson, S. W., J. Phys. Chem. 57, 535 (1953).

[3] Reid, A. F., and Sienko, M. J., Inorg. Chem. 6, 521

(1967).

[4] Geller, S., Romo, P., and Remeika, J. P., Z. Crystallogr.

124, 136 (1967); 126, 461 (1968).

[5] Norrestam, R., Arkiv for Kemi 29, 343 (1968).

[6] Weller, W. W., and King, E. G., US Bur. Mines Rept.

Invest. No. 6245 (1963).

[7] Pankratz, L. B., and Kelley, K. K., US Bur. Mines

Rept. Invest. No. 6198 (1963).

[8] Reid, A. F., and Ringwood, A. E., J. Geophys. Res. 74,

3238 (1969).

[9] Prewitt, C. T., Shannon, R. D., Rogers, D. B., and

Sleight, A. W., Inorg. Chem. 8, 1985 (1969).

[10] Noddack, W., and Walch, H., Z. Elektrochem. 63, 269

(1959).

[11] Schmalzried, H., Z. Physik. Chem. 38, 87 (1963).

[12] Tripp, W. C., and Tallan, N. M., Bull. Am. Ceram. Soc. 47, 355 (1968).

[13] Zyrin, A. V., Dubok, V. A., and Tresvyatskii, S. G., Khim. Vysokotemp. Mater., Tr. Vses. Soveshch. 2nd

Leningrad 59 (1965, Publ. 1967); Chem. Abstr. 69,

62562t (1968).

[14] Schneider, S. J., Roth, R. S., and Waring, J. L., J. Res. Nat. Bur. Stand. (U.S.), 65A (Phys. and Chem.), No. 4, 345-374 (July-Aug. 1961).

[15] Hazek, B., Petru, F., Kalalova, E., and Dolezalova, J., Z. Chem. 6, 268 (1966).

[16] Trzebiatowskii, W., and Horyn, R., Bull. Acad. Polon.

Sci., Ser. Sci. Chim. 13, 311 (1965).

[17] Tresvyatskii, S. G., Pavlikov, V. N., and Lopato, L. M.,

Izv. Akad. Nauk SSSR Neorg. Mater. 2, 269 (1966).

[18] Collongues, M. R., Queyroux, F., Perez, M., Jorba, Y., and Gilles, J.-C., Bull. Soc. Chim. France 4, 1141

(1965).

[19] Lefevre, J., Rev. Hautes Temp. Refract. 1, 229 (1964).

[20] Thornber, M. R., Bevan, D. J. M., and Graham, J., Acta Cryst. 24B, 1183 (1968)

.

[21] Spiridonov, F. M., Popova, L. N., and Popils’kii, R. Ya.,

J. Solid State Chem. 2, 430 (1970).

[22] Komissarova, L. N., and Spiridonov, F. M., Dokl. Akad.

Nauk SSSR 182, 834 (1968).

[23] Kalinovskaya, C. A., Spiridonov, F. M., and Komissarova, L. N., J. Le8s-Comm. Metals 17, 151 (1969).

[24] Trezebiatowskii, W., and Horyn, R., Bull. Acad. Polon.

Sci., Ser. Sci. Chim. 13, 303 (1965).

[25] Trezebiatowskii, W., and Horyn, R., Bull. Acad. Polon.

Sci., Ser. Sci. Chim. 13, 315 (1965).

[26] Noguchi, T., and Mizuno, M., Solar Energy 11, 90 (1967).

[27] Companion, A. L., J. Phys. Chem. Solids 25, 357 (1964).

[28] Petru, F., and Muck, A., Z. Chem. 6, 386 (1966).

8
1.2. Titanium Oxides

The Ti-0 system has been studied exhaustively by

various workers over the past several years. DeVries

and Roy [1] reviewed the available literature up to 1954 and gave a tentative phase diagram. Since then,

different groups of investigators [2-6] have contributed much to the understanding of the various

phases in the Ti-0 system. The important phases

of the systems are the following: TieO (reported by

Kornilov and Glazova [7, 8] and characterized by

Yamaguchi et al. [9]), Ti30 (reported by Kornilov

and Glazova [7, 8] and characterized by Jostsons

and Malin [10]), rhombohedral Ti20 [3-6, 8, 11], a cubic monoxide, TiO*, having a wide homogeneity

range (0.75 ^ 1.30) with disordered rock salt structure at high temperatures containing many

vacancies [3-6, 12-16], a low temperature ordered

monoxide recently characterized by Watanabe et al. [17], an ordered TiOi.2 B [18-20], a sesquioxide Ti2Os

with a narrow homogeneity range, Ti306, Magneli

phases of the general formula Ti n02n_i (4^71^10)

and Ti02 . The phase diagram of Porter and Roy [5]

is reproduced in figure 1.1. Recent studies have re- vealed the complicated nature of the Ti-0 phase

diagram; according to Hirabayashi and co-workers

[21], below 670 K, there is an almost infinite number

of ordered phases in the composition range 0.09 <

O/Ti<0.40 and also a number of higher Magneli

phases (n^ll-53; range, 1.90 <0/Ti< 2.00) (22, 23].

Titanium oxides show a wide variety of interesting

electrical, magnetic and optical properties and interesting phase transitions. We shall now critically dis- cuss some important aspects in this section.

Figure 1.1. Ti-TiO* phase phase diagram showing various oxide phases ( after Porter and Roy [5]).
TiOz (a: <0.70): Metallic titanium exists in two allo- tropic modifications, a (low temperature, hexagonal)

and (3 (high temperature, bcc). The a—>j3 transition occurs at 1155 K [2, 6]. Oxygen addition to Ti stabilizes the a phase and oxygen can be dissolved

interstitially up to ~30 atom percent [6]. From

measurements of resistivity [7, 24], Seebeck coefficient [7] and rate of oxidation [25], it was suggested that a stable phase, TieO, may exist. Recently,

Yamaguchi et al. [9] have fully characterized this

phase. Ti6 0 is hexagonal with ordered superlattice

structure. Electron/neutron diffraction and DTA

studies show an order-disorder transition at ~710 K;

the phase is stable up to ~1100 K.

Ti30 and Ti2 0 have a hexagonal ordered structure

and they differ from the solid solutions of Ti and

oxygen by the ordered arrangement of the oxygens

in the suboxide lattice and by the greater stability

of the chemical bond [3, 4, 7, 8, 10]. Andersson [26]

found that the so-called 5-phase of TiOx (* = 0.65)

has a hexagonal structure and the isotropy in TEC

values are probably connected with the occurrence

of the close metal-metal contacts, which are present

both parallel and perpendicular to the c axis.

TiOx (0.70 ^ ^ 1.30): The rock salt type TiO

phase exists over a wide composition range depending on the temperature, e.g., from TiOo .70 to TiOi .25 at 1673 K and TiOo.9 to TiOi.26 at temperatures

below 1260 K [3-6]. An important feature of this

phase, as shown from density and lattice parameter

measurements, is that its crystal structure has varying proportions of both titanium and oxygen vacancies [3, 4, 12-14, 16, 27-30]. At the composition

TiOo.70, the titanium lattice is almost perfect (-~4

percent vacancies, extrapolating to zero at ~TiOo.B)

and about 30 percent of the oxygen sites are vacant.

In TiOi.30, the oxygen lattice is perfect and about

25 percent of the Ti sites are vacant. Even in the

stoichiometric oxide, TiOi.oo* about 15 percent of

both Ti and O sites are vacant. These vacancies are arranged randomly at temperatures above the equilibrium temperature, 1260 K; at lower temperatures,

however, ordering of the vacancies occurs and the

cubic rock salt structure of TiOi .00 transforms to monoclinic phase [3, 4, 12, 17, 20, 31]. This monoclinic phase was first noticed by Naylor [32] from

enthalpy studies. The oxygen-rich TiOi .26 becomes

ordered after long annealing treatments at 1070 K

and the crystal symmetry changes from cubic to tetragonal [18]. There is another ordered structure,

the so-called 'transition structure’ [17] observed over

a wide composition range TiO0 . 7 -TiO1 . 2 B; this has an

orthorhombic structure and is observed in specimens

which were rapidly cooled from the melt (of TiO*;

cubic high temperature modification) and not well

annealed at low temperatures as well as in thin foils (of TiOx) annealed at temperatures greater than

870 K or heated by an intense electron beam [17,

31]. Thus, the high temperature cubic TiO0 . 7 -TiO1.26 phase transforms, on cooling, to various ordered

phases having different crystal symmetries by means

of vacancy (both cation and anion) disorder-order

mechanism. It is important to note, however, that

it is possible to retain the cubic phase at room

temperature by quenching from above 1270 K and

' a great deal of work has been done on these quenched

cubic phases.

Detailed investigations on well-characterized samples by Banus and Reed [28] and Taylor and Doyle

[29] have shown that the cubic lattice parameter

and the density increase with increasing * in TiO*

(in the range 0.86-1.24); p and dp/DT (negative) are small and increase slightly, a (n-type) increases

and xm is extremely small and unchanging, indicating that TiO is a typical metallic oxide [14, 28-

30, 33]. TiO becomes superconducting at liquid

helium temperatures [15-34]; Hulm et al. [15] found

that Tsc varies from 0. 1-1.0 K as * increases from

0.92 to 1.10; for other values of *, at atmospheric

pressure, the materials are normal conductors down

to T<0.08 K.

Annealing TiO* at 50-60 kbar pressure and at 1573 K decreases the total number of vacancies considerably (by ~17 percent) [28, 35]; according to

Taylor and Doyle [29], complete removal of vacancies in TiO* is possible by proper selection of temperature and pressure (e.g., 77 kbar and ~1850 K).

The removal of vacancies is accompanied by a rise

(for a given *) in lattice parameter, density and Tsc (from 0.7 to 2.3 K for TiOi.00)* slight increase in p

and a and decrease in compressibility and TEC.

These changes in properties are completely reversible and are unambiguously due to the pressure

effect because reheating the samples in the absence

of applied pressure (say at ^1570 K) causes them

to revert to the original vacancy concentration and

lattice parameter [28, 29]. Recently, Iwasaki et al. [36] have studied the effect of pressure on the lattice

parameters of TiOo .82 and TiOi.2 B and essentially

obtained the same results as those of Banus and

Reed [28] and Taylor and Doyle [29]. The change

in a parameter is small for both the samples; for

TiOo .82 the decrease is considerable for smaller pres- sures while TiOi .26 shows a decrease at relatively

higher pressures (10-60 kbar).

10
5 TiO* forms solid solutions with VO* and NbO*

but the solubility is low (^10 percent) [30, 37].

On the other hand, Gel’d et al. [37] noticed that the

oxygen-rich phases TiOi .20 and VO1.25 have an appreciable mutual solubility (~20-25%). BeO, MgO,

and CaO also appear to form solid solutions (up to ^50%) [37], Loehman, Rao, and Honig [30] found

that the vacancy concentration is decreased in TiOx after solid solution formation; both TiO and (Tio .96 Vo.os)0 show metallic behavior.

Since the low temperature ordered TiOz phases

have been characterized only recently, reliable data on the physical properties are lacking at present on

these materials.

Ti203

: Ti203 has a narrow range of homogeneity

(*=1.49-1.51 in TiOx) [3, 4] and has the rhombohedral crystal structure at room temperature

[12, 38, 39] and shows an unusual behavior in

the thermal expansion of the lattice parameters

in the range 450 to 650 K [12]. A more recent

study [40] on pure material shows that the abrupt

change in lattice parameters sets in at a lower

temperature (~400 K); the c/a ratio increases

and there is a 7 percent decrease in the volume

of the unit cell in the range 390 to 470 K. There

is no change in the crystal symmetry in this

temperature range. Magnetic susceptibility does not

show any significant anomaly in the range 400 to 550 K even though a two fold increase in the magnitude has been noted [12, 41-43]. The existence of

long range magnetic order in Ti203 has been much

debated on the basis of various experimental investigations [38, 44-46], but recent neutron diffraction study by Moon et al. [47] have clearly confirmed

the absence of antiferromagnetism in Ti2 03 below

400 K. A specific heat anomaly has been reported

in Ti203 in the range 450 to 600 K [32, 48]. Rao et al. [49] have found a broad endothermic peak (centered

around 410 K) in the DTA curve (see figure 2 in

introduction section).

Electrical properties have been examined by many

workers in the literature [12, 33, 38, 50, 51]. Morin

[33] found that below 450 K Ti203 is a semiconductor

with a small energy gap (Eff ~0.05 eV); above this

temperature it behaves like a metal with a ten-fold

drop in resistivity at Tt

. Yahia and Frederikse [51]

observed the semiconductor-to-metal transition at 450 K, the drop in resistivity being of a factor of 40.

Abrahams [38] noticed the transition, but found that

Tt was ~660 K. Honig and Reed [52] have carried

out a detailed investigation on highly pure and

nearly stoichiometric single crystals of Ti203 ; they

have noted that in the range 100 to 300 K, the material behaves as an intrinsic semiconductor with a band gap of 0.03-0.05 eV (depending on the sample)

and the drop in p beginning at ^400 K and extending up to 470 K. The drop in p in the 400 to 470 K

range was about hundred-fold. The resistivity of

Ti203 goes through a shallow minimum at ^800 K

and rises again with further increase in temperature.

Hall data [52] below 273 K indicated the charge

carriers to be positively charged and with high mobilities. Seebeck coefficient of pure Ti203 is positive at ordinary temperatures [51, 53] (Loehman [53] re- ports smaller values compared to Yahia and Frederikse [51] for pure Ti203 ) and drops to smaller

values in going through the transition region (400 to

470 K).

The nature of the semiconductor-to-metal transition in Ti203 has been discussed by various workers

in the literature [54-64]. The gradual decrease in p

and a in the region of the transition, the broad

nature of the anomalies in x, heat capacity and

DTA measurements indicate that Ti203 is a relatively

wide band material and that the semiconductor-tometal transition is of higher order [52, 54, 58-64].

Since the absence of antiferromagnetism is definitely

established in this material [47], a plausible mechanism appears to be that of band broadening or shifting [58, 61, 62, 64-66]. The presence of c axis pairs in

the corundum structure to produce a cation sublattice band [54] together with a strong trigonal

crystal field will produce a filled band with one 3d

electron per Ti3+ ion (figure 1.2). The rapidly inFigure 1.2. Schematic band strurture of T^Oa (after [65]).

11
creasing c a ratio in the vicinity of 400 to 470 K

[40] brings the anions and cations much closer and

thus provides for a progressively greater overlap

among the corresponding wave functions (Note that

c a ratio at room temperature is anomalously small;

see figure 1.5). This, in turn, leads to narrowing

and then to a disappearance of the valence bandconduction band separation (figure 1.2). The p-type

nature of the charge carriers can be explained by

assuming that either the carriers in the valence

band dominate the conduction or with a model in- volving joint participation of electrons and of holes

with comparable effectiveness.

Honig and co-workers [52, 67] have shown that a

slight amount of impurity remarkably affects the

electrical properties of this material. A variety of

values of Tt as well as the magnitude of the change

in p (at Tt) reported in the literature [33, 38, 51]

may as well be due to the small amounts of impurities

in the samples of Ti2 03 employed in the investigations.

Ti2 03 forms solid solutions with V2O3 throughout

the composition, but the solid solutions so formed

are not ideal and do not obey Vegard’s law [30, 68].

The interesting observation is that at rc — O.l in

(Tii-j Fz) 203 the values of o, c, and a are the same

as the corresponding values for pure Ti2 03 after it undergoes the electrical transition [30, 40]; the c parameter of Ti2 03 is increased by ^3 percent by

the incorporation of ~20 percent V2 C>3 while the

corresponding a parameter contracts by —T.5 percent. This strongly lends support to the idea [39]

that strong metal-metal bonding across the octa- hedral edges takes place in Ti2 C>3. Recent resistivity

data [69] on (Tii-zV^Os show that for 0^x^0.04,

p undergoes a change in almost the same temperature

range as undoped Ti2 03 ; samples with high x are metallic at room temperature and exhibit no transition. This observation is consistent with the crystal

data of Loehman, Rao, and Honig [30].

Ti3 05: Ti305 , the precursor of the Ti„02n _i Magneli

shear structures, is monoclinic at room temperature.

Asbrink and Magneli [70] reported that Ti305 transforms to a distorted-orthohombic (more exactly,

monoclinic) structure reversibly at ~390 K, where

as Naylor [32] from his enthalpy data, indicated a transition at ~450 K. Keys and Mulay [43], Mulay

and Danley [71], and Vasilev and Ariya [72] showed

that xm of Ti3 05 shows a sharp discontinuity at ~460 K, with a 30 K hysteresis on cooling; both

above and below the Tt , xm is temperature in- dependent. Keys, Mulay, and Danley [43, 71]

and Keys [73] have suggested that the transition

in Ti3 05 may correspond to a semiconductor-to- metal transition. X-ray studies of Asbrink and

Magneli [70] showed the presence of metal-metal

bonding in the low temperature form of Ti305 and

above Tt , these bonds get broken. Based on considerations of the localized versus collective electron

behavior of the d electrons in Tin 02n _i Magneli

phases, Goodenough [74] has suggested that a semiconductor-to-metal transition may exist in

Ti305 . Bartholomew and Frankl [75] made resistivity

measurements on flux grown Ti3 05 single crystals

and confirmed the semiconductor-to-metal transition

in this material. Recently, Rao and co-workers

[49] have examined the transition in Ti305 in detail

employing high temperature x-ray diffraction, DTA,

electrical resistivity, ESR and Mossbauer techniques.

These workers established the first order nature of

the transition in Ti305 with appreciable latent

heat and hysteresis; no magnetic ordering was

noted in the low temperature phase. At Tt (448 K),

p drops by two orders of magnitude and below

Tt , Ti3 05 is semiconducting with Ea of ~0.3 eV.

Slight amounts of Fe and W reduce the transition

temperature and the magnitude of resistivity

discontinuity and at about 5 percent Fe dopant

concentration completely suppress the transition.

Since metal-metal bonding in Ti305 has been

definitely established [70], it appears that Goodenough’s model [54, 55, 61, 74] would be applicable

in explaining the transition in this material. According to Goodenough, in the titanium and

vanadium oxides, in the high temperature high

symmetry phases, the cation-cation distance is greater than the critical cation-cation separation

(see [61] for details) and the charge carriers move

in partially filled cation sublattice d bands and

metallic behavior is exhibited only in the materials

with well-defined Fermi surface. As the temperature

is lowered through Tt , however, the band breaks

up into pairs of atoms that are bonded by homopolar

bonds in which the conduction electrons are trapped

and a change to lower crystal symmetry (and

semiconducting phase) occurs to produce a filled band separated from an empty band by a finite energy gap. The atomic rearrangements at Tt are subtle in Ti305 [49, 70] and it appears that these are

sufficient to bring about an electrical transition in

this material. According to Goodenough’s model,

we expect electrons to be the charge carriers in

Ti3 05 and the available Seebeck coefficient data

[76] seem to confirm this prediction.

12
Iwasaki et al. [77] have reported the existence of a metastable form of Ti305 (designated as D) which

is identical with the high temperature form of T13O5

1 [49, 70] (D' form), hut the two differ in their properties with respect to oxidation. D and D' forms can

be prepared by the hydrogen reduction of anatase

and rutile respectively at ~1520 K and give almost

identical x-ray diffraction patterns, but on oxidation

at ~920 K, D gives rutile while D' gives a mixture

of anatase and rutile modifications of Ti02. The low

temperature form (M form of Iwasaki et al. [77]) can be prepared by the vacuum sintering of TiCh

(anatase form) and Ti metal at ''-'1520 to 1570 K

and yields rutile and anatase on oxidation. For the

phase transition D'—»M, Iwasaki and his co-workers

report a Tt slightly greater than 373 K. The D form

could easily be changed to the M form by cooling

from 1620 K under vacuum. These observations suggest that differences in the oxygen coordination

around Ti may give rise to stable and metastable

modifications of ThOs and the Tt as well as electrical

and related properties may also differ accordingly.

From this point of view, careful reinvestigation of

the transition in Ti305 is worthwhile on well characterized samples of this material.

TinC>2n-i: The existence of Magneli phases in the

Ti-0 system was first reported by Andersson et al.

[3, 4] in 1957. These are a homologous series of tri- clinic phases of composition Ti„02n-i with 4 ^ n ^ 9. They are bounded by monoclinic Ti3Os on one side

and by reduced rutile (TiCh-x) on the other. The

structures with 4 ^ n ^ 9 can be described [3, 4, 22,

23, 78-84] as being built-up of slabs of rutile-type structure which have infinite extension parallel to the (121) plane and have a characteristic finite width

corresponding to n TiOe octahedra (where n is the

integer in the chemical formula, Tin02n-i) in the third

direction. The adjacent slabs are mutually related by

crystallographic shear (CS), the shear plane and

shear vector being (121) and [Oil] respectively in

the idealized Tin02n-i structure. This CS causes the

titanium atom positions in one slab to correspond

to unoccupied or interstitial positions in the next

slab and decreases the crystal symmetry from tetra- gonal to triclinic while decreasing the size of the

unit cell. However, the structure of TiioOi9 (

= 10)

cannot be explained in this way and the structure

is not yet known.

Anderson and Hyde [81] suggested that rutilebased shear structures other than the (121) family

probably exist, in particular, a (132) family in re- duced rutile. Detailed studies using electron microscopy and diffraction by Hyde and co-workers

[22, 23] showed the existence of a new family of

ordered phases, Ti„02 n-i (15^n^36) derived by

CS planes parallel to (132). Possibility of the existence of another Tin 02 „_i (n> 35) family (Oil) shear

planes was suggested by these workers since con- tinuous streaking along (Oil ) directions was observed

occasionally in the electron diffraction patterns of

reduced rutile and detailed studies are yet to be

made. Studies are also necessary in the composition

range 9<n<15, (i.e., TiOi. 8 89-TiOx. 933). The rutile based Ti n02 „_i series with 4 ^ n ^ 9 have

periodic stacking faults introducing extra planes of

metal ions for every n planes in one crystallographic

direction (121) and Magneli [79, 82] has pointed

out that the condensation of the defects into com- mon planes can minimize the elastic energy of the

crystal. Goodenough [74, 85] has recognized that

such systems exhibiting shear structures may contain collective d orbitals and that the regularity of

the spacings between these shear planes is due to the existence of a Fermi surface and to the energy

stabilization achieved by either creating or increasing the energy discontinuity across a Brillouin zone

surface at the Fermi surface. This stabilization is greater, the narrower the bands are, and hence

spontaneous crystallographic distortions could result at lower temperatures. Also, since the existence of a Fermi surface is a collective electron property [61],

it can be argued that crystallographic distortions in

the Magneli phases may also be associated with the

metal-to-semiconductor transitions (on cooling) . As a matter of fact, the available data indicate that elec- tronic transitions do occur in these materials associated with crystal distortions; however, the crystal

distortions appear to be minor (at least in Ti4C> 7 , for which detailed high temperature x-ray data are available [75, 86]) and do not involve structural

changes but only minor discontinuities in the lattice

parameters at the transition temperature.

Detailed x-ray [75, 86], xm-T [43, 71, 72], DTA

[75] and P-T [75, 87] studies show that there are two phase transitions in Ti407, at '—'125 and at 150 K. There is 8 to 10 K hysteresis associated with

the first transition and some structural changes;

the 150 K transition is associated with a change

from semiconductor to metal behaviour as well as magnetic susceptibility and DTA anomalies, but no

apparent structural change. TuOg exhibits transitions at 130 and 136 K and resistivity measurements

indicate discontinuities at these temperatures, but

the sample remains semiconducting above Tt [43,

71, 75]. DTA and xm-T measurements indicate a

13
transition in TigOn at 122 K [43, 71, 75], but the

resistivity data indicate a T; of ~130 K and semiconductor behavior above and below this temperature [75]. According to Bartholomew and Frankl

[75], Ti80i5 exhibits semiconductor behavior in the

range 78 to 295 K and no transitions exist in this

material, xm -T data are not available at present.

The detailed physical properties of Ti7 0i3 and TigOn

are not known at present.

Ti02 : Titanium dioxide, Ti02 , has a narrow homgeneity range ( x-— 1.983-2.000 in TiOx) [3-6, 88], and

exists in three polymorphic forms, namely, anatase

(tetragonal), brookite (orthohombic) , and rutile

(tetragonal) at atmospheric pressure. In each of

these, Ti4+ ion is surrounded by an irregular octa- hedra of oxide ions, but the number of edges shared

by the octahedra of oxide ions, but the number of

edges shared by the octahedra increases from two

(out of twelve) in rutile to three in brookite and to four in anatase. The rutile form of Ti02 is the most

stable form and both anatase and brookite transform

to rutile on heating. High pressure modifications of

Ti02 have been reported by Dachille and Roy [89],

Bendeliani et al. [90], and McQueen et al. [91].

Phase transformations involving various modifications of Ti02 have been extensively studied in the

literature.

Anatase transforms irreversibly and exothermally

to rutile in the range 880 to 1200 K depending on

the method of preparation of the sample and at atmospheric pressure, the transformation is time

and temperature dependent and is also a function

of the impurity concentration. Rao and co-workers

[92-96], Sullivan and Cole [97], Iida and Ozaki [98],

Suzuki and Kotera [99], and Shannon and Pask

[100] have carried out systematic investigations of

the anatase-rutile transformation. The rate of trans- formation of spectroscopically pure Ti02 in the anatase modification was found to be immeasurably slow

below 885 K and extremely rapid above 1000 K

[92, 101]. The transition is a nucleation-growth process, and follows the first order rate law with an

activation energy of ~90 kcal/mol (fig. 1. 3a). Impurities like CuO, CoO, Li2 0, and Na2 0 and re- ducing atmospheres accelerate the transformation

while sulphate, phosphate, and fluoride anions, P5+ and S

6+ ions and vacuum stabilize the anatase modification and inhibit the transition process [94-96, 98,

100, 102-104]. Values of Tt and activation energy

are generally higher in impure samples than in the

pure sample.

The defect structure of Ti02 is not yet fully understood. There is considerable evidence for the formation of both titanium interstitials and oxygen va- cancies [61, 104-108]; the atmosphere in which the

defects are formed seems to be an important factor.

Since the anatase-rutile transformation involves an

overall contraction or shrinkage of the oxygen struc- ture as indicated by a volume shrinkage of ^8 per- cent and a cooperative movement of ions (displacive

and reconstructive transformation [94, 109, 110]),

the removal of the oxygen ions would be expected

to accelerate the transformation; formation of tita- nium interstitials would inhibit the transformation.

This implies that a reducing atmosphere or doping

with an ion of lesser valence state (Cu2+ or Co2+

) ; would accelerate the transition as indeed found ex- perimentally. Vacuum or doping with anions will similarly give rise to Ti3+ interstitials and hence the

inhibition of the transformation process and would

increase the temperature of the anatase-rutile transition.

Yoganarasimhan and Rao [94] noticed that the

particle size and crystallite size of anatase increase

markedly in the region of the crystal structure trans- formation; smaller particle size and larger surface area of anatase samples favor the transition. Detailed x-ray studies indicated that the unit cell ex- pands prior to the transformation to rutile. Recently, Vahldiek [111] examined the effect of

pressure on the anatase-rutile transformation. Electrical resistance and quenching experiments indicated that Tt and AH for the transition decrease

with the applied pressure; at a pressure of 1 bar,

j AjFT-—

'

— 2.8 kcal/mol and his observations corrobo-

} rated the data of earlier workers. Discontinuous

l change in p was observed at Tt at various applied

pressures and the irreversibility of the anatase-rutile j transition at high pressures (up to 24 kbar) was

confirmed.

Kubo et al. [112] and Tekiz and Legrand [113] ob- j served the transition of anatase to rutile by grinding; anatase-type Ti02 obtained from TiCR method

is changed to rutile after grinding for 96 h, while

Ti02 (anatase form) obtained from the H2 S04 method

changed to an amorphous state. Brookite, the orthorhombic modification of Ti02 occurs in nature; it has not been possible to prepare

this polymorph in pure form in the laboratory.

Brookite-rutile transformation was first reported by

Schossberger [114] and examined in detail by Rao

and co-workers [115]. Below 990 K, the rate of

transition is extremely slow and sluggish and above

this temperature, it follows a first order rate law

with little or no induction time (fig. 1. 3b). The

14
Figure 1.3. Transformation of anatase (a) and brookite (6) as a function of temperature at different times (after Rao and co-workers

/A And /b are the fractions of anatase and brookite forms in the mixture. Kinetics of transformation of anatase and brookite to rutile are shown in (c) and (d) respectively (/r = fraction of rutile).

15
transition is irreversible with an energy of activation ~60 kcal/mol and — (100=fc75) cal/mol.

DTA studies give 7\« 1020 K.

As regards the comparison of the anatase-rutile

and brookite-rutile transformations, it may be mentioned that the kinetics and mechanism are exactly

similar (fig. 1. 3c and 1. 3d), but the energy of activation is larger in the anatase-rutile transition; the

entropy of activation is large and negative for the

brookite-rutile transition while it is small and has a positive value for the anatase-rutile transition. This

discrepancy may be understood in terms of the

change of symmetry (lower to higher) in the former

case and the absence of such a change in the latter case. A new high pressure modification of TiOa was

synthesized by Dachille and Roy [89] at pressure 15-

100 kbar and temperatures 298 to 773 K. It forms

easily from anatase, but appears to be metastable

with respect to rutile. The structure of this modification is not known. The new form converts slowly to

rutile on heating between 770 to 973 K via a shortrange order phase.

According to McQueen et al. [91], rutile at ordinary

temperatures transforms to an orthorhombic modification at 330 kbar pressure with considerable de- crease in volume (^21%). Bendeliani et al. [90]

have synthesized the orthorhombic modification at ~70 kbar pressure and 970 K (pressures 40-120

kbar and 670 to 1770 K, under hydrothermal conditions). This phase reverts to rutile on heating to 720 to 870 K. The detailed physical properties of

this phase are not known at present.

Even though the anatase and brookite modifications have not been investigated in detail, extensive

literature is available on various physical properties

of the rutile modification of TiC>2 [43, 116-120] and

have been treated in detail in many review articles

[57, 61, 63, 116, 121, 122]. Stoichiometric TiC>2 shows

a temperature independent susceptibility consisting

of a diamagnetic and a Van Vleck contribution [43,

121, 122a]. Ti02 has a high room temperature re- sistivity and exhibits semiconducting behavior; in- trinsic conduction seems to set in only at very high

temperatures (>1500 K) with a large energy gap

(~3 eV). Doped or reduced Ti02 has a lower re- sistivity and exhibits impurity conduction. Detailed

resistivity, Seebeck coefficient and Hall mobility

studies over a range of temperatures have not been

able to provide a clear understanding of the mechanism of conduction in pure and doped Ti02 [1 lb120; 122b—122d] ; multiple band conduction seems

to be conclusively proved, but the existence of band

or hopping (large or small polaron) type of conduction is still in dispute.

As mentioned earlier, the defect structure of Ti02

is not yet completely understood and evidence is in

favor of both anion vacancies and Ti interstitials

[61, 104-108, 123, 123a]. Hauffe [123a] and Rudolph

[123b] noted that in the range 882 to 1273 K,

p a Po2-1/n where n varies from 5.7 to 4.4 whereas

Greener et al. [123] obtained n = 4 above 1470 K

and no dependence of p on p02 below 1170 K (ideally, n should be 6, 5, and 4 for anion vacancy, Ti4+ and

Ti3+ interstitial mechanisms respectively). High temperature Ea values (1.8 eV) of both workers agree

very well. Internal friction measurements [123c] on vacuum-reduced rutile (for 1-3 h at 1530 K), on

the other hand, indicate that the predominant pointdefects are neither anion vacancies nor cation interstitials and the observed data in the range 50 to 300

K are best explained by assuming that Ti interstitials are associated in pairs in TiC>2 . Resistivity studies on rutile under various pressures [124] indicate a

large decrease in p up to ^6 kbar; at higher pres- sures (up to ^100 kbar), the resistance change was

small suggesting that Ti02 retains semiconductor

behavior over a large pressure range with the possibility of exhibiting metallic conduction at very high

pressures [124, 125]. Preliminary results of Kawai

and Mochizuki [125a] actually indicate a highly

conductive state in Ti02 at pressures greater than 2

Mbar.

TiC>2 in the rutile modification forms solid solutions with other transition metal oxides with rutile

related structures; thus, solid solutions with VO2 [126-131], Zr02 [132, 133], Nb02 [134-136], Mo02 [134], Ta02 [135], W02 [137, 138], Ir02 [139] are known and the physical properties are well investigated. In most of the cases, undistorted rutile struc- tures are formed and the properties of ZO2 (Z = transition metal) are modified by the presence

of Ti(>2 .

16
Titanium oxides

Oxide and description of the study

Data Remarks and inferences References

TiO* (*<0.70)

TieO

Crystal structure and

phase transformation.

TiaO

Hexagonal; space group, P31C; a = 5.14 A,

c = 9.48 A. DTA shows a transition at ~710 K; AH = 360 cal/mol; AS = 0.51

e.u.

The transition appears to be an

order -disorder type with respect

to the interstitial positions of the oxygens; no superlattice reflec- tions are noted in electron dif- fraction patterns of a sample

quenched from 970 K.

[9]

Crystal structure.

TijO

Hexagonal; space group, P312; a =5.1418

A, c = 14.308 A. The structure consists essentially of

a close-packed hexagonal arrange- ment of Ti atoms with every sec- ond layer of octahedral interstices normal to the c axis vacant; one

third of the oxygen sites in the

occupied layers are empty and

these vacancies have an ordered

arrangement in the direction of the c axis. Physical properties of Ti30 are not known in detail at present; it appears to be stable up to ~1670 K.

[10].

Crystal structure. Rhombohedral; space group, P3ml; a =

2.959 ±0.003 A; c =4.845 ±0.004 A. TEC

(293-773 K): ||“: 10.3 X 10-«/o C; || C :9.2X

10“«/o C.

The ordered structure, described as TioOi-y exhibits a homogeneity

range which increases as the temperature is lowered.

[4, 11, 26]

TiOo.es (5-oxide) Hexagonal; space group, P6/mmm; Z — 3; a=4.9915 A; c = 2.8794 A. TEC: (293-

773 K): ||° = 10.0Xl0-6/°C; || e =9.0X

io-«/ 0 c.

This is a deficient interstitial solu- tion of oxygen in a metal atom

lattice.

[26].

TiO* (0.70^ *0.25)

Crystal structure and

physical properties.

[28]

17
Titanium oxide—Continued

Oxide and description Data Remarks and inferences References

of the study

High temperature quenched (cubic, Fm3m) phases:

Composition

X

Density

(g/cm3 )

Lattice

parameter,

.(A)

% Vacancies

(Total)

Decrease in vacancies

(after pres- sure treat- ment), %

Tsc*

(K)

P (300 K)

(104 fi cm) (mV/°C)

0.86 5.057 4.189 31.5 14 <1.3 2.8 4 0.91 5.025 4.186 29.9 12 <1.25 ~3.0 5 1.00 5.000 4.184 28.0 17 1.35 2.8 8 1.18 4.907 4.173 24.7 11 1.7 3.4 11 1.24 4.870 4.170 23.3 22 2.0 3.6 12

• Pressure treated samples (at 50—60 kbar and 1370 to 2070 K). No magnetic susceptibility was observed for any TiOz samples down to the limit of the senstiivity of the magnetometer (Xji~150 X 10"6 emu); Hall I voltage observed was very small and no meaningful results could be obtained.

Vacancy filling in TiOi.oo and physical properties. [29].

Conditions of studyDensity

(g/cm3 )

Lattice para- meter, a (A)

% Vacancies

(total) Compressibility (X1013 cm2 /dyne)

TEC

(X106 deg-1

) at 298 K

7a Tsc

(K)

1773 K, 1 bar; 2 days,

quenched.

4. 97 ±0.003 4.1796 28.8 5. 5 ±0.08 6.73 1.18 0.7

1923 K, 77.4 kbar;

sudden quench.

5. 69 ±0.009 4.2062 0.0 4. 2 ±0.008 6.30 — 2.3

The gradual increase in density and lattice parameter with decrease in vacancy concentration under various pressure and

temperature treatments are studied in detail. An empirical equation correlating the lattice parameter, pressure and temperature i6 deduced from experimental data.

18
Titanium oxide—Continued

Oxide and description of the studyData Remarks and inferences References

Compressibilities of TiOz.

Experimental values are 1.6(4) and 2.8(7) X

10-13 cm2 /dyne for x=0.82 and 1.25 re- spectively.

These values compare well with

those of other transition metal

monoxides.

[34].

Seebeck coefficients of TiOz (0.8 1.2). Small and negative ( — 5 to —10 juY/°C) for

all x in TiOz • a increases with x and temperature.

These results are in agreement with

those of Banus and Reed [28]. Qualitative description of the band structure is given; Ef

changed appreciably with the composition x.

[140, 141].

Plasma resonance in TiOz.

Reflectance data show the plasma edges in TiOz in the range 3. 6-4.0 eV and the ac- tual value depends on x to some extent. The color of TiOz is correlated with these plasma edges.The studies indicate the metallic nature of TiOz. A band of 2.5 eV

in the reflectance spectrum is as- signed to an interband transition.

[14, 142].

Band structure of TiO. Tight binding and AP^ methods of theoretical calculation of the band structure

of TiO indicate that the 3d bands are wide (~6 eV) and the electrical conduction can be assumed to take place in the wide band. Most of the transport properties are predicted in good agreement with

experiment.

Calculations on TiOz taking into ac- count the vacancy concentration

and composition do not seem to be satisfactory in explaining the observed data. As pointed out by

Honig et al. (ref. [4], p. 130) cation- anion-cation overlaps also should

be taken into account in computing the band structure in addition to the direct cation-cation overlap in order to get a reasonable picture of the band model in oxide materials. Results of Kawano [141] indicate that the Fermi level and density of states

shift with the stoichiometry in TiOz.

[143-145].

Low temperature

ordered structures

Phase, stability range and conditions of observation:

TiOo.r-TiOo.g: thin

foils (of TiOz ) heated

above 870 K in the

electron microscope or heating by an intense

electron beam.

Orthorhombic; space group, 1222; Z=6;

a' = a V2; b —3a \/2; c' — a (

is the lat- tice parameter of the original, high temperature, cubic cell of TiOz).

This structure results from the or- dering of the oxygen vacancies

only since there are more oxy- gen vacancies than titanium

vacancies in the oxygen deficient

alloys; all oxygen vacancies are distributed on every third (110)

plane in the original rock salt type lattice while the Ti vacancies are distributed randomly.

[17, 19, 31].

19
Titanium oxide—Continued

Oxide and description of the study

Data Remarks and inferences References

TiOi.oo (composition Monoclinic; space group, A2/m; Z = 12; a = The transition temperature is [14, 17, 19, range TiO0 . 90-TiO 1 . 10 ); 5.855 A; 6=9.340 A; c = 4.142 A; 0 = around 1190-1220 K; but the 20, 28, 31]. quenched from 1773 K 107.53°. Vacancy concentration, 16.7%. process is slow (and requires long

and annealed for two p (300 K) = 2X10~4 V cm; p (77 K)=7X periods of annealing) since this is days (1170-1220 K).

TiOi .19

10-5fi cm. Positive temperature coeffi- cient; Absolute value of p is much lower

than that of the cubic phase.

%

a disorder -order transformation.

This structure can be described

in terms of vacancy ordering:

i.e., in every third (110) plane of the original cubic cell, half of the

titanium and half of the oxygen

atoms are missing alternately.

The spacing of this (110) plane

is slightly larger than that of the

other {110} planes, and as a result, the symmetry of the original

cubic structure is lowered to monoclinic. Essentially the same

structure has been observed in the composition range TiOo.90-

TiOj.io which implies that a small excess or deficiency of titanium and oxygen vacancies can be tolerated in TiOi.oo structure.

Samples annealed for Both types of ordered structures, TiO and Electron diffraction patterns indi- [18, 31]. long periods below

973 K.

TiOi .25

TiOi. 25 , are noticed. cate that the ordered phases coexist as alternate thin layers of

TiOi.oo and TiOi .25 parallel to (210 } c planes.

Alloy annealed for 7 Tetragonal; space group I4/m; Z = 10; a = The type of ordered structure in [18, 31,

days at 973 K and then ao-\/lO/2 =6.594 A; c = ao = 4.171 A. (ao is TiOi .25 is different from that in 146, 147]. at 673 K. the original cube cell edge). [Vacancy

concentration, ~22%].

TiOi.oo; the oxygen sublattice is fee and fully occupied while the titanium sites get ordered similar to Ni4Mo and Au4 Mn type super

structures.

Transition structure Both ordered TiO and TiO1.25 structures The appearance of the transition [31]. (range TiO0 . 7 -TiO 1 . 25). coexist, the amount of the latter increas- structure is of importance for Samples rapidly cooled ing with the oxygen content of the speci- understanding the process of from the melt and not men. The transition structure observed transformation from the dis- well annealed at lower occasionally in the TiOi .25 specimen seem ordered to the ordered form.

temperatures. to be due to insufficient ordering treat- ment and it is considered that the transi- tion structure would ultimately transform

to the ordered TiOi .25 structure.

When the disordered specimen of any composition is cooled to the equilibrium temperature, the vacancies concentrate firstly on

every third atomic plane parallel to the (110) plane of the original

20
Titanium oxide—Continued

Oxide and description of the study

Data Remarks and inferences References

rock salt structure, but are randomly distributed on these planes giving a diffraction pattern of the transition structure. On further annealing, the vacancies are distributed into the proper positions to make the

ordered structure of TiOi .26 type. Watanabe et al. [31] point out that the high temperature

cubic and orthorhombic TiO*

observed by Hilti [19] are the same as the 'transition structures.’

TisOj

Crystal structure and

high temperature

x-ray studies.

T = 300 K: Hexagonal (corundum); space

group, R3C; 6; a ^5.1572 ±0.0002 A; c* 13.600 ±0.001 A. T*540 K:

Hexagonal; space group, R§C; Z = 6; 5.122 A; c- 13.90 A. AV (on heating from

300 to 500 K) = -7%.

There is no change in the crystal

symmetry in the range 300=500

K but the thermal expansion

parameters behave in an anomalous way; the a parameter

decreases while c parameter in- creases. The unit cell volume

decreases.

[30, 39, 40].

Magnetic properties. Xm (300 K)~1.0X10-4 cgs units and shows

a two fold increase in the range 400—550

K. Xm below 400 K is almost temperature

independent.

Magnetic moment <0.03 /ubXm-T data and polarized neutron

diffraction data do not indicate magnetic order in T^Oa below

400 K.

[12, 41-43,

47].

Thermal properties. Cp (400 K)~0.2 cal/mol, deg. and shows a broad X-type anomaly in the range 450-

600 K. AH (calc.)~36 cal/mol [48]; 215

cal/mol [32], DTA shows a broad endothermic peak (centered at ~410 K).

These data show the second (or higher) order nature of the

transition

[32, 48, 49].

21
Titanium oxides—Continued

Oxide and description of the study

Data Remarks and inferences

Electrical properties of TiOi.501. Tempera- ture (K)

P

(ft cm)

Rc (cm3 / C)

Ap/

P

a

aWl

K)

4.2 K (1.3-1. 8) X104 — (b) 3-4

(^200

kG) —

77 K 0.2-0.

8

0.4-

1.5

<10“3

273 K ~10~2 0.08-

0.11

<10-3 56

600 K ~3 X 10~4 — — 45°

(a) Magnetoresistance; Ap/po^H2

. (b) Not measurable, (c) At 400 K.

Ea ( <20 K)~0.001-0.003 eV; Ea (100-300 K)~0.027-0.050 eV. T = 4.2 K: mh (calc.)~720-840 cm2 / Vs; ne ~1010

, cm-3

.

This is a careful study on well characterized pure single crystals.

Various ranges of p and Rc values

are encountered because of vari- able sample purity; common

impurities like C, N affect the

properties. The study by Honig

and Reed [52] emphasizes the need for painstaking efforts in the preparation and characteri- zation of oxide materials. Slight

anisotropy in p has been noted. Impurity conduction seems to be

predominant below 20 K; in the range 100-300 K, intrinsic be- havior is noted. Hall mobilities are the highest encountered

hitherto in pure oxide materials;

at higher temperatures, ph seems

to decrease. Magnetoresistance

data indicate that a multiband

conduction exists in Ti203. The

band model proposed (fig. 1.2) can explain the observed data

without invoking a magnetic

ordering in Ti203 at low

temperatures.

UHF Dielectric constant studies. UHF (~10 GHz): * (T = 77 K)=30;

p (UHF) >p (DC).

The imaginary part of the dielectric constant shows a maximum

at 225 K, but this is not con- nected with any transition in Ti203

. Raman spectroscopy Raman spectrum of Ti2C>3 exhibits seven modes as predicted for the corundum

structure. All the modes persist above the semiconductor-metal transition temperature indicating absence of space group

change. The Aig mode at 269 cm-1 (300

K) shows a 16% change in frequency and

increase in intensity at the transition.

The Raman results can be inter- preted in terms of the electronic and structural changes during the

transition.

TisOs

Crystal structure and x-ray studies.

Magnetic properties.

T = 300 K: Monoclinic; space group, C2/m;

Z = 4; a =9.80 A; 6=3.79 A; c = 9.45 A;

(3=91.75°. T>460 K: Monoclinic; a =

9.90 A; 6 = 3.78 A; c = 10.02 A; 0=90.75°.

The c parameter and the unit cell volume

show jump at T< (448 K) on heating.

There is not much change in crystal

symmetry at Tt but the unit cell volume increases. The transition

is of first order.

Xm (300 K)~10 8 cgs units; xm (600 K)~

2 X 10-4 cgs units, xm is temperature inde- Mulay and Danley [71] proposed a new model to explain the mag22

References

[52, 53].

[148].

[149].

[49].

[43, 49, 71].
Titanium oxides—Continued

Oxide and description of the study

Data Remarks and inferences References

pendent both above and below Tt ; large

jump at Tt is noted with hysteresis; Tt (heating) «462 K; Tt (cooling) as 432 K.

Xii-T data and Mossbauer studies do not

indicate long range magnetic ordering in the low temperature phase.

Rao et al. [49] have pointed out that the Mossbauer data on Fe-doped Ti3 Os

should be interpreted with caution since Fe is known to stabilize the high temperature phase of Ti3 Os. Neutron diffraction studies are urgently needed on Ti3 Os to

establish the presence or absence of magnetic order in this material.

netic behavior of Ti3Os and the Magneli phases. According to

this, below Tt , groups of cations

are distributed periodically

throughout the lattice; within

these groups, the d electrons are

localized. However, 'constrained

type’ antiferromagnetism sets in between specific neighboring d

electrons through homopolar

bonding of cations. Neighboring

groups interact via thermal ex- citation of electrons. Above the

transition, this type of ordering

is modified by crystal structure changes and 3d orbital overlap is large enough to bring about a nearly complete delocalization of

electrons over the entire lattice and the behavior is described by

the 'free electron gas’ model.

This approach is a slightly modified form of the one proposed by

Goodenough [74].

Thermal properties.

Electrical properties.

Heat content data on Ti3 Os exhibit an anomaly at ~450 K. DTA shows a sharp

endothermic peak at 450 K; the peak is shown in the cooling curve at ~415 K;

AH = 1.6 rt0.4 kcal/mol.

The first order nature of the transi- tion is established. Fe doping

reduces Tt and at higher concentrations (5%) suppresses the

transition; the transition at lower concentrations does not seem to be strictly first order.

[32, 49, 75].

p (340 K)«20 ft cm; p (500 K)«0.6 ft cm.

Semiconductor behavior below 450 K;

Ea = 0.29 eV. Large drop in p at T( (Bartholomew and Frankl [75] report a con- tinuous change in p through Tt). Metallic

behavior above Tt ; m* (calc., high temperature form) «20 m; a (340 K)~ —60

mV/°C [76].

The first order semiconductor-tometal transition is confirmed in Ti3 Os; Rao et al. [49] stress the

the importance of the purity of the material in determining the

resistivity discontinuity at Tt.

[49, 71, 75,

76].

ESR Studies. Multiplet structure is noted in the ESR

spectra at 77 K. Well defined spectra are not obtained at higher temperatures and

in doped samples (at 77 K).

The multiplet structure appears to be due to inequivalent cation

sites in the structure of Ti3C> 5 .

[49, 73].

23
Titanium oxides—Continued

Magneli phases, TinO„_i

n: phase Crystal structure, x-T and DTA Electrical properties References

studies

(T = 298 K: All phases are triclinic; space

group. Pi; Z = 2)

4; Th07 a = 5.600A; 6 = 7.134 A; c = 12. 468 A; a = Ti407 is metallic at 300 K and p [43, 71, 72, 95.05°; 0 =95.20°; 7 = 108.70°; (calc. increases by a factor of 103 at 75, 79, 84,

density, 4.32 g/cm3). 149 K on cooling and semicon- 86, 87]. T( = 125 and 149 ±2 K. Discontinuities ductor behavior in the range 130-

in the lattice parameters are noted at 149 K (Ea ~0.04 eV). At ~125

the 149 K transition; the volume change K, p shows an increase by about

is <0.001 and the extra reflections three orders of magnitude.

observed below 149 K suggest a doubling No Seebeck coefficient data

of the unit cell. The 125 K transition available.

is associated with ~8-10 K hysteresis At 300 K, m* «*13m and Hall data

and some structural changes seem to give a mobility of ~1 cm2 /V s, take place at this temperature. increasing to 4 at -~160 K; below

DTA (heating curve) indicates both the 149 K, no Hall voltage could be

transitions but the cooling curve indi- measured and hence ph<0.01

cates only the 149 K transition. AH val- cm2 /V s. ues not known. Least square refinement of the

X-T data do not indicate the low tempera- structure has shown that the

ture transition. Probably long range mag- sharing of octahedral faces, edges

netic order is absent. and corners between rutile layers

is similar to that in corundum

structures. Average Ti-0 and

0-0 distances are 2.012 and

2.826 A respectively. Average

Ti-Ti distances in a rutile layer

are 2.972 and 3.576 A across a shared octahedral edge and comer respectively.

5; Ti509 a = 5.569 A; 6=7.120 A; c = 8.865 A; <* = Discontinuities in p are noted at Tt ; [43, 71, 72, 97.55°; 0 = 112.34°; y = 108.50°. however, the sample appears to 75, 78, 79, T< = 130 and 136 K. be semiconducting above 136 K. 84]. DTA and x-T studies show anomalies at Ea (<130 K)~0.04 eV;

these phase transitions; no AH values are known. Probably long range magnetic

order is absent.

m* (>136 K) ~ 11m.

6; TifiOn a = 5.56 A; 6 = 7.14 A; c = 24.04 A; a =98.5°; p drops by a factor of 10 at 130 K [43, 71, 72,

0 = 120.8°; 7 = 108.5°. Tt = 122 K. and the sample is semiconducting 75, 79, 84]. DTA and x-T studies show this transition; both below and above Tt

. no AH is known. Probably long range Ea (<130 K)~0.08 eV; m*

magnetic order is absent,

o = 5.54 A; 6 = 7.13 A; c = 15.36 A; a = 98.9°;

( > 130 K) ~ 16 m. The Tt noted

here is less than that obtained

(122 K) from DTA and x-T data.

7; ThOn No data are available [79, 84]. 0 = 125.5°; 7 = 108.5°. No x-T, DTA data are available.

8; TigOis a = 5.57 A; 6=7.10 A;c = 37.46 A; a=97.2°; Semiconductor behavior in the [75, 79, 84]. 0 = 128.8°; 7 = 109.6°. temperature range 78-295 K; no DTA studies do not indicate a transition in indication of transformation.

the temperature range 78-295 K. x-T

data not available. Ea ~0.09 eV.

9; Ti90i7 o = 5.57 A; 6 = 7.10 A; c = 22.15 A; a =97.1°;

0 = 131.0°; 7 = 109.8°. No x-T, DTA data are available.

No data are available [79, 84].

24

n
Titanium oxides—Continued

Oxide and description of the study

Data Remarks and inferences References

Ti02 (anatase)

Crystal structure and

x-ray studies. T =298 K:

Tetragonal; space group, I4/amd; Z = 4; a =3.7845 ±0.0001 A; c = 9.5143 ±0.0004

A. TEC (300-985 K) ( /°C): || c

: 7.380 X

10-6 +6.620 X10"9 f +1.771 X 10-u t

2

; ±c

: 3.533 X lO-6 + 5.610 X 10"9 t + 4.315 X

10-12

1

2

, where t is the temperature in °C.

Careful work up to the transition point; the variation in the relative magnitudes of the values of TEC

are explained in terms of the inter- ionic distances. Earlier literature survey presented.

[150].

Kinetics of trans- formation of anatase —» rutile and thermal

properties.

The rate of transformation of pure anatase

is exponential (fig. 1.3); energy of activation is ~90 kcal/mol. The transition is immeasurably slow below 880 K and ex- tremely rapid above 1000 K and is accompanied by ~8% decrease in the unit cell volume. AH is ~ —2.8 kcal/mol (exothermic). Impurities usually stabilize the the anatase modification; Tt as well as the energy of activation are greater in these doped materials compared to pure

rutile. Smaller particle size and larger surface area seem to favor the transition.

The irreversible transformation of anatase to rutile is reconstructive

and involves changes in the secondary coordination. The particle size, the crystallite size and

the volume of the unit cell seem to increase prior to the trans- formation. The impurities seem to

act by way of creating anion

vacancies or cation interstitials in stabilizing the anatase phase.

[92-98, 100,

102-104].

Effect of pressure on

the transition and

resistivity studies.

Discontinuous changes in the resistance of the anatase samples noted at Tt

. In the range 3.8-24 kbar pressure, Tt decreases with the application of pressure; (dTt / dP) = —0.02° bar-1

, p (300 K, pre- pressed specimens, anatase) ~1012 Q cm;

p (300 K, pressed at 3.8-24) kbar ~107 Q cm.

Detailed work on the effect of pressure on the transition. Aff

for the transition also decreases with pressure; the oxygen defi- ciency seems to be responsible for the acceleration of the rate of

transition at higher temperatures. The results are in accord

with the volume change associ- ated with the anatase-rutile

transition.

[111].

Magnetic properties

of anatase.

Ti02 (brookite)

X=0.02X10-6 emu/g (essentially inde- pendent of temperature)

Not a very accurate value [122a]

because of the presence of impurities; the observed variation

of x with T is expalined on the

basis of the presence of Ti+3 ions

in the anatase samples.

Crystal structure T = 298 K

Orthorhombic; space group, Pcab? Z = 8; a =9.25±0.03 A; fi = 5.46±0.02 A; c =

5.16±0.01 A.

Data on the naturally occurring pure brookite sample; contains

references to the earlier data.

[151].

Kinetics of brookite

rutile transition and

thermal properties.

The transition is irreversible and time and

temperature dependent (fig. 1.3); below

990 K, the transition is extremely slow. Above 990 K, first order rate law is obeyed. Energy and entropy of activation

The enthalpy of transition is small compared to the anatase-rutile

transition. This indicates that the polymorphic transformation

may not be of the reconstructive

[115].

25
Titanium oxides—Continued

Oxide and description of the study

Data Remarks and inferences

are ~60 kcal/mol and ~ —18 cal/mol

(at 1070 K) respectively. DTA shows an

exothermic peak at 1020 K; A —(100

±75) cal/mol.

type but only a modified order- disorder type as applied to

diffusionless transitions in solids.

Ti02 (high pressure)

Crystal structure and

transformation to

rutile.

Ti02 (rutile)

Orthorhombic; space group, Pban; Z — 4; a =4.529 A; 6 = 5.464 A; c = 4.905 A. This

phase is formed from rutile at ~330 kbar

pressure at ordinary temperatures; it re- verts to rutile on heating at 720-870 K.

A lower bound of 60 kbar is necessary to

stabilize the higher pressure form with

respect to rutile at room temperature.

The high pressure phase reported

by two groups of workers pre- pared under different experimental conditions appears to be

identical. As expected, the high

pressure phase is denser than

rutile and can be retained with- out changes under normal conditions of temperature and

pressure after quenching for in- definite periods of time. Anatase

is metastable with respect to the

rutile and the high pressure forms. Reaction boundaries for these phases have been reported.

Crystal structure and

x-ray studies. T — 298 K; Tetragonal, space group, P42/

mnm; Z = 2; a =4.5941 ±0.0001 A; c =

2.9589 ±0.0001 A. TEC (303°-923 K)

( /°C): || c

: 8.816 XlO-«+3.653X IO

-9 t +

6.329 X10"12

t

2

; ±

c

: 7.249 X10~6 +2.198

X10-9

t +1.298X 10-12

t

2

, where t is the temperature in °C. High pressure x-ray

studies indicate volume change, (V/Vo)

of 0.91 at 102 kbar [152] and 0.94 at 158

kbar pressure [91].

Careful work up to the high temperatures. Slight anisotropy in the TEC parameters is obvious

and is explained in terms of the interatomic distances in the unit

cell of rutile.

Magnetic, optical and

dielectric properties. X = (0.067 ±0.0015) XlO-6 emu/g for highpurity single crystal rutile and temperature independent in the range 55-372 K;

negligible anisotropy.

Stoichiometric rutile shows continuous

absorption in the IR region [122b] but

reduced samples exhibit free career absorption and characteristic bands (at 0.75

or 1.18 eV) depending on the sample re- sistivity or degree of reduction. Optical energy gap = 3.05 eV. eo has a high value

and exhibits anisotropy; at 300 K, eo

(||<0 =170; eodl*) =86; n(|K> 2.903; n(||*) = 2.616. At low temperatures, eo approaches limit- ing values; at T—>0 K, «o(|

|

c

) = 257; e0 (||°) = 111. Parker’s data [153] in the range

1.6-1060 K indicate that rutile is not a

ferroelectric or antiferroelectric; however, the ionic polarizability of Ti is close

The temperature independent x is due to diamagnetic and Van

Vleck contributions. The free carrier absorption is due to the elec- trons released by the oxygen upon

reduction; the bands in the re- duced samples are believed to be

due to polaron absorption. The

environment of Ti sites in rutile, the anisotropy and high numerical

values of eo make an interesting

comparison with BaTiC>3.

References

[90, 91].

[150, 152].

[43, HO121, 122b,

123a, 153].

26
Titanium oxides—Continued

Oxide and description of the study

Data Remarks and inferences References

to the critical value for a ferroelectric polarization catastrophe at all temperatures.

Electrical properties

and high pressure

studies.

Thermal and

mechanical

properties.

p (300 K) ~108 ficm for stoichiometric rutile; £ff ~3.1-3.4 eV; impurity conduction

is always predominant at low temperatures; donor levels of 0.03-0.15 eV are en- countered. Reduced specimens have low resistivity

(~10-2 to 104 Slcm), are n-type and exhibit anisotropy in Rh, he and p. Anisotropy ratios, (po/pe) and (Ra /Rc)H vary

from 2.0 to 3.5 and 0.2 to 2.5 respectively

and exhibit interesting temperature de- pendence.

p.H~ 0.1 cm2 /V s. at 500 K; a (100 K) = — 1.0 mV/K. m* =20 mo. a shows large

phonon drag effect at T below 20 K.

In the range 882-1273 K, p

po2 ~lln, n 4.4 to 5.7; n =4 above 1470 K according

to Greener et al. [123]; both anion vacancies and cation interstitials appear to play

significant role. Ea (T>1000 K) =1.8 eV. High pressure studies indicate a large decrease in p up to 6 kbar (dp/dP = —0.2

XlO4 flbar-1

); in the range 6-100 kbar,

change in p is small {dp/dP = 0.08 fihar-1

). Data of Kawai and Mochizuki [125a] indicate that at pressures >2Mbar p of Ti02 drops by a factor of 10® and exhibits metallic behavior at room temperature.

Multiple band conduction is indi- cated by the p, Rh and a data

but many workers feel that polaron type conduction is pre- dominant at low temperatures.

The defect structure of Ti02 is not yet completely understood.

Kawai and Mochizuki [125a]

experiments at high pressures on

the p behavior of Ti02 are signifi- cant; x-ray data at these high

pressures are highly desirable and will indicate whether the

resistivity behavior actually

corresponds to the insulator- metal (Mott) transition or not.

[116-125a,

154].

Cp — 1 millijoule/K, mol at 4.2 K; CP ^T3 below 4 K with 0 d = 758 K.

Reduced Ti02 has large additional contribution and independent of T below 13 K and is attributed to electrons which do

not become degenerate above 1 K.

k (200 K)=0.1 W/cm, K and exhibits anisotropy above 25 K; goes through a maximum at ~15 K indicating phononphonon scattering in Ti02 . Internal friction (Q

-1

) goes through a maximum at 323° in pure rutile with Q~l

(| ;°) = 1 X 10-4 at 1842 Hz and activation energy of 14 ±0.4 kcal/mol. Reduced

rutile exhibits a pronounced maximum

(()-i(||«)=12xl0—4 at 394 K and 1838

Hz with activation energy = 23.5±1.0

kcal/mol) only along [100] direction and

not along [110] and [001] direction. The

results are discussed in terms of simple

and associated defects in pure and re- duced rutile.

It is evident that the degree and

type of reduction brings about

marked changes in these proper

ties in rutile.

[118, 123c,

155].

27
Titanium oxides—Continued

Oxide and description of the study

Data Remarks and inferences References

Band structure of Calculations using tight binding method in- Since there are no magnetic order [156]. rutile dicate relatively wide valence and con- duction bands (^-4 eV). The available complications in Ti02 it is worth

while to compute the detailed transport data can be interpreted assuming multiband conduction in Ti02 even at low temperatures (~100 K).

band structure of Ti02 using

various methods including spin- orbit coupling effects.

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P. A., Trans. Faraday Soc. 57, 504 (1961).

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(1968).

[121] Grant, F. A., Rev. Mod. Phvs. 31, 646 (1959).

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[122a] Senftle, F. E., Pankey, T., and Grant, F. A., Phys. Rev. 120, 820 (1960).

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91, 793 (1953).

[122c] Acket, G. A., and Volger, J., Physica 29, 225 (1963);

Phys. Letters 8, 244 (1964).

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J. Am. Ceram. Soc. 48, 623 (1965).

[123a] Hauffe, K., Reaktionen in und an festen stoffen, (Springer Verlag, Berhn, 1955), pp 135-140.

[123b] Rudolph, J., Z. ISaturforsch. 14a, 727 (1959).

[123c] Wachtman, J. B., Jr. and Doyle, L. R., Phvs. Rev.

135, A276 (1964).

[124] Vahldiek, F. H., J. Less-Comm. Metals 14, 133 (1968).

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11, 1635 (1957).

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allgem. Chem. 297, 1 (1958).

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[129] Sakata, K., and Sakata, T., Japan J. Appl. Phys. 6, 112

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Loehman, R. E., J. Phys. Chem. Solids 32, 1147

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(Gordon and Breach, Science Publishers, New York,

1964).

1.3. Vanadium Oxides

The vanadium-oxygen system has been extensively

studied by several workers in the different homogeneity ranges by employing a variety techniques.

In the range V-VO, by Seybolt and Sumsion [1], Schonberg [2], Rostoker and Yamamoto [3], Ozerov

[4], and Cambini et al. [ 5]; in the range V-V2O3, by

Klemm and Grimm [ 6], and Westmann and Nordmark [ 7]; in the range V2O3-V2O4, by Morozova et al.

[ 8], Katsura and Hasegawa [9], and recently by

Kimizuka et al. [10], Okinaka et al. [11], and Anderson and Khan [ 12]; in the range V2O3-V2O5, by

Hoschek and Klemm [13], Burdese [14], Grossman

et al. [15], Abei [16], and Kosuge [17]; in the range

V0-V2 06 , by Andersson [18] and Andersson [19] and

in the range VO2 -V2 O5 , by MacChesney et al. [20],

Theobald et al. [21], Sata et al. [22, 23], and Tilley

and Hyde [24]. The available data have been summarized by a few workers [25, 26]. Stringer [26]

constructed a schematic phase diagram and suggested a systematic nomenclature of the phases. A

modified phase diagram of Stringer [26] and

MacChesney et al. [20] (in the region VO2 -V2 O5) are shown in figure 1.4. The V-0 system is quite a complex one. In addition to the stable compounds like V2O3, VO2, and

V2 05 having narrow homogeneity ranges, compounds

with wide homogeneity range like VO, Magneli phases of the general formula Vn02n-i (4^n^8) and

Vn 02n+i (n = 3, 4, and 6) exist. Several new unidentified phases also exist in the region VO2-V2O5 [21-24].

Oxygen forms a solid solution with metallic vanadium up to about 11 mol percent (VO0.4) distorting

the cubic lattice of V [1, 2, 5]. Range VO0.6-VO0.8

has not been accurately studied.

Vanadium monoxide has a wide homogeneity

range (VO0 . 85 -VO1 . 27 ) and all the compositions have

been investigated experimentally. VO1.5 (V2 C>3 )appears to have a very narrow range of homogeneity

[27]; different phases have been detected in alloys

having compositions VO1.44 and VOi.es. The existence

and stability of the Magneli phases, VnChn-i* with re- spect to the oxygen partial pressure and temperature have been examined by various workers [9, 11,

12 , 17, 18]. VO2 (Vn02 „_i, n = 00

) has a narrow homogeneity range [10, 28]. Besides V2 O5 , in the

series V„02 „+i, V3O7 [29], V409 [30] and V60i3 [18]

have been prepared and characterized.

VO: VO is unique in having an extremely wide

composition range essentially symmetric about the

stoichiometric composition together with an extra- ordinarily high vacancy content (^20%). It has a cubic rock salt structure and the lattice parameter a

increases with increasing oxygen content but the

density decreases; however, the increase in a is not

linear with x in \Ox whereas the density versus x

plot is linear. According to the recent study on wellcharacterized and annealed VOx samples, the lower

limit for the single phase cubic region appears to be between 0.75 and 0.79 and the upper limit, 1.30

[31]. Mathewson et al. [32] first noticed that the

x-ray density of VO1.00 (6.50 g/cm3) is approximately 15 percent higher than the pycknometric

value (5.6 g/cm3) suggesting that the lattice is highly

defective at the stoichiometric composition. The

30
Figure 1.4. (a) V-0 phase diagram (after [26]). The various phases and stability ranges have been discussed in detail by

Stringer, (b) VO2-V2O5 phase diagram showing the VnChn+i

phases ( after [20]).

variation of the defect concentration of the cationic

and anionic lattices across the phase field has been

examined by many workers [7, 27, 31, 33-36]. At

the oxygen rich phase limit the anion lattice is nearly perfect with almost zero anion vacancies while

at the vanadium rich phase limit, however, the cation

lattice still contains ~13 percent vacancies. Banus

and Reed [31] and Taylor and Doyle [36] have found

that annealing the VOx at high temperatures (~1570

K) and high pressures (^50-60 kbar) decreases the

total number of vacancies (by about 6-13%) re- sulting in an increase in the density by ~1 percent

and a 0.1 percent increase in the lattice parameter.

These changes are unambiguously the result of the

high pressure treatment, since reannealing the samples at 1 atm pressure at 1570 K gave samples with

the same lattice parameter as the material before

pressure treatment. Loehman, Rao, and Honig [35]

have found that the vacancies in VO can be reduced

by solid solution with TiO.

Because of the wide homogeneity range and large

number of vacancies, VO* shows interesting physical

and chemical properties. With increase in * in VOx, p, dp dT, and xm increase linearly with abrupt in- creases in slopes at x = 1.00, while a changes smoothly

from n-type to p-type at *=1.02. The metal rich

VO is a semimetal with weak temperature-independent paramagnetism and higher * changes to a semiconductor with a small energy gap and a stronger temperature-dependent paramagnetism. Contrary to the earlier reports [37-40], recent studies on

well characterized samples of VOx indicated no

metal-to-semiconductor transition for any composition on decreasing the temperature to 77 K [31,

35, 41-43]. Oxygen rich VO shows [31] only a weak

temperature dependent paramagnetism in the range

4.3 to 273 K with no evidence of magnetic ordering,

in contrast to the findings of Kawano et al. [41]

who observed definite evidence of antiferromagnetic

ordering for *=1.15 and 1.26.

31
The decrease in vacancy concentration on anneal*

ing at high pressure results in slight changes in the

electrical and magnetic properties. The pressure

annealing of VO does not significantly change the

magnitude of p, the slope of p versus T or Xm. At all compositions, there is a 10-20 percent decrease in

Xm in the range 4.2-100 K; a decrease in anion

vacancies (caused by pressure anneal) affects the

Xm in an opposite direction to that caused by in- crease in x. This indicates that xm is more sensitive to the cation vacancies than the anion vacancies.

The magnitude of a increases after pressure annealing and VO1.02 changes from n-type to p-type behavior.

V2O3: Vanadium sesquioxide, V2O3, exhibits two

transitions. Because of the dramatic changes in

physical properties accompanying these phase transitions and the unique behavior with respect to pressure, temperature and impurity content, this oxide

has been studied extensively in recent years both

from theoretical and experimental points of view.

The metal-to-insulator transition exhibited by V2O3

is a field of intense research activity at the present

time (See fig. 2 in the introduction) and several

review articles have recently appeared [44-56] on

this aspect of the problem.

At room temperature, V2O3 has the corundum

structure with rhombohedral symmetry [57, 58].

There is a unique c axis perpendicular to the basal

plane which contains three identical crystal axes. Near 150 K, a phase transition occurs to the monoclinic structure [59-64] involving a slight tilting of

the hexagonal c axis of the rhombohedral phase.

The transition is accompanied by discontinuous

changes in the c/a ratio (fig. 1.5) and the unit cell volume (increase by about 1.4 percent) [63]. There

is also a high temperature anomaly in V0O3 around

500 K where there is no change in the crvstal parameters [63]; Kosuge [17], however, finds a DTA transition at 430 K. Recent neutron diffraction study on

V2O3 [65] has confirmed the presence of antiferromagnetic ordering in the low temperature monoclinic phase (earlier measurements using various

techniques gave contradictory conclusions; see, for

example, reference [53]). According to Moon [65],

the antiferromagnetic axis lies in a plane parallel to the c axis (in the hexagonal cell) and perpendicular

to one of the a axes and makes an angle of about 71°

to the c axis in this plane; the V moments are ferromagnctically coupled in monoclinic (010) or hexagonal (110) layers, with a reversal between

adjacent layers. The ordered moment is 1.2 pu per

Figure 1.5. Variation of the c/a ratio of V2O3 with temperature

[63]. At the transition temperature we see a drastic change in the ratio. Chromium

substitution markedly affects the c/a ratio. In the Insert we have shown

the variation of c/a ratio in various sesquioxides; the ratio is anomalously

high in V2O3 and low in T12O3. V atom. Neutron diffraction [65] and magnetic sus- ceptibility studies [66-68] indicate that the antiferromagnetic ordering in V2O3 disappears at the

transition point (154 K) where the crystallographic

transition also occurs. The high temperature anomaly

is associated with a discontinuity in the Xm, but the

substance remains paramagnetic throughout the temperature range, 150 to 1000 K [68].

Associated with the phase transitions in V2O3,

there occur dramatic changes in the electrical properties. At room temperature, V2O3 is a fairly good

metal (with the purest samples showing resistivities

of the order of 10-3 cm and a positive temperature

coefficient of p). When the temperature is decreased

to 150 K (crystallographic transition point), it undergoes a transition from the metallic state to an

insulating state with an accompanying increase in

resistivity by about seven orders of magnitude. This

transition has been first discovered by Foex [69]

and since then confirmed by many workers [37, 38,

63, 66, 70-78]. The characteristic metallic resistivity

in V2O3 shows an anomaly in the high temperature

region as well. Both the c axis and basal plane resistivities show a rapid increase with temperature

in the region near 525 K, but keep monotically in- creasing up to 800 K [76]. Foex et al. [66] on the

other hand, have noted a maximum at 525 K and

a semiconductor behavior in V2O3 above 525 K.

Discontinuity in a was noted at the low temperature

crystallographic transition in V2O3 [78, 79]. Above

150 K, a was positive and small and below this temperature, it changed sign and became very small.

32
Rh was negative in the entire range of temperature

studied (7-800 K) and did not indicate any anomaly;

a did not indicate anomaly at the high temperature

transition of V2O3 [78]. X-type anomalies are noted in the heat capacities

at the low and high temperature transitions in V2O3

[66, 80]; DTA studies indicate endothermic peaks

i corresponding to the transitions [17]. Anomalous

behavior of V2O3 in the region of the transition point

has been noted in the measurements made by a variety of techniques like NMR spectroscopy [81-

84], Mossbauer spectroscopy [74], positron annihilation [85], UHF (^10 GHz) dielectric constant [86]

and optical spectroscopy [76, 87].

The effect of pressure on the transition has been

studied by a few workers [38, 39, 61, 76, 77]. The

transition temperature (

\= Tn) decreases with pres- sure (~4 K/kbar). Application of pressures greater

than 25 kbar suppresses the transition completely

[77] and V2O3 remains metallic and Pauli paramagnetic down to 2.2 K without indication of any long

range magnetic order.

All the available data seem to establish that the

low temperature transition in pure V2O3 is of first order and the high temperature one is of second

order. The low temperature transition is sharp and

marked by a hysteresis. Tt is sensitive to stoichiometry [88] and the magnitude of the discontinuity

in p seems to depend on sample purity and stress. The properties of the metallic state of V2O3 are highly anomalous and they change rapidly with

pressure in the direction of making it more metallic.

It, therefore, appears that pure V2 Os is near a critical region and that the application of negative pressure

would induce a transition; doping with chromium

has an empirical correspondance with the negative

pressure, and indeed, it is found that a metal -to- insulator transition occurs with Cr doping in this

material.

As early as 1949, Mott [89] predicted that an

abrupt metal-insulator (collective electron-localized

electron) transition will occur in an ionic solid when

the interionic separation is forced to pass through a

critical value. Since each of the two resistance

anomalies in V2O3 is accompanied by an anomaly in

the thermal expansion, it would appear that they

may both be metal-insulator transitions of the type

envisioned by Mott. The basis for the ideas that the

V2O3 transitions are Mott-type transitions is contained in the recent work of McWhan and co-workers

[62, 63, 84, 90]. These workers have examined the

resistivity and the lattice distortion of V2 03 as a function of pressure, temperature, and chromium and

Figure 1.6. Pressure-temperature-composition (Cr) phase dia- gram (Vi_zCr*) 203 . M, I, and AFM insulator phases have been indicated; the metal-insulator phase boundary terminates at a critical point (~530 K) (after [63]).

titanium doping. Three types of phases have been

suggested to be present in V2O3: the metallic (M),

the insulating (I) and antiferromagnetic insulating

(AF) phases. Depending on the temperature, pres- sure and dopant concentration, all the three phases

and the transitions between any two (M^I; M^AF;

AF^I) can be realized experimentally. The temperature-pressure-composition phase diagram for

(Vi_.Cr.)i03 is reproduced in figure 1.6. At room temperature and atmospheric pressure,

pure V2O3 is situated in the metallic phase. As the

temperature is lowered, the undoped material passes

through the M-AF phase surface at ~160 K. Conversely, as the temperature is raised, pure V2O8 appears to pass through the extension of the M-I

phase boundary at ^550 K. However, the M-I

phase boundary terminates at a critical point (fig.

1.6), so that the high temperature transition in pure

V2O3 reflects a critical behavior. The phase transition

is no longer a well-defined, abrupt, first order transition, but is a gradual change from metallic to insulating behavior. When a small percentage of Cr

ions is substituted for vanadium in V2O3, a definite

first order phase transition between the metallic

and insulating phases exists, with the transition

temperature rapidly decreasing as a function of

chromium doping. At a critical concentration (^3

33
percent), the M—>1 and M—»AF transition temperatures are equal (~160 K). For Cr dopings greater

than 3 percent, an intermediate metallic state does

not exist and the only observed transition is a direct

I—>AF transition (at atmospheric pressure). Similarly, at a given temperature (say, 298 K) and Crconcentration (say, 4 percent), I—»M transition can

take place with the application of pressure without

the intervening AF phase. Doping with increasing

amounts of Ti has the empirical correspondance to the application of pressure (3.6 kbar/atom %) and

the system tends to become more and more metallic

[79]; at about 7 atom percent Ti, both the transitions

in V2 O3 are suppressed and the system is metallic in

the entire range of temperatures as is the case in

pure V2O3 above 25 kbar pressure [62].

Various theories have been put forward to explain

the metal-insulator transitions in oxide materials

([91-93]; for a review of various earlier theories see

[53]), but each of them is able to explain the experimental findings on a specific oxide material and fail to explain the behavior of other oxides. Any discussion of the theories would be outside the scope of

this review and it would suffice to point out that

there is still a need for unifying theoretical approaches to the understanding of metal-insulator transitions. In figure 1.7 we have shown the schematic

band structure of V2 O3 (following Goodenough) in

the rhombohedral, nonmagnetic monoclinic and

magnetic monoclinic structures to illustrate the nature of changes accompanying the low-temperature

transition.

Vn02 „_i: The Magneli phases, Vn02n-i, (4^n^8),

have a triclinic structure based upon the rutile struc- ture with periodic sheared planes resembling stacking

bgeji.j-ri.j — t i c*p

ttH « I — -iteTCH m "1—

I

c»d

monoclinic

Figure 1.7. Changes in the d-electron band strurture of V2O3 accompanying the transition at ~150 K (After Goodenough).

faults into which extra planes of metal atoms are introduced. Because of the shear planes, metal-metal

bonding can occur leading to interesting electrical

and magnetic properties. Whenever these metalmetal bonds are broken up, anomalies in the physical

properties occur, but there appears to be no case where structural transformation occurs; only a dis- continuity in the lattice parameters is noticed.

Nagasawa and co-workers have grown single crystals

of these Magneli phases by the vapor transport

method [94-97] and have carried out various measurements.

V3O5, even though it corresponds to Vn02n_i with n = 3, is not a Magneli phase since the structure is different (monoclinic). It shows a discontinuity in

the x-T plot around 133 K [95], but in contrast to the earlier studies [73] (which showed a semiconductor-to-metal transition in the range 150 to 180 K),

only a semiconductor behavior was noted in the

range 130 to 300 K [72, 98]. Kosuge [17] does not

observe a DTA peak, or a x-T anomaly. X-ray

studies of Asbrink et al. [99] up to 1270 K, also do

not indicate a phase transition.

Magnetic susceptibility and electrical transport

(on single crystal materials), x-ray, DTA and Mossbauer studies indicate semiconductor-to-metal transitions (on heating; first order) in V4O7 at 250db5 K

[17, 94, 100, 101]; in V509 at 135±5 K [17, 96, 102];

in V6Ou at 177 ±2 K [17, 95, 103]; in V8015 at 70 K

[97, 104, 105]. Detailed studies [17, 94, 103] indicate

that V7O13 is paramagnetic (Pauli ?) and metallic

in the range 4.2 to 300 K. In their recent study published in 1973, Kachi, Kosuge, and Okinaka [105]

have presented the data on all the Vn 02n-i (

n

=

3-9) systems and find that all but V3O5 and V7O13

show metal -sinulstor transitions. These workers

have established the phase relationships and struc- tures of these oxides.

It is difficult to rationalize this wealth of experimental data without a proper understanding of the

theory underlying the semiconductor-to-metal transitions; unfortunately, present day theory has not yet

developed to such sophistication, to satisfactorily

explain the transitions even in simple substances

like V2O3 and VO2. However, in the V„02n_i series,

two important generalizations may be noted: (i) all the phases are triclinic and detailed x-ray study on V4O7 indicate no crystal structure change at 7\, but

only discontinuity in lattice parameters. Probably

the same case may be true for other Magneli phases

as well, (ii) Recent studies [105] show that there is antiferromagnetic ordering in all the Vn02n-i phases

at low temperatures in addition to first order transi34
tions in many of them at higher temperatures; in

these respects, these phases are similar to V2O3. It appears that the first order transitions in these

Magneli phases are explainable in terms of Goodenough’s qualitative picture ([44, 106, 107]; for a detailed discussion, see [53]) of metal-metal bonding

and trapping of the conduction electrons in the

semiconducting phase and the breaking of these

bonds and creation of Fermi surface at Tt in the

high temperature metallic phase. Like VO2 , Vn02„_i

oxides also show only a single transition (V2O3 and

T14O7 on the other hand, show two transitions).

VO2

: Vanadium dioxide, V02 , exhibits a crystallographic transition at 340 K. The transition is accompanied by interesting changes in physical properties

(see fig. 2 in introduction). In view of the possible

practical applications of this material in electrical

switching circuits and in other devices, the crystal

growth and physical properties of this material have

been extensively studied and reviewed by various

workers in recent years [44-56, 108].

V02 is monoclinic at room temperature [101, 109-

113] and transforms to a tetragonal rutile structure

at 340 K [101, 114-116]. The transition is of first order and is accompanied by hysteresis effects and

changes in latent heat and volume [101, 114-119].

Magnetic susceptibility [15, 114, 117, 119-122],

Mossbauer [75, 123] and NMR [84, 124, 125] studies

have failed to show any evidence of long range

magnetic ordering in V02 in the temperature range

1.7 to 400 K; however, a jump in x is noted at Tt and x is almost temperature independent both above

and below Tt

. The electrical characteristics of VO2 change from those of a semiconductor to those of a metal at Tt with a large drop in resistivity (by a factor of ~105 for pure and single crystalline material) [37, 73, 75, 113, 114, 117, 126-134]. Seebeck

coefficient [117, 128, 135, 136] and Hall effect [130,

137] also show anomalies at Tt

. Studies of DTA

[17, 113, 116], heat capacity [114, 117, 138] as well

as infrared spectra and other optical properties [130,

139] clearly show7 the first order nature of the transition in this material, but thermal conductivity does

not show an anomaly at Tt [117].

Recently, some workers have found evidence for

the existence of an intermediate triclinic phase of

VO2 in the range 325 to 340 K by means of NMR

[125], DTA [116] and x-ray diffraction [115] studies.

The temperature range of stability of this intermediate phase may become vanishingly small for

pure V02 [101], but impurities like Al, Fe, Cr, Mo,

and hydrogen reduction appear to be effective in

the stabilization of this phase by way of Magneli

defects [115, 116]2

. The effect of pressure on the VO2 transition has

been examined by a few workers [39, 61, 140]. Tt increases linearly with pressure at a rate of 0.082 dt 0.005°C/kbar. The conductivity activation energy

in the semiconducting phase decreases linearly with

pressure typically at a rate of 1-2 mV/kbar. Assuming intrinsic behavior below Tt for V02 , this indi- cates that the carrier concentration increases with

pressure but no metallic conduction is encountered

up to pressures of ~300 kbar at 300 K.

The effect of various impurities on the temperature

and nature of the transition in VO2 has been investigated extensively in the literature. Many ele- ments like Ti, Nb, Mo, Tc, Ta, W, and Re, form

complete solid solutions with VO2 [113, 116, 120,

121, 133, 141-148] and at appreciable concentrations

stabilize the high temperature rutile from of V02 at room temperature; the Tt of VO2 is lowered

appreciably and the nature of the transition (at not

too high impurity concentrations) is changed from

that of a semiconductor-to-metal to that of a semiconductor-to-semiconductor. The same effect is produced by the addition of ions like Fe, Co, and Ni,

but Al, Cr, and Ge raise the Tt appreciably [133,

146, 148]. Unfortunately, each system is complicated in itself and no generalizations are possible on

the effect of impurities on the transition in VO2 until the detailed mechanism of the semiconductormetal transition in pure VO2 is clearly understood.

Since long range magnetic order is absent in VO2 , it might be considered that the Adler-Brooks crystal

distortion model ([149, 53]) would be applicable to the transition in V02 , but quantitative agreement

between theory and experiment is lacking [56, 108,

134]. Since the metal -metal bonding in V02 and

other rutile type oxides at low temperatures has been

established beyond doubt, Goodenough’s approach

seems to be applicable (for detailed discussion, see

[53]), but the treatment is only qualitative. Paul

[108], Hyland [56], and others [150, 151] strongly

argue for an electron-phonon interaction mechanism

and phonon instability [152] as the basic driving

force for the transition in V02 . In figure 1.8 we have

shown the elementary band structure scheme for

tetragonal and monoclinic VO2 to illustrate the nature of changes accompanying the transition.

2 Goodenough, [ 1 16a] J. Solid State Chem. 3: 490 (1971), has recently

identified two distinguishable mechanisms of the monoclinic-tetragonal

transition of VO2 : an antiferroelectric-paraelectric transition at a temperature Tt and a change from homopolar to metallic V-V bonding at Tt

• Both T

and T/ are at 340 K in pure VO2. In the presence of impurities the two

components seem to get separated with Tt <T(see ref. [136], and [148] for example).

35
w J { w

(a) (b)

Tetragonal V02 Monoclinic V

2 04

Figure 1.8. Schematic band structure of VO2 in the monoclinic

and tetragonal phases (after Goodenough)

.

Vn Oon+i 5 VeOi3 is monoclinic at room temperature

[16, 18]. Kosuge [17] reported an endothermic peak

in DTA at 177 K where a X-T anomaly has also been

noted. Recently, Kanazawa [153] has noticed a semiconductor-to-metal transition (on heating) in this

material at 149 K with a drop in resistivity by a factor of 104

. The transition may be first order but

the structure of the low temperature phase is not

yet established.

In the series Vn02n+i, in which V2O5 and V60i3 are known members, V4O9 and V3O7 have been re- cently prepared and characterized. V4O9 is orthorhombic [30] whereas V3O7 has a monoclinic crystal

symmetry [29, 154, 155] at room temperature and

decomposes at 930 K. No DTA peak was noticed

by Kosuge [17] in the range 130 to 950 K. The other

physical properties are not yet known.

Vanadium pentoxide has a narrow range of homogeneity [26]. It is orthorhombic at room temperature and has a corrugated sheet type structure and

is built up from distorted trigonal bipyramidal co- ordination polyhedra of oxygens around vanadium,

sharing edges to form zig-zag double chains along

[001] and are cross-linked along [100] through shared

corners, thus forming sheets in the xz plane [156-

159]. This special type of structure predicts anisotropy in thermal expansion of the lattice parameters

and is observed experimentally [160-162]. V2O5 does

not show any phase transformations up to the melting point (~940 K) at one atmosphere [17, 163].

Minomura and Drickamer [39] have noticed a phase

transition in V2 05 at pressures ~100-105 kbar,

associated with a large increase in the resistivity.

V2O5 remains semiconducting both below and above

the transition. Electrical properties and the mechanism of conduction in V2O5 have been discussed

by a few workers [164-169]. Small amounts of alkali

and other metals form 'vanadium bronzes’ with

V2 05. These important materials have been discussed

in detail in the literature [53, 170].

Vanadium oxides

Oxide and description of the study

Data Remarks and inferences References

Oxide

0 a, A

(cubic,

rock salt)

Density

(g/cm3 ) % Vacancies

(Total)

P

(300 K)

(Ocm)

a

(mV/

°C)

Xm

(273 K)

VO

Crystal structure and VO0.86 4.034 5.736 37.0 6 .OXIO-4 -10.9 0.5X10-3 [31, 35]. physical properties of VO* VO0.99 4.068 5.602 30.8 2.2 X 10~3 -4.2 O. 6 XIO-3 (0.8 <x< 1.25). VO1.02 ^Oi.23 4.077

4.133

5.583

5.329

28.9

21.2

3.5X10-3 -2.0

+22.1

O.6 XIO-3 0.8X10-3

36
Vanadium oxides—Continued —

Oxide and description of the study

Data Remarks and inferences References

Melting point (VOZ)~2170 K Most comprehensive study on 30

well-characterized samples; contains literature survey up to 1970; High pressure annealing

studies conducted; VOz (0.86 ^

x ^ 1.02) is metallic and show

temperature-independent xm

while samples of VOz (1.02 ^ x ^

1.23) are semiconducting and

show weak temperaturedependent xm- No metal-to- semiconductor transition nor magnetic ordering noticed in any

sample in the range 4.2-300 K

for all x.

Vacancy annealing in VO. High temperature (~1920 K) and

high pressure (~56 kbar) pro- duced substantial decrease in the

vacancies (vacancy filling); In common with other workers,

difficulties were encountered in the preparation of pure single

phase VOi.ooA critical value of PAT (>90,000

kbar. °C) where P is the applied pressure and AT is the temperature above ambient at which the pressure is applied, seems to eliminate the vacancies

completely in TiO. A similar (but higher) value is predicted

for complete vacancy filling in VO.

[36].

NMR studies of VO. V51 NMR studies on single crystals

and pressure annealed VOz (x=0.86, 1.02 and 1.23) using cw and spin echo techniques in

fields of 9 -50 kOe in the range

1.4-300 K. Resonance was observable in all compounds at all temperatures indicating the absence of metal-insulator transi- tion in the observed materials.

Results indicate that the bulk of the temperature-dependent

magnetization comes from local moments on a minority of sites whose nuclear resonances are unobservable.

[42].

Resistivity and Hall

coefficient studies. VOz (0.82 ^ x ^ 1.0) samples studiedat 4.2, 77, and 300 K. p (300 K)

is in the range 10-3-10-4 flcm with small negative values of dp/dT; Ru is ~5X10-4 cm3/C

and positive Ap/p at 4.2 K. The

samples did not exhibit metalinsulator transition.

An overlapping band model is suggested to explain the observed properties.

[43].

37
Vanadium oxides—Continued

Oxide and description of the study

Data Remarks and inferences References

Seebeck coefficient studies on VO*.

For VO* samples with a: <1.0, a is negative; for *>1.0, a is positive

and for * * 1 .0, a ~ 0. The

numerical values are small ( — 5

to +20 mV/°C).

The data indicate that electrons and holes are competing to- gether for the contribution to a

and the consequent complications in the band structure

related to a large concentration

of metal and anion vacancies.

[171].

Crystallography and

defect chemistry of VO

and solid solutions -with TiO.

The solubility of TiO in VO is small (~10%). The concentration of defects are lower than in pure VO* at any given composition. Semiconductor behavior was

noticed for VO (* ^ 1.0) and its solid solutions in the range 100-

300 K. No metal-semiconductor

transition was noticed for VO or

in the solid solutions in the temperature range investigated.

It appears that VOo.8 does not be- long to the same family of cubic

VO* with (0.9 ^ ^ 1.2) and

higher disorder in cation sublattice exists compared to anion

sublattice. The low solubility of TiO in VO is explained in terms

of the different degree of disorder

in the two crystal lattices.

[35].

Electronic structure and

solubility of other oxides

in VO.

Mutual solubility of VO and TiO at 1870 K is ~18-20%; in the oxygen rich phases (*>1.0),

solubility is greater (20-25%).

Solubility of BeO, CaO, and

MgO are small in VO.

The observed behavior is explained

in terms of the electronic struc- ture and valence states of the metal in these compounds.

[172].

Plasma resonance in VO. Reflectance minima are found in the range 3. 8-4.0 eV in VO*

depending on the composition.

The reflectance minima correspond

to the plasma edges in these

materials.

[173].

Infrared studies of VO*. VOi.od and VOi.u give rise to bands at 1075±15 and 1080±10 cm-1 respectively.

The band corresponds to the metaloxygen vibrations and results indicate that the band position is not much sensitive to the nature of the cation or the phase

composition.

[174].

Band structure of VO.

v2 o3

Band structure calculations by the

tight binding method indicate that the d bands are about 7 eV

wide, lying below the vanadium

4s band so that conductivity in .VO is primarily due to the d

electrons. The Fermi level falls in the d band.

The energy bands in VO seem to be

sensitive to the degree ionicity

assumed in the calculation; the

density of states at the Fermi

level is large, suggesting that VO

might exhibit some sort of magnetism. See also (ref. [9], p. 130)

for band structure calculations.

[175].

Crystal structures of the phases and x-ray studies. Range 77-160 K:

Monoclinic; space group, 12 /a; Z = 4; a = 7.255±0.003, A; b =

5.002 ±0.002 A; c = 5.548 ±0.002;

0 =96.75 ±0.02°; E = 99.97±0.08

A». TEC of this phase is <2X10-6 K-1

.

Earlier workers [59, 176] interpreted the low temperature monoclinic

distortion in terms of a ortho

hexagonal cell or c-centered monoclinic cell but the present

choice in terms of a hexagonal

cell appears to be the most

easily conceived. Single crystal

[63, 64]. j

38
Vanadium oxides—Continued

Oxide and description Data Remarks and inferences References

of the study

Range 160-700 K: growth conditions established. Rhombohedral (pseudo hexa- V2O3 has an unusually high c/a [58, 62, 63]. gonal); space group, R3 C; Z = 2, ratio (at 298 K); detailed studies oh =4.9515 ±0.0003 A; ch = indicate that in going to the

14.003 ±0.001 A; c/a =2.8281; monoclinic structure, nearest V=98.61 ±0.05 A3

. neighbor V-V distances show a TEC (/K) at 298 K: ||“ = (20.2± discontinuous expansion while

0.3) X 10-6

; || c = -(8.6 ±0.3) X the average V-O distance re- io-«. mains unchanged. Contrary to The c/a ratio (pseudo c/a for the earlier interpretations [45, 76,

monoclinic cell) shows a sharp 177], pair --wise contraction of the jump at ~160 K and slight basal plane cations does not, in nonlinear decrease in the region fact, occur but the oxygen octa- 450—600 K; the 4% Cr doped hedra become skewed up about

samples show the exact opposite the central metal atom and the

behavior. triply twinned monoclinic struc- AF (mono.—»rhombo.) = —(1.4 ture is a logical consequence of ±0 . 1 )%; the value is much less for doped samples (4%). With

pressure, volume decreases gradually with increasing pres- sure but doped samples give dis- continuity depending on the

concentration.

Lattice parameters vary smoothly

with the application of pressure; din (c/a) /rf (In V) = -0.7.

Melting point of pure V203 ~

2240 K.

the distortion.

Magnetic susceptibility. Range 4.2-1000 K: The ordering of magnetic dipoles [65, 67, 68, magnetic ordering and Xxi = 1.0X10~4 (cgs units) in V2O3 is of special type com- 76, 77, 79, neutron diffraction studies. (T <150 K) pared to the other transition 178-181].

xxi=2.1X10-4 +1.40/(T+600) metal sesquioxides.

(180° <T<350 K); xxi =2.1 X The estimated latent heat at low

10_4 +1.78/ (T+600) (580 <T< temperature transition is ~700

1000 K). xxi shows little aniso- tropy (< 20 %) and is essentially

temperature independent, xxi shows a jump in going through

Tt while heating and shows

Curie -Weiss behavior; a hysteresis of ~12° was noted. Tt = 155

K. A shallow maximum is indical/mol.

cated in xxi-T curve in the range 400—500 K. This seems to corre- spond to the high temperature

anomaly in pure V2O3 . The magnetic moment changes from 2.37 MB to 2.69 mb in going through this transition (spin only value for V3+ is 2.83 mb).

Xxi -T studies were also made

on doped samples. Internal

field value ~175±15 kOe was

obtained in V2 03 at 105 K

using inelastic spin -flip

39
Vanadium oxides—Continued

Oxide and description Data Remarks and inferences References

of the study

scattering of neutrons. The

ordered moment (calculated) is 1.25 hb- The existence of antiferromagnetic ordering

in the low temperature phase

is confirmed. The observations are in accord with a model in which the vanadium

moments are ferromagnetically

coupled in monoclinic (010) layers with a reversal between

adjacent layers. The ordered

moment is ( 1.2 ±0. 1 ) mb per V

atom. Neutron diffraction study

shows that the magnetic ordering

disappears at 154 K which is the Neel temperature (Tn). Application of pressure decreases the

transition temperature (dT^/dP = —3.78 K/kbar). Above 26

kbar pressure, the magnetic order

disappears and the material re- mains paramagnetic down to 4.2 K.

Electrical properties. Semiconductor behavior below Tt The low temperature behavior is [38, 63, 71, (151±3 K); P (125 K) ~ 106 usually interpreted in terms of 76-78, 182; ficm; Eo~0.12-0.18 eV; below intrinsic behavior. According to 40 K, evidence of impurity con- McWhan et al. [63], the proper- duction with Ea ~0.001 eV. Be- ties of both the high temperature

low Tt, a is ~ —3 nV/°C and phases (metallic, M and insulat- f?n<0.3 cm8 /C (at p~108 ftcm). ing, I, phase) are anomalous and

Metallic behavior above T<; are modified by the impending

p drops by a factor 107 at 7\ while transition in the range 500--600 heating; hysteresis of ~12 K K. The order of the transition noted, a becomes positive and may be second or higher order

jumps to ~12 mV/0 C and shows but can be made first order under

slight decrease with temperature proper conditions (by doping

thereafter, p shows an anomaly with Cr). A generalized picture

(broad maximum) in the range of the transitions in V2Q3 has

500-600 K. No anomalous be- emerged from the work of these havior in a and Rh in this temperature region.

Resistance decreases with the

application of pressure:

1 atm: p=227±1.4 T (150 < T<

350 K). 26 kbar: p = 182 ±0.7 T

(200 <T<300K);p= 3.1 ±0.042 T2 (T<50 K). (d Inp/d In V) =30

(basal plane); =16 (parallel to

c-axis). Rh=2.3X10-4 cm8 /C (p-type); no change in Rh at the high

temperature transition, mh =

investigators.

I 0. 2-0.6 cm2 /V s in the range 300-600 K; ra*»50 m; Mean

free path ~2 A.

!l
Vanadium oxides—Continued

Data Remarks and inferences References

Endothermic peaks in DTA were

noted at 168 and 430 K; no AH

value reported. X-type anomaly was noted in the heat capacity of V2O3 at 169 K; AH = 692 cal/mol;

AS for the transition = 4.1 e.u. McWhan et al. [62] estimate a value of AS = 2.6 e.u. A nuclear

specific heat measurement in the temperature range below 0.5 K

indicated a Schottky anomaly

varying with T~2 for pure V2O3.

The thermal properties are showing

up the two transitions in V2O3

but there is need for quantitative

studies on the AH and AS of

transition.

[17, 62, 76,

80, 183].

Energy gap of ~0.1 eV is obtained

from the optical measurements

at 77 K in pure V2O3. The re- flectivity of pure V2O3 at 300 K

is small but has the general shape

characteristics of a metal; plasma

edge is at ~1 eV.

The fundamental absorption edge

in the antiferromagnetic phase

appears to be very soft and

shifts to lower energies on

entering the high temperature

insulating phase.

[76, 87].

Knight shift (%):

fc = 1.52 - 18.1/(T +684) (T>150

K). An abrupt change in k is noted at the high temperature

transition, while the resonance

signal vanishes at the low- temperature transition (150 K).

No anisotropy in k is noted nor any observed nuclear quadrupole

splitting.

Antiferromagnetic state is sup- pressed in pure V2O3 at high

pressure and low temperatures.

V51 NMR signal is observable

down to 4.2 K. k = ( — 1 .0 ±0.2) +

(0.01 ±0.005) P (20<P<65

kbar and T = 4.2 K). The effect of doping V2O3 with Cr and Al

has also been studied in detail.

This study is a strong evidence for magnetic ordering at low temperatures in pure V2O3 . From the 'NMR-phase diagram’, Rubenstein [83] concludes that the

metal-insulator (high temperature region) phase boundary is not sharp and uniform below the

critical point, but corresponds to an inhomogeneous transition. It

is postulated that the high temperature phase transition is produced by an instability in a normal mode vibration of the

crystal lattice and that the low- temperature transition (150 K)

is driven by large magnetostriction by the antiferromagnetic

ordering aided by the presence of

this ’soft’ acoustic phonon mode.

The lattice instability is believed to be caused by the close proximity of a van Hove singularity to the Fermi energy in the

metallic state of the crystal.

The pressure dependence of the Knight shift implies that the d

spin component of the susceptibility (xd) is unusually strongly

dependent on volume with

(d In xd/d InK) =8±5.

[81-84].

Studies of Fe-doped V2O3 in the range 4.2-300 K: Magnetic

hyperfine splitting was noted be

The calculated Ts was ~200 K. It

is inferred that the larger dis- crepancy between Tt and Tn

[ 74].
Vanadium oxides—Continued

Oxide and description

of the study

Data Remarks and inferences References

low T

t

(~140 K) which disappeared above T< and the substance was paramagnetic. The

internal field decreased with temperature (400 and 315 kOe at 4.2 and 131 K respectively) and

abruptly disappeared at Tt ; isomer shift showed a jump at Tt

.

(calc) might indicate that magnetic ordering is not a major

cause of the transition.

UHF dielectric properties. Wave guide method using a fre- quency of 9.5 GHz: e~18 (T<

160 K); e~36 (T>160 K). e slowly varies with temperature;

in the region of the transition a jump by a factor of 2 is noted. Complex conductivity also exhibits an anomaly at Tt

.

The high value of e indicates the

relative importance of nonpolar

type of bonding in V2O3 and the

transition is accompanied by a change in the type of chemical

bonding, electron -energy spec- trum and consequently the

dielectric properties.

[ 86]

Electric field effect on the

transition. When the applied field reaches acritical value (~104 V/cm at 77 K) the current increases

rapidly; this threshold field de- pends weakly on the sample

thickness. Switching behavior

also is noted in the samples of pure V2 O3 .

[184]

Effect of stoichiometry on

the low temperature

transition.

For low stoichiometric deviations

in VO1 . 5+X, both a and c parameters of hexagonal form at room

temperature, decrease linearly

and c/a is a constant. With large x (x> 0.015), a decreases more

rapidly than c suggesting a probable change in the distribu- tion of defects involved in the

crystals.

The magnitude of the discon- tinuous change in xm at TV and

Tn become smaller with increas- ing x; no transition is observed

observed for x =0.034 either in terms of a change in the crystal structure or in Xm-

[88]

Band structure of V2O3 .

V3O5

The band structure of the hexagonal V2O3 lattice is examined by

the tight -binding method. The

stability of the structure in rela- tion to the distortions and the

splitting of the d bands is investigated.

The model appears to account for a transition from a distorted,

insulating state to an undistorted semimetallic one, for certain values of the d-d

interactions.

[185]

Crystal structure Monoclinic; space group, C2/c or Cc; Z = 4; a =9.98 A; 6 = 5.03 A; — (99)

42
Vanadium oxides—Continued

Oxide and description of the study

Data Remarks and inferences References

c = 9.84 A; 0 = 138.8°. X-ray studies do not indicate any phase

transition up to ~1270 K.

Physical properties. X-T plot shows an anomaly at ~133 K, but DTA does not

indicate a peak at this temperature. Probably the substance is paramagnetic in the range 77—

300 K. Semiconductor behavior

in the range 130-300 K; Ea =0A

eV. a data (~280 MV/°C at 273

K) indicates re-type behavior; a

decreases with T.

Earlier studies [73] might have

contained some V5O9 impurity;

the cause of x-T anomaly is not known.

[17, 72, 95,

98, 105].

Magneli phases, VnOin-i Crystal structure and transition temperature

Magnetic, electrical, DTA

and Mossbauer studies References

; phase All phases are triclinic; space group. Pi; Z—2.

4; V<07 o = 5.502 A; b =6.997 A;

c = 12.248 A; a =95.09°; 0 =

95.17°; 7 = 109.28°. 7\ = 250±

5 K. Discontinuity in the lattice parameters at Tt ; no change in the crystal structure; the volume

change for the semiconductor-to - metal transition = +0.0011 ±

0.0006; the pressure dependence

of T( is —0.2d=0.1 K/kbar.

X-T anomaly at Tt

. Semiconductor -to -metal transition; Ap =

102 at Tt ; a shows discontinuity

at Tt ; a is negative in the range 120-300 K; a above Tt is ~10-12

mV/°C. Endothermic DTA peak.

No magnetic order below Tt ; Isomer shift and quadrupole

splitting show discontinuities at Tt ; Tt decreases by Fe3+ doping

but jump in x remains almost the same.

[17, 94, 100,

101, 105,

186].

5; Vi09 0 = 5.475 A; 6=6.994 A; c = 8.718 A; a =97.53°, 0 = 112.44°; 7 =

108.99°. Tt = 135 ±5 K.

X-T anomaly at Tt . Semiconductorto-metal transition at T<; A

104—106

; Ea below Tt = 0. 1-0.2 eV. a decreases with increase in T; anomaly at Tt ; above Tt , 10-20 mV/°C (re-type). Endothermic DTA peak at Tt

. No

magnetic order below Tt ; Isomer

shift and quadrupole splitting

show discontinuities at T<; Fe3+ doping decreases T< slightly.

[17, 96, 102,

105, 186,

187].

6; VgOn o = 5.44 A; 6=6.99 A; c = 23.66 A; a =98.5°; 0 = 120.9°; 7 = 108.9°. Tt = 170-177 K.

X-T anomaly at Tt

. Semiconductorto-metal transition at T Ap~

104

; Ea below T<=0.12 eV. a is negative in the range 120-300 K

and goes through a sharp maximum at Tt

. Above Tt , a = 10-13

mV/°C. DTA endothermical peak.

[17, 95, 103,

105, 186].

43
Vanadium oxides—Continued

Oxide and description of the study

Crystal structure and transition temperature

Magnetic, electrical, DTA

and Mossbauer studies References

7; V7 O1 , a = 5.43 A; 6 = 7.00 A; c = 15.16 A; a =98.9°; /3 = 125.5°; 7 = 108.9°. No transition.

X continuously decreases with in- increase in T. Metallic in the range 4.2-300 K; p (300 K)-~

10-3 Dcm. a is negative and

small in the range 4.2-300 K,

decreases slightly with T and

below 120 K, nearly constant (~1 pV/°C). No endothermic

peak in DTA.

[17, 94, 103,

105, 186].

8; VgOi5 a = 5.43 A; 6=6.99 A; c = 37.08 A; a =99.0°; /3 = 128.5°; 7 = 109.0°. 7\ = 70 K.X-T anomaly at Tt . Semiconductorto-metal transition at Tt: Ap ~

10; p (200 K) ~10-3 ftem. a is negative and drops from —70 to — 5 /zV/°C at Tt ; Above Tt , a is constant.

[97, 104,

105, 186].

The general characteristics of all the Magneli phases, Vn02n-i are: (i) All have triclinic crystal structure at room

temperature, (ii) Antiferromagnetic ordering is present at low temperatures and semiconductor-metal transitions are present at higher temperatures, Tt [105]. Ty is 70, 40, 30, 23, 43, and 7 K respectively, when n is 3, 4, 5, 6, 7, and 8 . V0O3 (n— 2 ), V„02n-i (3<n<8) and VO2 (n = «) make an interesting comparison: n= 2 , Tw = Tj; n = 3-8,

Tat<T,; n = «, 0 = Tff<Tt . (iii) a of all the phases is negative below and above Tt which shows that the predominant carriers are electrons. Above T

a

is ~10-12 pV/°C in all the phases which is characteristic of

metallic conduction. Below Tt , a-T curve is complicated, (iv) DTA [105] gives AH (cal mol-1

) and AS (e.u.) values at Tt as follows: n=4, 142, 0.57; n= 5, 215, 1.59; n= 6, 222, 1.31; No data for n = 8 ; V3O5 and V7O13

show no transitions and are semiconducting and metallic respectively in the ranges 77 to 127 K and 4.2 to 300 K.

VO*

Crystal growth. Single crystals of V02 are grown

from V206 at high temperatures

under various oxygen partial pressures.

Depending on the conditions large

single crystals of varying stoi- chiometry can be grown.

[ 10, 28,

133].

Crystal structures of the phases and x-ray studies. T <340 K:

Monoclinic; space group, P2i/c;

Z = 4; density, 4.65 g/cm3 (obs.); 4.67 g/cm3 (cacl.). a = 5.7517 ±

30 A; 6=4.5378±25 A; c =

5.3825 ±25 A; /3 = 122.646±96°.

T = 346 K:

Tetragonal; space group, P4/mnm; Z = 2; a =

4.551 ±0.001 A; c = 2.851±0.001

A. AV at the transition = 0.02

±

0.05 cm3 /mol.

TEC (350-690 K)/°C:

|| e

: 29.638 XIO-9 -2.930 X10-8 1+

2.576 X10-11

1*; ||«: 5.828 X10-6 -

7.091 XIO-

* t+6.946X 10 — 12 t

2

. 325 < T <340 K: Triclinic, a —

5.80 A; 6 =4.52 A; c = 5.38 A. a =91.5(5)°; /3 = 122.7(8)°;

7=90.0°.

Crystal symmetry changes at the

transition; volume change is very

small. Above Tt both a and c parameters increase but the rela- tive increase in the c parameter

is greater. NMR evidence shows

the triclinic phase formation at 325 K. Cr, Al, and Fe are sup- posed to stabilize this phase.

Also see [116a] for structural aspects

of V02 .

[61, 101 , 109, 111,

112, 115,

116, 118,

125, 189].

44
Vanadium oxides—Continued

Oxide and description of the study

Data Remarks and inferences References

Magnetic properties. Paramagnetic in the range 1.7-400 K; x below and above T

are almost temperature independent;

slight anisotropy in x is shown.

X» 1.0X10'" (emu/g) (T<340K);

X-8.5X10"6 (emu/g)

(T>340 K).

Jump in x at Tt is noted; the exact cause of this is not understood

clearly.

[17, 117,

120-122].

Electrical properties.

Optical properties.

T <340 K: Semiconductor behavior. p~10-3 — 10 ftcm; Ea ~

0.1-0.65 eV; a~30-1000 n\/°C

(negative); mhM). 1-1.0 em“2 /V s; nfl ~10l8-1019 cm"3 (T <340

K); m*~1.6-7 m; ap*^2.

T>340 K: Metallic behavior

2-5 (X10-4

) ficm; p linearly increases with T; a^23 /*V/

0 C

(negative); mh~1-10 cm_2 /V s; n«'*^1021 -1023 cm-3

; m*«0.5 m. dT/dP m + (0.082 ±0.005) °C/

kbar; ~dEa /dP~(0.001-0.002)

mV/kbar. p decreases with

pressure at 300 K but no metallic behavior noted up to 300 kbar.

The wide variation in the properties of VO2 in the low and high

temperature region are believed

to be due to the purity, method

of preparation etc. Slight aniso- tropy exists in all the properties.

Discontinuities at Tt in p, a, and

Rn are noticed by various

workers. A hysteresis of '>-'2° is noticed in p-T data. The pres- sure effects on T

indicate qualitatively different behavior than

that encountered in V2O3.

Slight increase in carrier concentration is noticed with pressure but the effect is small and

superpressures seem to be re- quired to produce metallic con- duction in VO2 at room

temperature.

[39, 61,

120, 128,

130, 134-

137, 140].

V02 exhibits infrared active phonon

modes for T<Tt ( = 341 K) in the energy range 0.02-0.09 eV. Above Tt , the infrared spectra

show sudden increase in reflec- tivity. Reflectivity and trans- mission measurements in the range 0.25-5.0 eV below Tt indi- cate prominent absorption peaks

at 0.85, 1.3, 2.8, and 3.6 eV. Above Tt , metallic free carrier absorption is observed below 2.0 eV, but the same two absorption

peaks near 3 and 4 eV are present

in the high temperature phase.

e. = 5.54 (300 K); 4.17 (355 K).

According to Kawakubo [150], e of VO2 measured at 24 GHz in- creases monotonieally from 22 to 26 in the range 100=300 K and

attaing a high value of 60 within

a few degrees below T< (340 K).

This seems to indicate the onset

of an intermediate ferro- or

anti -ferroelectric phase [151] possessing a distinct triclinic structure [115, 125],

The data are consistent with the assumption of the appearance of ~1021 quasi free electrons per cm3 in the metallic state and a

filled oxygen 2p band ~2.5 eV

below from the partially filled bands arising primarily from

vanadium 3d orbitals. Transitions

from the filled 2p bands are re- sponsible for the high-energy

peaks in optical absorption in both the high- and low-temperature phases. In the metallic phase

there is evidence of overlap

among the 3d bands such that at

least two bands are partially

occupied by the extra d electron per vanadium ion. In the semiconducting phase, a band gap of ~0.6 eV opens up within the 3d

bands separating two filled bands

from higher-lying empty bands.

[130, 139].

45
Vanadium oxides—Continued

Oxide and description of the study

Data Remarks and inferences References

Thermal properties. DTA of pure VO2 shows an endothermic reversible peak at 340 K.

Some w'orkers [116] have noticed

two peaks at 342 and 346 K with

slow heating rates (0.5°/min).

AH = 750 cal/mol; Cp = 13 cal/

mol, °C (300 K) and shows a X- type anomaly at Tt ; 0d = 75O K;

k~40-70 mW/cm°C at 300 K

and is independent of temperature in the range 300-360 K.

First order nature of the transition

is confirmed. DTA evidence is presented for the existence of a

triclinic phase of VO2 just below

the transition.

[17, 80,

113, 116,

117, 119,

138].

Mossbauer studies. No magnetic hyperfine splitting is noticed at any temperature in Fe67 doped VO2 . Isomer shift shows an anomaly at T< while the quadrupole splitting goes to zero.

No long range magnetic order exists

in VO2 and the absence of quadrupole splitting in the high temperature phase indicates shielding of the nucleus by conduction

electrons.

[75, 123,

190].

Electron and optical

microscopic study of the phase transition.

The boundaries between the monoclinic and tetragonal phases during the transition in VO2 have

been observed in an electron microscope by the diffraction contrast at the interfaces which

are revealed as fringe patterns.

The growth of transformed regions

from sites of nucleation is observed in micrographs.

This is a direct observation of the

transition and various domains

formed according to the different modes of transformation can

easily be distinguished.

It appears that the T< and the

rate of growth of domains are dependent on the mechanical

strain.

[115, 191].

Infrared study of VO2 . Bands at 720 and 1115 cm-1 are noted. These seem to correspond to metal-oxygen vibrations.

[174].

Vn02n+i phases v«o13

Crystal structure and

properties.

V4 O9

T = 300 K: Monoclinic; space group, C2/m; a = 11.90 A; fe =

3.67 A; c = 10.12 A; /3 = 100.87°. Low temperature structure not known. DTA endothermic peak

and x-T anomaly at 177 ±2 K;

semiconductor-to -metal transition

at 149 K.

The transition may be first order

but the detailed properties are not known at present.

[16-18,

153].

Crystal structure. T = 300 K: Orthorhombic; space

group, Pnma; Z = 4; a = 17.926 =fc 0.004 A; fe =3.631 ±0.001 A; c= 9.396 ±0.002 A.

The structure appears to be closely related to that of V2O6 . The

physical properties are not known at present.

[30].

v8 o7 '1

Crystal structure and

properties.

T = 300 K Monoclinic; space

group, C2/C; Z = 12; a =21.92 A;

Detailed physical properties are not known at present.

[17, 29,

154, 155].

46
Vanadium oxides—Continued

l

MagnSli phases, Vn02n-i Crystal structure and transition temperatureMagnetic, electrical, DTA

and Mossbauer studies References

v2o5

6 = 3.68 A; c = 18.34 A; 0 =95.62°.

DTA does not indicate any

transition in the range 130-

950 K.

Crystal structure and

magnetic properties.

T = 300 K: Orthorhombic; space

group, Pmnm; Z = 2; density, 3.357 g/cm3 (obs.); 3.37 g/cm3 (calc.); a = 11.519 ±0.006 A; b =

4.373 ±0.002 A; c = 3.564±0.002

A. x = 3.7X10-® cgs/g. TEC

(300-870 K) (X10®/°C): ||“, 2.0;

|| 6

, 55.4; || c, 8.0; average, 21.8; polycrystalline, 13.0.

V2O5 has a corrugated sheet type

structure and predicts anisotropy

in the TEC values; this is observed experimentally.

[157, 162].

Electrical properties. Range, 77-450 K: Semiconductor;

p~103 Ocm; a~10 pV/°C (negative). p-T plot shows breaks at 250 and 390 K. a goes through a maximum at 250 K. Anisotropy

in p, Ea and a are noted, po in- creases with T in the range 350-390 K.

Anisotropy in electrical transport

properties is understandable be- cause of the peculiar sheet type

structure of V2O6. The breaks in p-T plots may correspond to the change in the mechanism of conduction (extrinsic to

intrinsic).

[168, 192].

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in non-Metallic Solids, Eds. L. Eyring and M.

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Tskhai, V. A., Russ. J. Inorg. Chem. 12, 1053 (1967).

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J. Solid State Chem. 2, 315 (1970).

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(1970).

[176] Abrahams, S. C., Phys. Rev. 130, 2230 (1963).

[177] Goodenough, J. B., Magnetism and the Chemical Bond

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627 (1965).

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49
1.4. Chromium Oxides

Many binary oxides are known in the Cr-0 system.

Schonberg [1] described the existence of C^O, but

detailed investigations have not yet been carried

out. CrO does not exist in the stable state and at- tempts have been made to stabilize it in the form

of solid solutions [2]. The known oxides of chromium

are Cr2 03 [3], Cr02 [4], CrBOi2 [5], an oxide of the

composition Cr02>44 [6], Cr60i5, or Cr2 05 [6-8], a nonstoichiometric oxide Cr02 . 6 5 [6], Cr3Os [8] and

Cr03 [6-9]. Cr304 has been reported by a few workers

[10, 11], but recent work [12] has failed to indicate

the existence of this material as a stable phase.

Cr2 03 and Cr02 have been extensively investigated

in the literature.

Cr2 (>3: Cr2 C>3 is rhombohedral (pseudo hexagonal)

and is antiferromagnetic below 308 K. Even though it has the same corundum structure as a-Fe2 03, the spin

structure differs and precludes the possibility of

superimposed ferromagnetism below Tn [13-19]. The

crystal structure does not change at Tn, but discontinuities in the lattice parameters are noted

[20]; in particular, the body diagonal c parameter is contracted slightly on heating through Tn. Below

Tn, x (perpendicular to the [111] direction) is almost

temperature independent whilst x parallel to the

[111] direction approaches (but does not become)

zero at 0 K [14, 21]. Cr2 03 exhibits magnetoelectric

effect below Tn and various workers have studied

this phenomenon [22-27]. Anomalies in p [28, 29],

ultrasonic sound velocity [30-32], Young’s modulus

[33] and dielectric constant [25, 26, 34] have been

noted in Cr2 03 at ~Tn. High pressure x-ray studies [35] indicate no

crystallographic transition in Cr2 03 ; however, Tn

decreases linearly with pressure up to ~13 kbar

[36]. T

n

also seems to depend on particle size of

Cr2 03 [37] to a small extent. Recently, Kawai and

Mochizuki [38] noticed a transition from insulator

to highly conductive state in Cr2 03 under static

high pressure conditions (>2 Mbar).

Cr2 C>3 forms solid solutions with other oxides of

corundum structure like Al2 03 [3, 39-41], Fe2 03 [18,

42] and Mn2 03 [42] and the nature of magnetic interactions and other physical properties have been in- vestigated in detail.

Cr02

: The dioxide of chromium, Cr02 , has a number

of interesting properties which include ferromagnetism and metallic electrical conductivity. Many

methods have been reported in the literature for the

preparation of Cr02 [2, 6, 8, 43-52] but the material

is actually a metastable form and will, on heating to ~720 K, transform irreversibly to Cr2 (>3 [53]. Recent

work has shown [51, 54] that Cr02 can be held for considerable period of time without decomposition at temperatures 1170 to 1770 K at pressures of 60-65

kbar.

Cr02 has a tetragonal rutile structure and its lattice constants do not vary with slight changes in the O/Cr ratio [48, 51, 52] and the rutile structure

exists in the range 300 to 673 K [55, 56]. Magnetic

measurements [4, 57-59] and Mossbauer data [60]

indicate that the oxide is ferromagnetic with Tc ~

395 K. p-T data in the range 80-575 K indicate

that it exhibits a positive temperature coefficient of p and is a typical metal [4, 48, 49, 51, 61-64]; dis- continuities in p and a are noted at Tc . Optical

and heat capacity data [65, 66] are also consistent

with the metallic nature of Cr02 . Cr02 forms solid solutions with V02 [67, 68] and

Mn02 [69] with the associated changes in the physical properties.

Cr5Oi2 : Cr60i2 is formed under high pressure conditions at ~500 K and has orthorhombic symmetry

[6, 7]; the substance appears to be nonferromagnetic

but detailed physical properties are not known at present.

Cr02 44 : This oxide is obtained by the decomposition

of OO3 at a pressure of 600 bar and ^510 K [6]; the structure and other properties have not yet been

investigated.

Cr6Oi5 (Cr2 0B) : Kubota [8] reported the formation

of Cr2 0B by the decomposition of CrOs at high oxygen pressures but the structure is not known. On

the other hand, Wilhelmi [6, 7] reported the formation of orthorhombic Cr60i6 at pressures > 1 kbar

and 490 K; pertinent physical properties, however,

are not known at present.

Cr02 .65: This nonstoichiometric oxide reported by

Wilhelmi [6] is obtained by the decomposition of

Cr03 in air at 540 to 570 K. Detailed data are lacking.

Cr3 Og: This oxide is formed by the decomposition

of anhydrous Cr03 [8]. It is not very stable and

decomposes at 650 K. The crystal structure and

other properties are not known.

CrOs: Chromium trioxide, CrC>3, is a stable material

but absorbs moisture. It is orthorhombic and the

structure consists of linear chains of corner-sharing

Cr04 tetrahedra [6, 9, 70]. Electrical transport studies [71] indicate it to be a semiconductor over a wide

temperature range. No crystal structure transitions

at ordinary pressures are known but p studies at high pressures [72] indicate a first order transition

at ~140-145 kbar pressure and the sample retains

the semiconductor behavior. Other properties of Cr03 have not been investigated in detail.

50
Chromium Oxides

Oxide and description of the study

Data Remarks and inferences References

Cr203

Crystal structure and

x-ray studies. Hexagonal; space group, R3C;

Z = 6. T = 291 K: a=4.9575 A;

c = 13.5976 A. T = 313K: a =

4.9605 A; c = 13.5945 A. Discontinuities are encountered in a and

c parameters at ~Tn (308 K);

c axis contracts on heating

through Tn. Pressure also decreases c parameter.

No change in the crystal structure takes place through the transi- tion region.

[6, 20, 35,

73].

X-T data. T = 4.2 K: x=22.4XlO-6 emu/g

(parallel to [111] direction). The

sublattice magnetization drops

from ~1.5 to zero at 7’n- Magnetic moment = 2.76 ±0.03 mb- Anisotropy in x is shown. Tn =

308 ±1 K; Tn decreases linearly

with pressure; dTx/dP = — 1.6±0.3 K/kbar.

X-T plot is typical of an antiferro- magnetic material. Increase of pressure seems to bring about the

collapse of the antiferromagnetic

spin structure at a given

temperature.

[14, 21, 36,

74, 75].

Electrical properties. Semiconductor behavior in the range 300-1600 K. p~105 flcm

at 300 K; Eo ~0.4 eV (T<1500

K); £a ~2.5 eV (T>1520 K). a — 0.2-0.4 mV/°C at ~500 K;

p type; md~105 cm2 /V s at ~500 K.

The available data indicate that

the conduction is extrinsic below

1200 K dominated by impurities

or native defects with hopping of

holes between Cr4+ and Cr3+

sites. Intrinsic conduction sets in at T ^ 1500 X.

[28, 29, 71,

76-82].

Dielectric properties. e«*8 (300 K) and the e value at zero and high magnetic fields and at high frequencies show

anomalies at Tn.

The magnetoelectric effect associ- ated with Cr203 below Tn is because of the magnetic ordering

of the Cr3+ spins.

[25, 26, 34].

Mechanical properties. Young’s modulus « 96 dyn/cm2 at 273 K and shows anomalous

variation in going through Tn.

Compressibility ~4.5 X 10-4 (kbar) -1 at 300 K.

The anomaly in the Young’s

modulus in Cr2C>3 is quite differ- ent from the one encountered in CoO and NiO and is attributed

to the domain phenomena and

differences in the magnetic order.

[33, 40].

Optical properties. Bands are noted at 14 390, 16 610, 21 690, 27 030 and 39 220 cm"1 and are attributed to the crystal

field transitions. Well defined

infrared bands are encountered in region 300-750 cm-1 and are due

to vibrations of the Cr-0 bond.

10 Dq = 16 610 cm-1

; 5=478

cm-1

.

The data indicate the localized nature of the electrons in

Cr2C>3.

[83].

C1O2

Crystal structure and

x-ray studies. Tetragonal; space group, P4/mnm;

Z = 2; a=4.421 A; c = 2.916 A. Typical rutile structure; TEC

shows high anisotropy and in

[4, 8, 51,

52, 55, 56].

51
Chromium oxides—Continued

Oxide and description of the studyData Remarks and inferences References

TEC (/°C): T = 303 K: ||‘»

-14.84X10-*; || a = +18.61 X

10"*. T-640 K: ||

fl - -0.1 X

10-«; || a = +13.52X10-*.

fact || e axis is negative. No

crystal structure change in going

through T«.

Magnetic properties. Ferromagnetic; T«~392-398 K

depending on the sample. Magnetic moment =2.07 ±0.03 mbSpecific magnetization = 138.3

at 0 K.

X^2X10-4 emu/g. at 500 K.

Typical ferromagnetic behavior

exhibited by single and polycrystalline samples. The properties of Cr02 are adequately explained on the basis of Goodenough’s

model [48, 84, 85].

[4, 48, 57-

59, 84, 85].

Electrical properties. Metallic in the range 80-575 K;

p~10-4 Qcm (300 K); a* —10

pV/K (300 K); exhibits discontinuities in p, a at Te . Semiconductor behavior is not en- countered above Te contrary to the expectations. Magnetoresist- ance effects noted; at 298 K in a

field of 29 kOe (applied parallel to [001] axis), (Ap/p) = ( — l.Bsfc 0.2)%.

Typical metallic behavior; the

electrical properties are ex- plained in a rational manner by

the Goodenough’s model [48, 84, 85].

[4, 48, 49,

51, 61-64,

84, 85].

Mossbauer studies of 87 Fe

doped CrOj.

Fe atoms exist as Fe3+

; hyperfine

splitting of the spectra dis- appears at To (397 K). Mag

netic moment = 2ps per Cr4+ ion at 0 K; internal magnetic

field = 530 kOe.

A novel study of the transition in Cr02 ; Te agrees well with other

measured values; internal field drops to zero at T« as expected.

[60].

CnOu

Crystal structure at 300 K. Orthorhombic; space group, Pbcn;

Z = 4; o = 12.044 A; b = 8.212 A;

c = 8.177 A. The structure contains pairs of CrOg octahedra

joined by sharing edges; by

sharing corners, the pairs of octahedra are linked with CrCh

tetrahedra to form a three di- mensional frame work. Trivalent

and hexavalent Cr atoms seem to be present.

[6, 7].

Cr«Oi6 (C^Os)

Crystal structure at 300 K. Orthorhombic; space group, Cmcm

or C2cm or Cmc2; Z = 4; a =

8.47 A; b = 12.90 A; c = 10.08 A. Kubota [8] reports that C^Og has

a high value of p (~1010 ftcm)

at 300 K.

Detailed structural and other data

are lacking.

[6-8]

CrO,

Crystal structure at 300 K. Orthorhombic, space group, C2cm;

£ = 4; a -4.789 ±0.005 A; 6-

8.557 ±0.005 A; c = 5.743 ±0.004 A.

CrOg has a chain structure. [6, 9, 70].

52
Chromium oxides—Continued

Oxide and description of the study

Data Remarks and inferences References

The structure consists of

linear chains of comer-sharing

CrO, tetrahedra.

Electrical properties. Semiconductor behavior; p~105-

106 ficm, p studies at high pres- sures indicate a transition at ~140-145 kbars with consider- able decrease in the value of p but

C1O3 remains semiconducting up

to very high pressures.

Detailed data are, however, lacking on Cr03 at present.

[71, 72].

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1.5. Manganese Oxides

Detailed phase relation studies of the Mn-0 system

[1-5] have shown that MnO, Mn304, M^Oa, and

MnOa are the stable solid oxides. All the oxides show

interesting crystallographic and magnetic transitions

and have been widely investigated in the literature.

MnO: MnO shows slight deviations from stoichiometry (<2%) at high temperatures (~1400 K)

[5-10]. It is cubic with the rock salt structure at room temperature and undergoes a trigonal distortion as well as a volume contraction on cooling to ~118 K and changes from a paramagnetic to an

antiferromagnetic state [6, 11-15]. The structure in

the antiferromagnetic phase has been examined by

a few workers [11, 15, 16-24] and detailed x

m T

measurements made. The magnetic moments in

MnO are arranged in ferromagnetic sheets parallel to {111} planes, with neighboring planes coupled

in an antiferromagnetic arrangement. The long range

order parameter increases progressively with decreasing T below Tn. Neutron diffraction measure- ments show considerable short range magnetic order

in MnO above Tn [16, 22]. Even though many workers have argued for a continuous (higher order)

transition in MnO because no hysteresis effects are noted at TN, the available x-ray diffraction [14],

X-T [21], exchange striction [14, 15], TEC [25], Mossbauer [26], internal friction and Young’s modulus

[27], acoustic attenuation [28], elastic moduli [29]

and NMR [21, 30-32] data indicate that the antiferromagnetic-paramagnetic transition is indeed first order. High pressure studies on MnO have been

carried out and the x-ray data [33] indicate a transition in MnO at ~100 kbar to a phase which is tetragonal or of lower symmetry. Bartholin et al. [34] found that the Tn increases with P up to ~3

kbar.

Electrical properties of pure [9, 35-41] and doped

(with Li or Cr) [9, 42-44] MnO have been examined

in detail. MnO is unique among the transition metal

monoxides in that there is a maximum in the p versus po2 plot indicating a transition from p to n type

semiconducting behavior at ~1370 K; changes in

sign of ph and a have also been noted [5, 35, 38-40]

to corroborate the resistivity data. The electron

mobility in MnO is higher than the hole mobility

at high temperatures (p£/p^«10 at ~1450 K) [45,

46] and the defect structure seems to involve doubly

ionized cation vacancies at high temperatures and

at low oxygen partial pressures [5, 9, 35-37, 41, 42,

45, 46]. The charge carriers are localized in pure

and doped MnO and at low enough temperatures

(<700 K) hopping type of conduction occurs by

holes in the 3d-band with a small Ea whereas the

dominant mechanism of conduction is by holes in

the oxygen 2p-band [9, 47, 48]. The EMF data on MnO at high temperatures [10] indicate second or higher order transitions within the homogeneity

region.

Optical properties of MnO have been studied in

the literature [49-51]; the absorption edge appears

to be ~3.8 eV. Far infrared antiferromagnetic resonance studies [52-54] show that the resonance frequency at 27.5 cm-1

is dependent on temperature

and impurity content.

MnO forms solid solutions with other transition

metal monoxides having the rock salt structure [55,

56] and the physical properties show variations with

the composition.

54
M113O4

: M113O4 has a tetragonal structure at room

temperature and can be considered as a distorted

spinel because of the Jahn-Teller distortion at the

Mn3+ sites [57]; the ionic formula is probably very

close to Mn2+ [Mnj!+

] O4 corresponding to a 'normal’

spinel. Preparation of Mn304 in both polycrystalline

and single crystal forms has been reported in the

literature [6, 58-65]. The degree of tetragonal dis- tortion in Mn304 decreases with increase in T and

a rapid structural transition to cubic phase takes

place at ~1435 K [66, 67]. The high temperature

cubic form can not be preserved by quenching and

!

the thermodynamic parameters indicate the first order nature of the transition [10, 66, 67].

II Mns04 is ferrimagnetic below 43 K which is the

curie temperature [65, 68, 69]. At 33 K a rearrangement of the moments occurs such that the chemical

and magnetic unit cells become identical. Measurements of Cp [70] in the range 20-49 K show that

there are no anomalies except for a peak at Tc. The material is a semiconductor [71-73] and has a higher resistivity than the analogous material Fe3C> 4 . Mn304 forms complete series of solid solutions

with Fe304 and C03O4 [67, 74].

Mn2 03 : The common form of manganese sesquioxide

is the a-modification which is closely related to the

bixbyite, (Mn, Fe) 2 03, structure [6]. 7-Mn203 having

a structure nearly identical to that of Mn304 has

been reported [6, 75], but detailed data are lacking.

Moore et al. [6] have noted that 7-Mn2 03 transforms

to the a-modification either by heating in vacuum

for 48 h at ~770 K or by keeping at room temperature for one year. Among the transition metal sesquioxides, a-Mn203

is the only oxide which does not possess a corundum

structure and instead, it assumes a deformed cubic

C-type rare-earth oxide structure [76-78] possibly

as a result of the large Jahn-Teller distortion associated with the Mn3+ ion [79, 80]. Geller and co- workers [79-84], employing x-ray diffraction, x-T

and Mossbauer studies have shown that a-Mn2 03

undergoes a crystallographic transition (cubic to orthorhombic) at 302 K, a paramagnetic-antiferromagnetic transition at 80 K and another magnetic

transition (of the Neel type) at 25 K. Crystal struc- ture of a-Mn203 does not change below 300 K and

x-ray data at 6.5 K show no additional lines or symmetry change implying that the transition at 25 K

involves a shift in the symmetry center of the struc- ture (characteristic of a diffusionless, but probably,

first-order transition).

Mn2 C>3 behaves as a semiconductor in the range

400 to 1000 K [85, 86]. It decomposes at high temperatures.

Trivalent cations can be doped successfully into

Mn203 and the structural and magnetic ordering

aspects have been examined by Geller and co-work- ers [80-84] and by Hase et al. [87].

Mn02 : Mn02 apparently exists only in the rutile

modification and found in nature as pyrolusite [6]. An orthorhombic modification has been reported

in the naturally occurring samples, but detailed data

are lacking; laboratory preparation of the orthorhombic form has, however, not been successful [6]. Mn02 is always associated with slight nonstoichiometry [6, 88, 90] and decomposes above 800 K to Mn203 . The usual wet chemical methods of preparation

of Mn02 result in nonstiochiometry and water of

hydration and are less characterized [6, 88, 89].

Atomic or molecular oxygen is evolved on heating

these 'disperse’ forms of Mn02 and serve as good

oxidizing agents in organic chemical reactions; most

probably the 'active’ oxygen is present in the lattice as defects or adsorbed on the surface. The activity 'seems to depend on the particle size and the method

of preparation etc. No crystal structure transitions are known in

Mn02 . It is antiferromagnetic below 92 K [90-94]

and has a screw-type spin structure such that the

spins, aligned parallel in the c plane, screw along the

c axis; the magnetic unit cell is as large as 7 times

the chemical unit cell. Mn02 has a low resistivity (~0.H2 cm at 300 K)

and shows anomalous temperature dependence in

the range 4 to 300 K [90]. Anomalies in p and Cp have been noted at Tn [90, 94, 95]. Bhide and

Damle [96] reported Mn02 to be ferroelectric with a Tc of 325 K and Samokhvalov [97] finds a (UHF)

dielectric anomaly at around this temperature; however, the anomaly does not correspond to that of a

typical ferroelectric material. Ferroelectricity in

Mn02 is doubtful since no other relevant data are available in the literature.

Solid solutions, (Mn1_a: Cra.)02 , have been reported

by Siratori and Iida [98].

55
Manganese oxides

Oxide and description of the studyData Remarks and inferences References

MnO

Crystal structure and

x-ray studies. T = 296 K: Cubic; space group, Fm3m; Z = 4; a =4.4457 ±0.0002

A. T ~i K: Rhombohedral; o =

4.4316 ±0.0003 A; a=90.624±

0.008°. Tt (= Tn) = 118±2 K.

TEC (/°C): 64X10-9 at ~TN. High pressure x-ray studies at 300 K indicate a phase transition at ~100 kbar to a tetragonal or lower symmetry phase.

The cubic lattice distorts to rhombohedral structure at Tt on

cooling; slight volume changes

also occur. TEC also touches

maximum at Tt. The structure of the high pressure form is not known.

[6, 11-15,

25, 33].

Magnetic properties. x (122 K)= 83Xl0-« emu/g;

X (4.2 K)=79X10-6 emu/g.

Typical antiferromagnetic be- havior with Tn = 118±3 K.

Tn increases with pressure; (dTn/dP) =0.3X10-3 deg/bar. In

the presence of a magnetic field, below Tn, the direction of anti- ferromagnetism is modified which

leads to an increase in xIn the antiferromagnetic state the magnetic moments are arranged

in ferromagnetic sheets parallel to (111) planes and the direction of magnetization in neighboring

planes is antiparallel. Neutron

diffraction shows the persistence of short range magnetic order

above Tn. The magnetic transi- tion appears to be first order.

[11, 15, 16-

24, 34].

Electrical properties. Pure MnO:

p (300 K)~1016 ftcm in stoichio- metric MnO. Semiconductor be- havior up to ~1500 K; Ea ~l.Q

eV; a (500 K)~400 pV/K;

Changes from p- to n-type be- havior as a function of poa (at ~10“12 atm), at ~1370 K; a

changes sign at this temperature;

(Md/md) * 10.

Li doped MnO:

p~10-3-10-1 ficm; £a ~0.7 eV; Mh =0.007 cm2 /V s (400-700 K);

Md = (calc.)~0.03 cm2 /V s; m* =

7 mo at 700 K. p decreases with

pressure and no transitions are indicated up to 200 khar.

The data on pure MnO can be in- terpreted as due to hopping of charge carriers with a band gap

of ~2.2 eV or with a band con- duction for electrons with a

smaller band gap. The change

from p- to n-type behavior is due

to the greater mobility of elec- trons compared to holes at high

temperatures and low oxygen

partial pressures and due to the

cation vacancy defects. In doped

materials, conduction is always

extrinsic at ordinary temperatures

and mainly occurs by hopping;

intrinsic conduction seems to set in at very high temperatures.

[5, 9, 35-

37, 42, 45,

46-48, 99].

Optical properties. Dq = 1010 cm-1

; B = 601 cm-1

; Absorption edge is at ~3.8 eV. Far infrared antiferromagnetic resonance is at ~27.5 cm-1 at 2 K; the frequency decreases with rise in T and varies with the impurity content.

Optical data indicate the localized nature of the charge carriers in MnO.

[49-54].

Mossbauer studies of 67 Fe

doped MnO.

Tn = 116.9 K. Magnetic hyperfine

splitting noted below Tn. Internal magnetic field is maximum (170

kG) at 90 K and decreases to 97

kG at 4.2 K.

The magnetic transition appears to be first order but no temperatures hysteresis is noted in the

transition.

[26].

56
Manganese oxides=C©ntinued

Oxide and description of the study

Data Remarks and inferences References

NMR studies of MnO. The 66Mn NMR frequency shifts are found to be temperature de- pendent and proportional to x* magnetic field dependence also is noted. Hyperfine coupling constant = — (81.5±2.5)X10-4 cm-1

.

Guenther et al. [32] speculate that weak ferromagnetism may exist

in MnO at very low

temperatures.

[21, 30-32].

Mechanical properties.

M113O4

Compressibility = 6.48 X10“7 bar-1. Elastic moduli (measured at 30

MHz): Cn = (1.768 ±0.006) X1012 dyn/cm2

; Cu = (6.8±0.1) X 10u dyne/cm2 (at T>Tn). At Tn,

Cu drops by ~19% and greater decrease is shown by Cu. Young’s

modulus (at Tn)=2X10u dyn/cm and internal friction « 20

X 10~3

; both the properties show

anomalous behavior near Tk.

The discontinuous changes in the

elastic moduli, Young’s modulus

and internal friction are taken as strong evidence for a first order magnetic transition in MnO.

[14, 15, 27,

29].

Crystal structure and

x-ray studies. T = 300 K: Tetragonal; space group, I4i/amd; Z = 4; 0 = 5.762 A; c = 9.463 A. T, = 1435 K.

T>T(: Cubic; a = 8.42 A.

MmO* can be considered as a dis- torted spinel because of the Jahn -Teller distortion at the Mn3+ sites; the ionic formula can

be written as Mn2+/[Mn2

8+

] 04. The transition is first order and

the high temperature phase can- not be quenched to room

temperature.

[6, 58-67].

Magnetic properties. Ferrimagnetic below 43 K (To); a rearrangement of the spin moments occurs at 33 K such that

the magnetic and chemical unit

cells become identical, xm (290

K)~12XlO-3 emu; Magnetic

moment = 5.27 mb below To.

The observed ferrimagnetism is consistent with the spinel structure. 1/x-T behavior is of the Neel type above Tc and consistent with the Yafet-Kittel

model.

[60, 65, 68,

69].

Electrical and thermal

properties.

Semiconductor behavior; p (300 K)

~107 ficm; a«0.2 mV/°C in the range 500-900 K (p-type); AH = — (5.9±0.7) kcal/mol; AS =

(4.2 ±0.5) et at T<; Cp shows an anomaly at Tt

.

Detailed electrical data are lacking; Mna04 has a higher value of

p than FesCh. The enthalpy and

Cp data indicate that the transi- tion is first order.

[10, 66, 67,

70, 71-73].

Infrared spectra.

MniOs

Bands at 474 and 550 cm-1 are noted. The data are correlated with the

spectra of other oxide spinels.

[100].

Crystal structure and

x-ray studies. T = 298 K; Orthorhombic; space

group, Pbca; Z = 4; a =9.414 A; 6=9,424 A; c = 9.405 A. T< =

302 K. T = 314 K: Cubic; space

group, Ia3; Z = 16; a =9.414 A.

Earlier workers concluded that only cubic structure exists in MnjOs; this oxide does not have

a corundum structure and in- stead has a distorted C-type rare- earth oxide structure. The transi-

[76-84],

57
Manganese oxides—Continued

Oxide and description of the study

Data Remarks and inferences References

tion appears to be first order but

detailed data on other properties

are lacking.

Magnetic properties. Xm (100 K)~7X10-3 emu. Para- magnetic—antiferromagnetic

transition, Tni, at 80 K and

another magnetic transition at 25 K (Ts2 ). No crystal structure

transition below 300 K up to 4 K;

Both the low temperature phases

are antiferromagnetic. Tni and

Tn2 are greatly affected by

impurities.

Geller et al. [82] proposed a magnetic structure in the antiferro- magnetic phase. Chevalier et al. [102] find that the low temperature (25 K) transition is at 50 K

and is of second order.

[79-84, 101,

102].

Optical and electrical properties.

Band at ~20,000 cm-1

is ascribed

to the spin allowed transition. Infrared bands are in the range 320-670 cm-1 and are ascribed to the stretching and bending vibrations of the Mn-O bond. Semiconductor behavior in the range 400-1100 K: p (200 K)~7X103 Qcm; p (500 K)~50 £2cm; Ea =

0.6 eV.

Detailed electrical data on single

crystal material are lacking.

[85, 86,

103].

M11O2

Crystal structure and

x-ray studies. T = 298 K: Tetragonal; space group P42/mnm; Z = 4; a =4.3980

±0.0002 A; c = 2.8738 ±0.0002 A. TEC (300-670 K) (X107/°C);

1

1

c = 69 ; || a =67. With pressure, c

axis expands at low pressures, then passes through a maximum

(at ~40 kbar) and ultimately

contracts; a axis, on the other

hand, decreases smoothly with

pressure. The compressibility is low at low pressures and then

increases at pressures beyond the

maximum in c axis.

No crystal structure transitions are known and it is very difficult to prepare exactly stoichiometric

MnC>2 .

[6, 33, 88-

90, 104,

105].

Magnetic properties. Antiferromagnetic with Tn= 92±2

K. x (100 K) «36X 10-6 emu/g;

X (300 K) —32X10-8 emu/g.

Curie-Weiss Law is satisfied above 150 K. A screw type spin structure exists below Tn and the magnetic unit cell is 7 times the chemical unit cell.

The characteristic screw structure

is exclusive to MnC>2 .

[90-94].

Electrical and other

properties.

Resistivity is low; p (4.2 K) ~2.6X

10"2 ficm; p (300 K)»l.lXl0~k ficm. Anomalous T dependence

in the range 4-300 K. Anomalies

The anomalous p-T behavior is not

clearly understood; Mn02 may

be classified as a degenerate

semiconductor [57]. The magnetic

[90, 94, 96].

58
Manganese oxides—Continued

Oxide and description Data Remarks and inferences of the study

in p and Cp noted at Ts- Ferro- transition may be classified as

electricity with a Tc~325 K second order. The details of the

reported. ferroelectric behavior are not understood and ferroelectricity is doubtful.

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1.6. Iron Oxides

The Fe-0 system has been a subject of extensive

investigation over the past several decades [1-4]. The

available data up to 1957 has been summarized in the literature [2]. The most important iron oxides are the oxygen deficient ferrous oxide (wiistite),

Fei-zO, magnetite, Fe304 and hematite, Fe2 03 . The

dioxide, Fe02 has been recently reported [5], but

the existance of stable Fe02 still appears doubtful II [6].

'I Fei_zO: Wiistite at ordinary pressures is always I nonstoichio-metric and the homogeneity range ex- tends from Fe0 . 84O to Feo.gsO at ~1620 K and the

material is unstable below 840 K with respect to decomposition into Fe304 and Fe metal; the metastable phase can, however, be retained by quenching i [1, 2, 7-10]. The thermodynamics and the defect

structure at high temperatures has been examined

[4, 11-20]. The deviation from stoichiometry in ' wiistite is due to cation vacancies, with each cation

vacancy requiring the presence of two Fe3+ ions to maintain charge balance; thus, Fei_xO may be re- i garded as a solid solution of Fe2+ and cation vacancies in FeO. The average structure at room temperature is cubic [7-10] and the material is paramagnetic.

Magnetic ordering into antiferromagnetic state oc- ; curs at 198 K accompanied by a change from cubic

to rhombohedral structure. Below Tn, the magnetic

moments are arrayed in ferromagnetic sheets parallel to (111) planes and the direction of magnetization

in neighboring planes is antiparallel; the magnetic : axis in Fei_xO is perpendicular to the ferromagnetic

sheets [9, 10, 21].

Detailed studies employing x-ray and neutron

diffraction techniques of the quenched phases of

Fei_xO indicate defect cluster formation [10, 18, 22-

24]; it appears that the oxygen sublattice is continu- ous and the defect cluster is a region in which there

are vacancies in cation sites and interstitial ions in some tetrahedral sites. The results are in agreement | with a model which assumes that the average defect ' cluster contains about 4 vacancies and 2 interstitials

in powder samples and 13 vacancies and 4 interstitials in single crystalline samples. Order-disorder

transitions have been noted in Fei_ xO at high temperatures [19].

The cubic lattice parameter of Fei-zO shows varia- ; tion with the stoichiometry and generally, a increases

with increasing Fe content [8]. Magnetic cluster

60
formation [22] also affects the spin structure and

directions in Fei_xO below Tn, but Tn is almost

unaffected by variation in x [21]. This behavior is indicative of the existence of defect clusters of cation

vacancies by virtue of which the remaining part of

the cation sublattice is left intact thereby not weakening the antiferromagnetic interaction; random

vacancies, on the other hand, would be expected to

1 weaken the antiferromagnetic interaction and lower

the Tn.

Stoichiometric FeOi.oo lies outside the phase field under ordinary pressure conditions [24], but under

high pressure conditions and in the presence of excess metallic iron, the cation deficient structure of

wiistite might not be formed; Katsura et al. [25]

have indeed successfully synthesized FeOi.oo from

FeOo.95 and Fe at a pressure of 36 kbar and at 1040 K. Hentschel [26] has recently concluded on

the basis of x-ray and Mossbauer data that FeOi.oo

is formed as a metastable intermediate in the decomposition of wiistite at relatively low temperatures (~500 K).

Resistivity measurements [4, 13, 14, 27-29] on

Fei_;rO indicate that p decreases with temperature

as well as the oxygen content. Low oxygen content

samples are p type as evidenced by the a data; a p

- to n-type transition is noted at higher oxygen

content or with higher po2 in the system. The defect structure is interpreted as due to doubly ionized

cation vacancies.

Anomalous behavior of TEC [30], Cp [31] and

UHF dielectric constant [32] have been noted at Tn in Fei-xO. High pressure x-ray studies [33] do

not indicate a transition up to several hundred

kilobars.

Solid solution formation of Fei_xO with MnO and

MgO has been reported in the literature [8, 34].

Fe304 : This oxide is encountered in nature as the

magnetic oxide of iron or magnetite; it can also be

prepared in the laboratory and has a narrow range

of homogeneity [1-3]. Fe304 has the cubic inverse

spinel structure [35] at room temperature where

Fe 3+ ions occupy all the A sites and one-half the B

sites, while the Fe2+ ions occupy the other half of

the B sites. The material is ferrimagnetic with Tn~

858 K [36-38].

On cooling to 119 K, Fe3 04 undergoes a crystallographic transition to an orthorhombic modification

[39-44]. This has been explained by Verwey et al. [39, 40] as due to the ordering of the octahedral Fe2+ and Fe 3+ ions into perpendicular rows with the

associated reduction in crystal symmetry. This

transition is commonly referred to as the Verwey

transition and detailed investigations have been carried out to understand the nature of the transition.

Magnetic properties of Fe304 are not affected

below the Verwey transition, but x shows anisotropy [41, 43-46]. Fe304 has a low resistivity at room

temperature and the ordering of the ions below Tt (119 K) appears as a sudden increase of p by a factor

of >90 and also causes an anisotropy in p [41, 43].

Verwey et al. [39, 40] have explained this observation

as due to the fast electron exchange in Fe304 which is inhibited below Tt because of the ordering process.

Many workers [47, 48] have proposed that this

phenomenon is similar to a metal-semiconductor

transition, but the actual electrical behavior above

Tt is more complex and does not correspond to the

typical metallic behavior. Semiconductor behavior

is noted in the range 80 to 119 K and 119 to 250 K;

p is essentially constant in the range 250 to 1700 K

showing slight maxima and minima at 350 and ~850

K respectively [43, 49]; the low value of p and con- stancy over a wide temperature range (250 to 1700

K) seem to indicate that Fe304 can be classified as a degenerate semiconductor above room temperature

[35, 50-53]. Discontinuities in a, Rh and mh have

also been noted at Tt [53-56].

Mossbauer studies of the ordering transition in

Fe3 04 have been reported by many workers in the

literature [57-67], Careful studies on untwinned

crystals of Fe3 04 have revealed that the Mossbauer

spectrum is composed of five components below Tt corresponding to a tetrahedral Fe 3+ site, two octa- hedral Fe 3+ and two octahedral Fe2+ sites. One

would expect, on the basis of the Verwey mechanism, a three component spectrum of equal intensity

(due to octahedral Fe2+ and Fe 3+ and tetrahedral

Fe3+ sites). In order to explain the observed spectrum, it is proposed [64, 65] that the Verwey-ordering is not entirely correct and the unit cell must at

least be doubled to enable the different sites to contribute to the hyperfine component. This is cor- roborated by the recent studies employing neutron

diffraction [68] and electron microscopy [69].

Several properties of Fe3 04 exhibit discontinuities

at Tt ; among them are TEC [70], saturation magnetization [41], magnetostriction [70], Young’s modulus and internal friction [71], elastic constants and

sound wave velocity [72], magnetocaloric effect [73]

and NMR relaxation [74, 75]. However, Tt determined by different techniques differ slightly, possibly

due to slight nonstoichiometry or sample purity

[66]. Discontinuities in the ultrasonic sound velocities

have been noted near Tn in Fe304 [76]. High pres61
sure studies [47, 77] indicate that Tt decreases whereas Tn increases in a linear fashion.

Fe3C>4 forms solid solutions with Mn304 and C03O4

giving spinel type structures [34, 78].

Fe20s: Iron sesquioxide, Fe2C>3 (hematite), exists in

several modifications: a-Fe2 03, the most stable form

having a corundum structure [1, 2, 4], 0-form (supposedly metastable) possessing a cubic bixbyite

structure [79, 80] and the 7-modification having a tetragonal structure which is metastable at all conditions of temperature and pressure [2, 79, 81-84].

The structure of 6-Fe203 is not well established and

even its composition is open to doubt (a better description might be 5-FeOOH) [85-88]; the c-form

having a monoclinic structure has been recently

characterized [89] and a high pressure form of Fe203

having a hypothetical perovskite structure has been

deduced from shock wave experiments [90].

7-Fe2 03 : 7-Fe2 03 has a tetragonal structure and

consists of three spinel blocks stacked on top of

each other; the unit cell has the formula Fe6 4096 instead of Fe72096 to be expected with a spinel compound. The eight vacancies are distributed among the

octahedral spinel sites in an ordered manner [79, 83,

91]. The material is ferrimagnetic [91, 92] and trans- forms to <*-Fe2 03 above ~670 K [2, 79, 81, 92-94];

the transition occurs gradually and it is easy to control the conversion. Goto [95] investigated the

phase transformation of y-Fe203 up to 33 kbar

pressure and up to 570 K. The rate constant primarily depends on the thermo-dynamic parameter

PA V where A V (~7%) is the volume difference

between a- and 7-modifications.

e-Fe2 03 : The monoclinic e-Fe203 has a dark brown

color and is ferromagnetic with Tc ~485 to 500 K

[89, 96]. It decomposes to a-Fe2C>3 above 770 K.

a-Fe2C>3: This is the most stable sesquioxide of iron

and has a very narrow range of homogeneity [1-4,

97]. It has a rhombohedral structure [98] and is antiferromagnetic with a high Neel point (Tn~960

K) [99-104]; discontinuities in the lattice parameters

are noted at Tn [105]. Of special interest in Fe203 is the magnetic transition which takes place at ^263 K

(Tm) and is usually referred to as the Morin transition [106] and has been extensively studied in recent

years. Below T

m

and in the absence of magnetic

field, the Fe3+ spins are aligned along the (111) or z axis of the crystal and the magnetic structure is

that of a pure antiferromagnet; above TM , the crystal exhibits weak ferromagnetism with canted spins

lying in the (111) plane. On application of a suffici- ently large magnetic field in the 2-direction below

Tm, a field induced first order spin-flip transition is observed. The Morin transition in Fe2 03 has been

examined by a variety of experimental techniques

like neutron diffraction [100, 107, 108], static magnetization [106, 109-113], AFMR [110, 114, 115],

Mossbauer spectroscopy [102, 116-120], NMR [121],

ultrasonic attenuation [122], Cotton-Mouton effect [123] and inelastic neutron scattering [124-127]. The

effect of pulsed magnetic field has also been studied

[128].

Several theoretical approaches have been attempted to gain an insight into the mechanism of the

Morin transition [113, 128, 129-132]. The effect of

pressure on the transition indicates that T

m

in- creases in a linear fashion; (dTu/dP) = +3.6°/kbar

[133-135]. Tm also depends on the particle size, the

method of preparation and the purity of the sample

[136, 137]. The effect of doping Fe203 (with impurities) on Tm has been studied by a few workers;

Al3+

, Ga3+

, Cr3+, Sc3+

, Ti4+, and Sn4+ ions seem to lower Tm (broadening the range of existence of weak

ferromagnetism) [112, 138, 139] while Rh3+ raises

the Tm considerably [140-142].

Electrical properties of pure [143-146] and doped

[147-150] Fe203 have been examined by various

workers in the literature. The material is a semiconductor in the entire temperature range investigated (200 to 1600 K). Highly pure Fe2 C>3 shows some what complex behavior with different activation energies in the region 300 to 700K, 700 to 950K,

and 950 to 1600 K whereas impure Fe2 03 shows

linear variation of lnp with 1/T in the range 400 to 1500 K with Ea^T.O eV. Anomalies in p and a at Tt and Tn have not been noted, but n-type Fe2 C>3 samples show a sign reversal in Rn at ~Tn while

Ph goes through a minimum around this temperature. Because of the ease with which the valence

state of Fe can be changed by slight impurities, it is highly improbable that the conduction is intrinsic

in Fe203 . High pressure x-ray studies do not indicate any

phase transition [151], but recent resistivity studies

at ultrahigh pressures seem to indicate a transition

from the insulating phase to a highly conducting

state (at ^2 Mbar pressure) [152].

Solid solutions of Fe2C>3 with Mn2 03 [153-157],

Cr2 03 [158], Ga2 03 [159, 160], and ln2 03 [160-162]

have been examined in detail in the literature.

62
Iron oxides

Oxide and description of the study

Data Remarks and inferences References

Fei_*0

Crystal structure and

x-ray diffraction studies.

Magnetic properties.

Electrical properties.

TEC and UHF dielectric constant studies.

T = 300 K: Cubic; space group, Fm3m; Z = 4. a varies from

4.2886 to 4. 3012 A with in- creasing Fe content in Fei_x O

(Feo.8*0 to Feo.950). For FeOi.oo synthesized at high pressure, a =

4.323 ±0.001 A. Tt = 198 K.

Below Tt, the structure is rhombohedral. The rhombohedral

distortion is sensitive to the Fe

content; at 90 K, a «60° for high

Fe content Fei_xO and « 59.04°

for low Fe content sample. High

pressure x-ray studies do not

indicate any phase transition up

to several hundred kilobars

pressures.

At ordinary pressures, FeO is always nonstoichiometric due to the cation vacancies. Studies on

quenched high temperature

phases (Fei_xO is unstable below

850 K) indicate defect cluster formation. Below Tt , the rhombohedral distortion varies with the stoichiometry but Tt itself remains unaffected. Hentschel [26] found evidence for the formation of intermediate

stoichiometric FeOj.oo in the de- composition of wiistite at rela- tively low temperatures.

[1, 2, 7, 8, 10, 18, 19, 22-25, 33].

Antiferromagnetic below 198 K

(= Tn) and the magnetic moments are arrayed in ferromagnetic sheets parallel to (111)

planes and the direction of magnetization in neighboring

planes is antiparallel; the magnetic axis is perpendicular to the ferromagnetic sheet, xm obeys a Curie-Weiss law; xm =3.56/

(T+136). Magnetic cluster for- mation is noted anologous to the

cationic defect clusters which

affect the spin structure and spin

directions below Tn. Tn is not much affected by variation of x

in Fei-zO.

The spins in FeO are perpendicular [9, 10, 21, to the (111) plane unlike in NiO 22]. and MnO which also show anti- ferromagnetism and have rock

salt structure above Tn.

Semiconductor behavior (100-1100

K); p (300 K) «0.05 ficm; p decreases with increase in the oxy- gen content in Fei_xO. Three

distinct regions in lnp-T behavior: Ea = 0.008 eV (<100 K); Ea =

0.3 eV (125-175 K); Eo =0.07

eV (200-1100 K). The samples

are p-type but high oxygen content samples are n-type. The defect structure is of the doubly

ionized cation vacancy type.

Unusually low resistivity exhibited

by Fei_x O compared to other 3d

transition metal monoxides and

controlled by stoichiometry;

p-T behavior is extrinsic.

[4, 13, 14, 27-29].

TEC and UHF (~10 GHz) dielectrie constant data indicate anomalies at Tn. TEC (186 K) =

41X10~8 /°C; e« = 12.7.

[30, 32].

63
Iron oxides=Centinued

Oxide and description

of the study

Data Remarks and inferences References

FeiO«

Crystal structure and

x-ray data. T = 295 K: cubic; space group, Fd3m; Z = 8; a =8.394 ±0.0005 A. T« = 119 K. T = 78 K: Orthorhombic; space group, Imma;

a=5.912 A; 6 = 5.945 A; c =

8.388 A. TEC (128-295 K): 7.7X

10_6 /°C. TEC is negative near T( ( —20 X 10-6 /°C) with a peak

at Tt. T

t

decreases with pressure; (dTt /dP)= — (0.46 ±0.02)

K/kbar.

The substance is an inverse spinel.

The Verwey transition in FegC^

changes the lattice symmetry

from cubic to orthorhombic and

causes an ordering of the Fe2+ and Fe3+ ions on the B sites which are randomly distributed above the transition temperature.

The octahedral ferric ions lie along the orthorhombic a axis and the ferrous ions along the 6

axis in alternate rows that are mutually perpendicular.

[40-43, 47,

70].

Magnetic properties. Ferrimagnetic; Tn«860 K. x does not show anomaly at Tt (119 K)

but anisotropy behavior noted

with or without a magnetic field. (Aff /Mo) =0.026 at TN . Magnetic moment = 4 mb* Tn in- creases with pressure; ( dTn/dP) = (2.05 ±0.10) K/kbar.

The magnetic behavior is unaffected

at the ordering transition. The

magnetic moment corresponds to the spin-only value (since FegO*

is an inverse spinel), the contri- bution from the Fe3+ ions must

vanish and the entire net mo- ment must be due to the Fe2+ ions.

[36-38, 41,

43-46, 77].

Electrical properties. Below T

t

(119 K) semiconductor

behavior; Ea increases from 0.03

to 0.15 eV just below T<. At Tt

(on heating), p drops by a factor of 102 to the order of 10-2 flcm. Above Tt , p continues to decrease (with Ea ~0.06 eV) and in the range 250-1700 K, p is es- sentially constant ('~4X10-8 12cm) with a slight maximum at ~350 K and a minimum at ~Tn.

a (500 K) « —60 mV/°C. Rh, however, is +ve. At 300 K,

Rh ~5 X 10-4 cm3 /C; mh~0.1 cm2 /V s. a shows discontinuities

at T

t

and Tn. Rh and mh decrease with T below Tt and

remain constant or show an in- creasing trend above Tt

.

The electrical properties of FegC^

are a little bit complex; some

workers [47, 48] assume that it exhibits a semiconductor-metal

transition at T

t

(119 K). However, the p-T behavior above T

t

indicates that of a typical de- generate semiconductor. The

maxima and minima at 350 and

860 K in the p-T plot may be

spurious and requires confirmation by a detailed investigation on well-characterized, stoichio- metric FeaCh. The abrupt drop

in p at Tt is explained as due to the rapid electron exchange be- tween the octahedral Fe2+ and

Fe3+ ions; this is inhibited below

T( because of the ordering

process.

[39-41, 43,

49, 53-56].

Mttssbauer studies. Careful results on untwinned crystals of FeaO« indicate that the spectrum

can be resolved into five indi- vidual components corresponding

to a tetrahedral Fe2+

, two octa- hedral Fe,+ and two octa- hedral Fe2+ sites. The lines below

Tt are broadened. The results are interpreted in terms of a modiMossbauer studies have provided

the most direct evidence of the

electron exchange and ordering

process in FegO* at Tt<

[57-67].

64
Iron oxides—Continued

Oxide and description

of the study

Data Remarks and inferences References

fied Verwey model consisting of twice as large as the unit cell originally proposed. The relaxa- tion time of the electron hopping

is estimated to he 1.1 ±0.2 ns.

Mechanical properties. Young’s modulus drops by ''-'30% at 108 K; internal friction exhibits a peak at ~95 K. At 300

K, cl = 6.75 X 105 cm/s; is —

3.69 X105 cm/s. Both cl and cs along [100] axis show sharp in- creases of 7.5 and 0.2% respectively at 119 K. cl also exhibits an anomaly at 130 K where there

is an easy axis change in FejCX

from the [100] to the [111] axis. cl and cs also go through a minimum at ~Tn.

These observations indicate that the 119 K transition is of first order.

[71, 72, 76].

Magnetocaloric effect. The temperature change of a sample upon applying magnetic field has been examined through Tt in FejO*. AT is negative below Tt and reaches a maximum at T<; AT passes through zero and be- comes -fve. The sign reversal

shifts to lower temperatures as the applied field gets stronger.

[73].

Infrared spectra.

-FesOj

A band at 595 cm-1

is noted and

correlated with the bands noted

in other oxide spinels.

[163].

Crystal structure and

properties.

FesOj

Tetragonal; space group P4j; Z =

32; a =8.33 A; c = 24.99 A. 7-FejOa is ferrimagnetic and

transforms to a -modification above 670 K. AF~7% and high

pressure studies indicate no

abrupt transition at any given P

and T. Detailed physical properties not known.

The structure consists of three

spinel blocks stacked on top of each other; the unit cell has

cation vacancies arranged in an

ordered manner.

[79, 83, 91,

92—95].

Crystal structure and

properties.

•FejOj

Monoclinic; Z = 20; a = 12.97 A;

6 = 10.21 A; c = 8.44 A; 0 =95.33°. Ferromagnetic; Tc~485-500 K.

Transforms to a -form above

770 K.

Detailed properties not known. [89, 96].

Crystal structure and

x-ray studies. T = 300 K: Rhombohedral

(pseudo-hexagonal); space group, R3c; Z = 6; a = 5.0351 ±

Typical corundum structure as- sumed by many transition metal

sesquioxides and a-AljOj.

[98, 105,

151, 160].

65
Iron oxides—Continued

Oxide and description of the study

Data Remarks and inferences

0.0003 A; c = 13.750 ±0.001 A.

Discontinuities in lattice parameters noted at Tn (»960 K).

High pressure studies do not

indicate any phase transitions up

to several hundred kilobars.

Magnetic properties.

Electrical properties

(200-1600 K).

Mossbauer studies (100-1050 K).

Optical properties.

Antiferromagnetic; Tn~960 K.

X~1.7-2.3X10-6 emu/g. Magnetic moment = 2.45 mb- Above

263 ±2 K (Tm), Fe203 exhibits weak superimposed ferromagnetism due to slight canting of the spins in (111) plane. The

spin -flip transition has been

observed in the presence of a magnetic field. Careful neutron

diffraction studies show that the

transition is spread over a 25-30°

range, centered at 261 K. Tm de- pends on the particle size and

stoichiometry of the sample. T

m

increases with pressure; (dTu/dP) =3.6 ±0.3 K/kbar.

Semiconductor behavior; p (300 K)

~103-106 ficm. Pure Fe2C>3 shows

a three-region behavior with

various Ea values in the range 300-1600 K; usually p type be- havior but some samples show

change of sign of a on heating.

Impure or doped samples exhibit

linear lnp-T plots with Ea ~1.0

eV. Both p- and n-type samples

can be obtained by doping. For

n-type Fe2C>3, at 1000 K, p~l-10

12cm; a~ — (400-700) pV/K;

Ph~T0-2 cm2 /V s. Rh changes

sign at Tn.

Characteristic six -line spectra noted

below Tn. Anomalies in isomer

shift and quadrupole splitting

noted at ~Tm. The electric field gradient shows an abrupt in- crease at ~Tn. The temperature

dependence of the internal field

is in agreement with the predictions of the molecular field theory.

Absorption hands noted at 1.4, 1.9, 2.4 and 3.2 eV. Morin [164] observed a band at 0.7 eV which

The superimposed weak ferro- magnetism in the range 260-960

K is characteristic of only a-Fe203 and the mechanism

which causes the Morin transition

is the change in the spin- direction from parallel to per- pendicular to the c-axis. The

observed pressure dependence of Tji can be explained if we as- sume a change in sign of the

total magnetic anisotropy energy.

The conductivity behavior appears to be extrinsic or controlled by native defects in the

entire temperature range investi- gated. The reason for the sign reversal of Rh is not exactly

known.

Results indicate that Fe203 in the

form of fine particles exhibit superparamagnetic behavior.

The 0.7 eV peak observed by

Morin seems to be associated

with donor levels. The higher

References

[99-104,

106-113,

133-137].

[143-150].

[102, 116-

120].

[164-166].

66
Iron oxides—Continued

Oxide and description of the study

Data Remarks and inferences References

increases strikingly as Ti is added. Infrared hands noted in the range 285-560 cm-1 and are associated with the metal

oxygen vibrations.

energy hands are ascribed to the

crystal field transitions [166].

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1.7. Cobalt Oxides

In the Co-0 system, CoO and C03O4 are the stable

oxides which have been investigated in detail. Although many workers have reported the preparation

and properties of a sesquioxide, Co2 Og, characterization with respect to stoichiometry and method of

preparation seems to be still in doubt and many a

time, hydrated forms of the oxides are obtained.

Cobalt monoxide, CoO, has a narrow range of

homogeneity [1-4]. It has a cubic rock salt structure

at room temperature and is paramagnetic [5-8]. Below 289 K, it becomes antiferromagnetic followed

by a crystal distortion to tetragonal symmetry [5-7,

9] and the tetragonality increases with decrease in

temperature. Detailed x-ray diffraction studies [10] at low temperatures indicated that a rhombohedral

deformation is superimposed on the tetragonal distortion and at 123 K, the true symmetry is monoclinic. The magnetic structure in the low temperature phase of CoO has been examined by various

workers [11-13]. Although a number of non-collinear

spin-axis structures are possible in CoO [12, 14],

available experimental evidence points to a collinear structure [10, 13, 15] with the moments tipped out of

the {111} planes 7.85° toward the c axis. The transition in CoO has been studied by a variety of techniques in recent years. x

m T data on powder and single crystalline CoO indicate typical

antiferromagnetic behavior below Tn [16-19]. CoO

is semiconducting at all temperatures and discontinuities in p, a, and Rn are exhibited at the N6el

temperature by pure and Li-doped CoO [20-22],

Similarly, anomalies in Cp [23-26], k [25, 27], elastic wave velocity [27, 28-30], elastic constants, internal

friction and Young’s modulus [27-29, 31] and an

[25] have been noted at ^Tn in CoO. The continuous

nature of the transition as indicated by k, Cp and

neutron scattering experiments [9, 23-27] seem to

infer that the antiferromagnetic—^paramagnetic transition in CoO is of the order greater than unity. CoO

was one of the first compounds to be studied by

Mossbauer spectroscopy and many workers have

examined the spectrum as a function of temperature

[32-39]. Below Tn, a six-line pattern indicating magnetic hyperfine splitting is seen; this disappears in

the paramagnetic state. The Neel temperatures obtained from Mossbauer studies agree excellently with

those obtained by other techniques.

Effect of pressure on the magnetization and magnetic ordering temperature of CoO has been in- vestigated by a few workers [40, 41]; the results

indicate that Tn increases with increasing pressure

and decreasing volume. X-ray diffraction [8] and

resistivity [42] studies do not indicate any high pres- sure transitions in CoO up to pressures of ~250

kbar.

According to Ok and Mullen [37, 43], there exist

two forms of cobalt monoxide, CoO (I) and CoO (II).

Both have cubic rock salt structure at room temperature but CoO (II) has a lesser density and the

lattice has ^25 percent vacancies. CoO (II) can

only be prepared at room temperatures and is very

reactive with atmospheric oxygen at ordinary temperatures. The two forms show different Mossbauer

patterns and their Neel temperatures differ. CoO (II)

transforms to CoO (I) above '—'573 K with a trans- formation rate that increases with temperature; bn

the other hand, the transition is not complete on

heating CoO (II) in an argon atmosphere up to ~1270 K for several hours and generally CoO (I, II) is obtained which can be treated as a mixture of

forms I and II in varying amounts. Ok and Mullen

point out that pressures of ~10 kbar are not

sufficient to transform II—*1 at 300 K in an inert

atmosphere. These authors indicate that the usual

chemical methods of preparation of CoO yield al- ways CoO (I, II). It should be mentioned that the Mossbauer spectra of the oxides reported by Ok and Mullen [37] differ nontrivially with the work of others; complete

characterization of CoO (I) and CoO (II) is less

69
unequivocally established [43]. Further detailed work

is urgently needed.

Optical properties of CoO have been examined by

many workers over a wide energy range (0.02-26.0

eV) [44-53]. Studies in the UV region have provided an understanding of the density of states in

CoO [44, 45, 49, 50, 52]; some of the bands observed

in the far infrared region have been ascribed to the

antiferromagnetic resonance [19, 46, 47, 53] but the resonance frequencies are insensitive to temperature

[53].

Small amounts of Li and Ti can be incorporated

in CoO and a great deal of work has been carried

out on these doped materials [3, 20-22, 50, 54, 55].

Generally, doping with Li decreases the resistivity

compared to the pure stoichiometric material. CoO

forms solid solutions with other metallic oxides like

MnO, FeO, NiO and MgO [24, 51, 56, 57]. The

magnetic, electrical, optical and thermal properties

show variations depending on the composition of the

solid solution.

C03O4 is a normal spinel and has the ionic structure

Co2 +[Co3+

] 2 04; it is cubic [58, 59] and paramagnetic

but not ferrimagnetic, although it crystallizes in the

magnetite structure. The Co3+ ions have zero permanent moment as a consequence of the splitting of

the 3d levels by the octahedral cubic field. At ~40 K,

C03O4 undergoes a transition to form an ordered

antiferromagnetic state [58]; the magnetic structure

is due to antiferromagnetic ordering of spins in the

A sites of the spinel structure [60] and each Co2+ ion in an A site is surrounded by four nearest neighbors with oppositely directed spins, xm -T curve fol- lows a Curie-Weiss law with a deviation at high

temperatures in the range 90-500 K and the abnormally low value of xm in C03O4 is ascribed to the

fact that the material is a 2-3 spinel in which Co8+ give no contribution to the magnetic moment [61,

62].

C03O4 is a semiconductor [42, 63, 64] at ordinary

and at high pressures [42]; however, detailed studies on single crystal materials have not been made.

Mossbauer studies on C03O4 have been reported by

a few workers [38, 39, 65]. Above Tn ( = 33±1 K),

two quadrupole split lines corresponding to A and B

site Fe3+ are noted while below Tn the A site Fe8+ spectrum is magnetically split due to the antiferromagnetic Co2+ sublattice. NMR studies [66, 67] of

C03O4 fully confirm the spinel structure and different

oxidation states of cobalt ion at the A and B sites.

Cobalt oxides

Oxide and description of the study

Data Remarks and inferences References

CoO

Crystal structure and

x-ray study.

T = 290 K: Cubic, space group, Fm3m; Z = 4; a =4.260 A. T =

250 K: Tetragonal; a =4.263 A;

c = 4.247 A. T = 123 K: Monoclinic; space group, C2/m; Z = 2; a = 5.18±0.03 A; 6 =3.01 ±0.05

A; c = 3.01±0.07 A; /3 = 125.55±

0.01°. X-ray studies conducted in the range 210-330 K indicate the change in crystal symmetry at 284 ±1 K which is recognized as the Tn.

The low temperature structure (at and below 123 K) is shown to be

strictly monoclinic and not

tetragonal.

[5-7, 10].

Magnetic susceptibility in the range 90-500 K and the

effect of pressure.

Xm~10-3 cgs units; slight aniso- tropy noted. Tn =287.25 K and

Tn increases with pressure; (dTu/dP) =0.6d=0.03 K/kbar.

The calculated values of xm on the

basis of Kanamori’s theory [19] are in agreement with the ex- perimental values.

[16-19, 41].

70
Cobalt oxides—Continued

Oxide and description of the study

Data Remarks and inferences References

Electrical properties. Pure oxide: p~107-10n flcm in the range 200-300 K; p(1300 K)~

0.5 ficm; pd (1300 K)~0.4 cm2 /V

s; Ea changes at Tn and slightly

depends on the stoichiometry.

Li doped CoO (0.05-0.15%):

p~0.5 ficm; a~+500 pV/K;

p D = 0.25 cm2 /V s (1200 K);

pd~6 cm2 /V s (300 K); Rn i9 +ve; p and Rn show discontinuities at Tn. ph is constant in the range 200-1500 K (~0.1 cm2 /V

s); p type behavior,

p decreases with pressure but no

transition occurs up to 250 kbars.

Typical semiconductor behavior in the temperature range investi- gated. Band and polaron theories have been applied to explain the mechanism of conduction in pure and doped CoO and the weight

of experimental evidence seems

to point out the validity of band

model in this oxide.

[1, 3, 20-

22, 42, 68].

Optical reflectance spectra

(range 1-26 eV).

Optical reflectance spectra give

bands at 5.5, 7.5, 12.6 and 17.5 eV. The observed bands at 0.97 and 2.28 eV are attributed to

4 Tlc —*T, ff and 4 Tlff -»«Tlff (P)

respectively [49].

The low energy bands are inter- preted as the exciton transitions whereas the higher energy hands

may be due to the transitions in the conduction band. The data

show the localization of the d

electrons in CoO and the 2p hand

of oxygen seems to lie ~8.5 eV

below the vacuum level.

[44, 49, 50,

[52].

Infrared spectra

(100-300 cm"1 ) Prominent bands are at 146, 215,

222, 233, 243, 250, 260, and 297 cm-1

. Most of the bands are pre- sumed to be the antiferromagnetic resonance modes but temperature variation in the range 2-300 K shows the band to be

insensitive.

The data are in agreement with the

theoretically predicted values

using time-dependent molecular

field theory.

[19, 45-49,

53].

Mossbauer studies of 57 Fe

doped CoO at ordinary and

high pressures.

1 atmospheric pressure: The hyperfine splitting disappears at Tn

(291 K); Isomer shift, screening

parameter and magnetic hyperfine field show discontinuities at Tn. High pressures: Tn increases with pressure; (d \nTn/d \nV) =

— 3. Isomer shift and screening

parameter show small changes on

the application of pressure. Ok

and Mullen [43] report different values of Mossbauer parameters

for CoO(I) and CoO(II).

Quantitative features of the spectra seem to differ for various investi- gations but the general agreement

with the values of Tn is excellent.

[32-40, 43].

Thermal properties. Dilatometric study shows that the

coefficient of expansion passes through a sharp maximum at 292 K for CoO; TEC at 292 K =

3.0Xl0-8/K. Cp and k show

anomalies at Tn; Cp (290 K) =32

cal/mol, K.

The discontinuous nature of the

transition indicates that it is probably of a higher order

transformation.

[24-27, 69].

71
Cobalt oxides—Continued

Oxide and description of the studyRemarks and inferences References

Mechanical properties.

C03O4

Crystal structure and

magnetic properties.

Electrical properties.

Mossbauer studies.

Sound velocities in various crys- tallographic directions change dis- continuously above Tn by 10-

20%. Elastic moduli (X1011 dyn/cm2

) (300 K): cn=26.17±

0.03; C12 = 14.5zt0.2; C44 = 8.32ifc 0.02. During phase transition, Cn

and C12 exhibit jumps. In the

region of the transition. Young’s

modulus increases rapidly from

6.3 to 17X1011 dyn/cm, with in- crease in temperature and the moduli are approximately step

functions of temperature; on the other hand, the internal friction in CoO falls from 91 to 2 (X10-4

) at Tn for a constant

strain amplitude of 10-7

. In- creasing the strain amplitude has

little effect on the Young’s

modulus and internal friction.

T = 300 K: Cubic; a = 8.0835 A;

space group: Fd3m (T>Tn);

F43m (T < Tn). Tn =40 K. xm

(100 K) =1.4X10-2

; xm (300 K) = 0.71X10~2

. The Co2+ moment

below Tn is 3.02 mbSemiconductor; p~103-104 f2cm;

p decreases with P and a minimum is noted at 215 kbars in the p-P plot. Detailed data not

available. Tn is found to be 33 ±1 K; above

Tn, two quadrupole split lines are observed corresponding to A and

B spinel sites whereas below Tn,

the A site Fe3+ spectrum is magnetically split due to the anti- ferromagnetic Co2+ sublattice.

[28, 29, 31].

The crystal structure does not change in going through Tn but

the space group changes. The low

value of xm is believed to be due

to the absence of permanent

moment of Co3+ and the magnetic structure is due to the anti- ferromagnetic ordering of the

spins of Co2+ ions on the A sites of the spinel structure.

[58, 59, 61,

62].

[42].

Tn obtained from Mossbauer work

is less than that reported by

Xm-T measurements. The be- havior of ultrafine particles of C03O4 towards Mossbauer spectra

is interesting and needs further

detailed study.

[38, 39, 65].

72
Cobalt oxides—Continued

Oxide and description of the study

Data Remarks and inferences References

Above Tn, the spectra of the

ultrafine particles (diameter =

100 A) of C03O4 is found to be

identical to that of the bulk

material. Below Tn, the fine particles display superparamagnetic

behavior and the B site spectral

broadening is enhanced. The

anisotropy constant estimated

from the superparamagnetic - antiferromagnetic transition is 4X104 erg/cm3

. Effective 0 d =

185 K.

NMR studies of 59 Co in the paramagnetic state. The Co signal is identified with the Co3+ ions at the B sites and the

line shift is separated into T- dependent and T-independent

terms; the line shapes are studied

and the thermal relaxation time

is estimated to be ~1.5X10-5

s.

The T-independent shift (1.5%) is interpreted as the second order chemical shift due to the mixing

of the low-lying excited state; the T-dependent shift is at- tributed to the hyperfine coupling

between the Co nuclear spin and

the electron spins on the Co2+ ions at the A sites in C03O4 . The

super -exchange mechanisms are examined and it is concluded

that 2 % fractional spin density in the Co3+ e g orbital mainly contributes to the super-exchange.

[66, 67].

Diffuse reflectance spectral studies. Absorption bands at 0.81 and 1.85 eV are noted. These are attrib- uted to Co2+ at A sites (

4A2 —

4 T2 ) and Co3+ at B sites QTig—*

4Ai g ) transitions respectively;

corresponding Dq values are 0.08 and 0.19 eV.

It is concluded that the forbidden

band widths in CoO and C03O4 are almost identical.

[49].

Infrared spectral studies. The prominent bands are at 570

and 665 cm-1

; other bands are at 350, 460 and 635 cm-1

.

The spectra have been discussed in relation to other oxide spinels and

and covalency effects.

[48, 70, 71].

C02O3

Crystal structure and phase

transition Low-spin phase (with t

2ff es

° configuration) is obtained at high pressures. This phase has

corundum structure (a =4.782 A

and c = 12.96 A). The low-spin

phase transforms to the highspin corundum phase (

2ses

2 ) around 670 K (a =4.8882 A,

c = 13.34 A).

The high -spin phase appears to be

the stable phase at atmospheric

pressure.

[72].

73
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74
1.8. Nickel Oxides

Nickel monoxide, NiO, is the only stable oxide in

the Ni-0 system. Earlier literature [1, 2] contains

reports of the existence of some 25 different oxides

of nickel including Ni304, Ni2 03 , and Ni02 and

Kuznetsov [3] has recently suggested that all the

higher oxides and oxide solid solutions are merely

partial hydroxide decomposition products. Detailed

studies made recently [4-6] have shown that the

monoxide is the only material known in the Ni-0

system and slight deviations from stoichiometry are allowed [4-7]. The possibility of synthesizing Ni304 at high pressure has been pointed out [8]. NiO is one of the extensively studied transition

metal oxides. In the paramagnetic state it has the

fee rock salt structure [9-17] and as the temperature

is lowered through T(~523 K), there is a trigonal

distortion in the unit cell [9-14, 16, 17] and the

material becomes antiferromagnetic [11-13, 18]; in

addition, there is an isotropic volume contraction

associated with the magnetic behavior. In the antiferromagnetic state, the spins are ferromagnetically

alligned in sheets parallel to (111) planes and antiparallel to those in adjacent sheets [11-13, 18]. This

magnetic structure is characteristic of all the transition metal monoxides and is explained as due to the

antiferromagnetic superexchange through the p orbitals of the intervening oxygen anions [19]. Below Tn, the unit cell contracts along the [111] axis perpendicular to the ferromagnetic sheets and the crystal

becomes rhombohedral; the distortion from cubic is, however, small and the rhombohedral angle increases

from 90° at Tn to 90.06° at 300 K [17]. x-T data

definitely indicate the antiferromagnetic behavior

of NiO [20]. Magnetic anisotropy and antiferromagnetic domains in NiO have been studied by a few

workers [13, 19, 21-24].

Pure, stoichiometric NiO is green in color and the

reported black and grey colors are due to slight deviations from stoichiometry or the presence of impurities [25-27]. Stoichiometric NiO is a good electrical

insulator (p^l013flcm) but the p decreases drastically by slight changes in the stoichiometry or by

lithium doping at small concentrations. Electrical

properties of pure and doped NiO have been investigated by various workers for the past few years

[4, 5, 28-48]. Discontinuities in p, E0, a and jRh are encountered at Tn', Rh changes sign from positive

to negative values at around Tn, but the exact cause

of this behavior is not understood. Measurements in

the range 10 to 1300 K and at various dopant concentrations (up to 40% [49]) and at high pressures

[50] indicate that the samples of NiO remain semiconducting and that the d electrons are localized.

Even though it is definitely known that the d

electrons in NiO are localized, the detailed mechanism of electrical conduction is not yet understood;

the various properties of this material have been

reviewed in several articles [51-56]. The available

data on transport properties indicate that: (a) between 200 to 1000 K, the predominant conduction

mechanism is the motion of holes in a band with an m* = 6m; (b) the carrier concentration in lightly

doped samples is thermally activated with an acceptor ionization energy of 0.3-0.4 eV; (c) Li acceptors

are always self-compensated by donors; (d) hd

(«0.5 cm2 /V s at 300 K) decreases with rise in T

and ph is proportional to hd in the range 300 to 600

K and (e) intrinsic conduction is observable at T> 1000 K in the more highly compensated samples.

It appears that small polaron hopping by 3d-holes

(with low Ea) contributes little to the total conductivity at ordinary temperatures (100-1000 K)

whereas the major contribution is from the holes in

the oxygen 2p-band of NiO. We may note here that

until 1963 it was generally taken for granted that

charge -transfer in doped NiO involved activated

mobilities. Since then the weight of the evidence seems to have shifted in the other direction; for

example, the studies of Bosman and Crevecoeur [42] are in favor of the band regime. Since most of the

studies have been on doped or nonstoichiometric

NiO, we have to view the various claims with some

caution. Needless to mention, there is continuing

need for careful experimental studies on pure NiO.

Optical absorption measurements have been carried out on pure and doped NiO from far infrared

through x-ray frequencies [44, 57-68] and the results

indicate that the d electrons are localized. There is a peak at 0.24 eV which begins to broaden and decrease in energy above 400 K and disappears above

600 K [58, 63]; it appears to be connected with antiferromagnetism since the band vanishes just above

Tn and moves towards lower energies with decrease

in sublattice magnetization by either increase of

temperature [58] or dilution with MnO or CoO [59].

The main feature in the optical spectrum of NiO is the absorption edge at ~3.8 eV [57, 58, 64, 67].

Infrared studies show restrahlen bands at ^0.05 eV

the antiferromagnetic resonance peak is at 0.04 eV

below Tn [46, 69-71]. The band structure that emerges from the optical absorption, electroreflectance

[72] and photoemission [73] studies is consistent

with the theoretically calculated band structure of

NiO [74-76].

75
Moiibauer studied of 67 Fe doped NiO have been

reported in the literature [77, 78] and accurate value

of Tn obtained. Digcontinuitiei in k, Cp, am. Young’s

modulus and internal friction and TEC have been

noted in pure and Li doped NiO [79-83],

NiO forms solid solutions with other oxides of

rock salt structure [59, 66, 84, 85] and the properties

are slightly modified depending on the nature of the

oxide.

Nickel oxides

Oxide and description of the studyRemarks and inferences References

NiO

Crystal strucrure, x-ray studies.

X-T studies and observation

of the magnetic domains by

optical and neutron

diffraction.

Electrical properties.

T—>0 K: Rhombohedral; a =4.1705

A; a =90.075°. T = 298 K:

o Rhombohedral; a =4.1759 A; a =90.058°. T = 525 K: Cubic;

space group, Fm3m; Z = 4; a =

4.177 A. Slight volume change at Tn (523 K). High pressure studies up to several hundred kilobars do not reveal any phase transition. TEC (/K): 7.93X10“®;

0 d = 9OO K.

x(Tn) =11.7X10“® emu/g. The

antiferromagnetic T (twin) and

S (spin -rotation) walls studied. Crystals with only T or S walls can be produced; these can be

displaced by the application of small mechanical stress or magnetic fields. Magnetic field de- pendence of x also is noticed.

Pure NiO: p (300 K)~1013 ftcm; pap0s “1/4 (po2 . 10-M atm.; 1170-1473 K); p«por1/6 (po«, 10-2-10+3 atm; 1320 K); p«

Po»~

lli (pog^lO-1 atm; 1070-

1373°K). The data are inter- preted in terms of the singly and

doubly ionized cation vacancies; the conduction is of p type. Impurities in the range 100-500

ppm seem to affect the intrinsic

electrical properties. Ea ~0.9 eV

(200-500 K); above Tn, Ea drops

to 0.6 eV and increases to 1.0 eV

at 1000 K; Range 10-100 K, Ea ~

0.004 eV. mh~0.5 cma /V s, at 200

K and hole like behavior; no

striking temperature variation in the range 200-400 K but mh

gradually decreases with rise in T. Above 400 K, rapid decrease

is shown and at ~600 K, mh re- verses sign, a ^0.5 raV/K at ~200 K and above 300 K, has

The rhombohedral distortion is small and the transition at Tn

appears to be second order.

The antiferromagnetic-paramagnetic transition appears to be of order greater than unity and

slight magnetic order has been

noticed above Tn by neutron

diffraction. The anomalous x in (111) at higher magnetic fields is caused by spin rotation.

At very low temperatures, impurity conduction is noticed

whereas in the range 100-1000 K

3d holes contribute to conduction

(only a small part of the total <r ) by hopping of the small polarons.

Major part of the conductivity comes from the holes in the oxy- gen 2p band in pure and slightly

doped NiO. The high temperature Hall data anomalies are sug- gested to be due to the formation or small phase separation of n type Ni30, or small regions of

metallic Ni in the samples.

[9-17].

[11-13,

19-24].

[4, 5, 28-

50].

76
Nickel oxides—Continued

Oxide and description of the study

Data Remarks and inferences References

the same slope as log p as a function of T. Doped NiO: p~l

12cm at 300 K and Ea is much

smaller than in pure samples; Ea varies with dopant concentration

and slightly with T. ph and a

behave in a qualitatively similar manner as the undoped samples.

Application of pressure decreases

p but the sample remains semiconducting up to the highest

pressures measured (~500 kbar).

Neutron irradiation seems to in- crease p and decrease a of pure NiO.

Optical properties

(0.03-26 eV).

The main features are (i) anti* ferromagnetic resonance peak at

'**'0.04 eV observed in NiO below

Tx (and which disappears above

Tx), (ii) restrahlen peaks in the

vicinity of 0.05 eV, (iii) a peak at 0.24 eV which begins to broaden

and decrease in energy above

400 K and disappears at ~600

K, (iv) absorption edge at ~0.38

eV and (iv) several high energy peaks in the range 5-18 eV con- nected with interband transitions. eo»12; e«»5.

Most of the observations are consistent with other body of data

and the band picture that emerged is in reasonable agree- ment with the theoretical predictions. The peak at 0.24 eV

appears to be connected with

antiferromagnetism in NiO.

[44, 57-72].

Mossbauer studies of 67 Fe

doped NiO (4.2-550°K).

An electric held gradient which

appears at the 67 Fe nucleus below

Tx grows as T decreases. The

internal magnetic held is maximum (216 kG) at ^300 K and

decreases to ~55 kG at 4.2 K.

Tx is found to be 524 K and the

direction of spins of Ni*+ is found to be close to <112>.

The order of the transition in NiO

appears to be greater than unity.

[77,-78].

Thermal and mechanical

properties.

Cp shows a X-type anomaly at Tx; maximum value of Cp = 65 cal/ mol. aid and k also show anomalies at Tx. Large increase in Young’s modulus noted at ~Tx

(Maximum value = 8.5 X10-11 dyn/cm2 ). Elastic wave velocity also increases at ~Tx in NiO.

Striking anomalies are noted inspite of the negligible volume change

and only smooth variation of the

lattice parameter at Tx in NiO.

[79-83].

77
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(a) 1, 597 (1970).

[55] Bosman, A. J., and Van Daal, H. J., Adv. Phys. 19, 1 (1970).

[56] Adler, D., and Feinleib, J., Phys. Rev. B2, 3112 (1970).

[57] Doyle, W. P., and Lonergan, G. A., Disc. Faraday Soc. 26, 27 (1958).

[58] Newman, R., and Chrenko, R. M., Phys. Rev. 114, 1507

(i959).

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(1959).

[60] Stephens, D. R., and Drickamer, H. G., J. Chem. Phys.

34, 937 (1961).

[61] Haber, J., and Stone, F. S., Trans. Faraday Soc. 59, 1 (1963); 59, 192 (1963).

[62] Gielisse, P. J., Plendl, J. N., Mansur, L. C., Marshall,

R., Mitra, S. S., Mykolajewycz, R., and Smakula, A.,

J. Appl. Phys. 36, 2446 (1965).

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(Ser. 2) 1, 1418 (1968).

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2295 (1969).

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[66] Jacono, M. L., Sgamellotti, A., and Cimino, A., Z. Phys.

Chem. (N.F.) 70, 179 (1970).

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(1970).

[68] Brown, F. C., Gahwiller, C., and Kunz, A. B., Solid State Commun. 9, 487 (1971).

[69] Kondoh, H., J. Phys. Soc. Japan 15, 1970 (1960).

[70] Sievers, A. J., and Tinkham, M., Phys. Rev. 129, 1566

(1963).

[71] Richards, P. L., J. Appl. Phys. 34, 1237 (1963).

[72] McNatt, J. L., Phys. Rev. Letters 23, 915 (1969).

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Bulgakov, N. N., Sov. Phys.—Solid State (English

Transl.) 11, 506 (1969).

[74] Yamashita, J., J. Phys. Soc. Japan 18, 1010 (1963).

[75] Switendick, A. C., MIT Quart. Progr. Rept. *No. 49,

1963 (unpublished, quoted in [51]). [76] Wilson, T. M., J. Appl. Phys. 40, 1588 (1969); Intern.

J. Quant. Chem. IIIS, 757 (1970).

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(1951).

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42, 209 (1967).

78
1.9. Copper Oxides

Phase equilibrium studies [1-3] on the Cu-0 sys- tem indicate that Cu2 0 and CuO are the only stable

oxide phases in the solid state; both have narrow- ranges of homogeneity. Frondel [4] has reported the

existence of a tetragonal nonstoichiometric oxide of

the formula (Cu2+

i_2 xCu+2z)Oi_*, x= 0.116, but details are not know-n.

Cu2 0: Cuprous oxide, Cu2 0, has a cubic structure

[5] and no phase transformations are known up to the melting point [3, 5]. Because of the extensive

use of Cu2 0 as a barrier layer photoelectric cell,

many workers have successfully grown single crystals

of the oxide [6, 7]. CU2 O has considerable covalency

associated with the Cu-0 bond and exhibits plastic

behavior above 870 K [5, 8]. The material is a semiconductor and the bulk and surface resistivities and

photoconductivity have been examined by several

workers [9-18]. Optical absorption studies [19-22]

indicate several reflection bands with an absorption

edge, corresponding to the optical energy gap, of

~2.02 eV. Cu2 0 exhibits the interesting phenomena

of exciton spectra at low temperatures which appear

as hydrogen-like series of narrow- lines at the absorption edge and towards the long-wavelength re- gion (yellow series). Similar series of lines (called the

green, blue-green and blue series) also appear near

the other interband transitions [23-25] and are due

to the optical excitation of the excitons in Cu2 0.

Detailed investigation of these excitonic spectra contributed to the understanding of the photoconductivity and band structure of Cu2 0 [23, 26].

CuO: Cupric oxide, CuO, has a monoclinic structure

[27, 28] and has considerable covalencv associated

wdth the Cu-0 bond [28]; no phase transformations

are known at ordinary pressures. CuO is a semiconductor [29, 30] and resistivity studies at high

pressure [29] seem to indicate a phase transition;

however, detailed data are lacking.

CuO forms solid solutions with PdO [31, 32] and

f PtO’ [33] having a tetragonal structure and with

MgO, NiO and CoO having cubic rock salt structure

[32]; the lattice constant-composition curve extrapolated to pure CuO suggest the existence of a tetragonal or cubic metastable (crypto-) modification

of CuO [31-33].

Copper oxides

Oxide and description of the study

Data Remarks and inferences References

Cu2 0

Crystal structure and

x-ray studies. Cubic; space group, Pn3m; Z — 2; a =4.268 ±0.001 A. TEC (120-

470 K), /°C): 2X10-0. Melting

point ~1510 K.

Cu20 has an unusual structure de- scribed as two interpenetrating

and identical frame works of copper and oxygen atoms which

are not cross connected by any

primary Cu-0 bonds. Considerable convalency is associated with

the Cu-0 bond.

]5, 8].

Electrical properties. Semiconductor (300-670 K); p (400

K)~2X103 flcm; Ea = 0.26 eV; a (400 K)~+l mV/°C; mh

(300 K)«0.08 cm2 /V s. Cu20

may be nearly an ionic conductor

and extrinsic hole conduction can occur by activated hopping.

The conductivity behavior in Cu20

is complicated by surface and

photoconductivity effects.

[9, 15].

Elastic properties by pulse

echo technique

(4.2-300 K).

Room temperature values of elastic constants (XlO12 dyn/cm2 ): Cn =

1.165 ±0.005; ci 2 = 1.003 ±0.04;

c4 4 = 0.121 ±0.003. Limiting low- temperature values (~4.2 K)

(XlO12 dyn/cm2 ): Cn = 1.211±

0.005; Ci 2 = 1 .054 ±0.04; C44 =

[34].

79
Copper oxides—Continued

Oxide and description of the study

Data Remarks and inferences References

0.109 ±0.003. 0d = 181 K; compossibility (300 K) = 0.946 X

10“6 cm2/kg.

Optical properties and

band structure.

CuO

Reflectivity bands at 2.67, 3.85, 4.64 and 5.00 eV are noted. Energy gap is estimated to be

2.02 eV. €o = 7.60; €» = 6.46. Exciton spectra are noted at the band edge at low temperatures.

The data are discussed in terms of the band structure of CU2O.

[14, 19-

26].

Crystal structure and

x-ray studies. Monoclinic; space group, C2/c;

Z = 4; a = 4.6837 ±0.005 A; b =

3.4226 ±0.0005 A; c = 5.1288

±

0.0006 A; /3= 99.54 ±0.01°. TEC

(293-973 K, /°C) «6.5XlO-«.

Each copper atom is coordinated

to four coplanar oxygen atoms

situated at the corners of an al- most rectangular parallelogram;

the oxygen atom is coordinated to four copper atoms situated at the

corners of a distorted tetra- hedron.

Estimated ionic character 40%; hence the bonding in CuO

appears to be predominantly

covalent.

The structure can be described in the following manner: the build- ing elements are the oxygen co- ordination parallelograms, which

form chains by sharing edges.

Such chains traverse the structure

in the [110] and [110] directions, the two types of chains alternat- ing in the [001] direction. Each

type of chain is stacked in the

[010]

direction with a separation

between the chains of ~2.7 A;

each individual chain in a group

of stacked chain of [110] type is linked to each chain in the two

adjacent groups of [110] type by

corner sharing.

[3, 27, 28].

Electrical properties. p (296 K)~2-4X10

3

Slcm at 20

kbar pressure. High pressure

studies indicate no transition up

to 450 kbar at 300 K or up to 419 K at 17 kbars; however, at 391 K, p shows discontinuous

drop at 187 kbar or 462 kbar at 360 K. a data indicate p type behavior. Detailed data are lacking.

The high pressure data seem to indicate a solid -solid phase

transition in CuO but detailed

studies are yet to be made.

[29, 30].

References

[1] Roberts, H. S., and Smyth, F. H., J. Am. Chem. Soc. 43, 1061 (1921).

[2] Gadalla, A. M. M., and White, J., Trans. Brit. Ceram.

Soc. 63, 39 (1964).

[3] Pranatis, A. L., J. Am. Ceram., Soc. 51, 182 (1968).

[4] Frondel, C., Am. Mineral. 26, 657 (1941).

[5] Suzuki, T., J. Phys. Soc. Japan 15, 2018 (1960).

[6] Toth, R. S., Kilkson, R., and Trivich, D., J. Appl.

Phys. 31, 1117 (1960).

[7] Brower, W. S., Jr. and Parker, H. S., J. Cryst. Growth

8, 227 (1971), and references therein.

[8] Vagnard, G., and Washburn, J., J. Am. Ceram. Soc. 51,

88 (1968).

[9] Toth, R. S., Kilkson, R., and Trivich, D., Phys. Rev.

122, 482 (1961).

[10] O’Keeffe, M., and Moore, W. J., J. Chem. Phys. 35,

1324 (1961); 36, 3009 (1962).

[11] Campbell, R. H., Kass, W. J., and O’Keeffe, M., in Mass Transport in Oxides, Nat. Bur. Stand. (U.S.),

Spec. Publ. 296 (1968), p. 173.

[12] Young, A. P., and Schwartz, C. M., J. Phys. Chem.

Solids 30, 249 (1969).

80
[13] Zielinger, J. P., Tapiero, M., Roubaud, Mme. C., and

Zouaghi, M., Solid State Commun. 8, 1299 (1970).

[14] Kuzel, R., and Weichman, F. L., Canad. J. Phys. 48,

1585 (1970); 48, 2643 (1970).

[15] Kuzel, R., Cann, C. D., Sheinin, S. S., and Weichman,

F. L„ Canad. J. Phys. 48, 2657 (1970).

[16] Fortin, E., Zouaghi, M., and Zielinger, J. P., Phys.

Letters 24A, 180 (1967).

[17] Zouaghi, M., Caret, A., and Eymann, J. 0., Solid State Commun. 7, 311 (1969).

[18] Zouaghi, M., Tapiero, M., Zielinger, J. P., and Burgraf,

R„ Solid State Commun. 8, 1823 (1970).

[19] Balkanski, M., Petroff, Y., and Trivich, D., Solid State Commun. 5, 85 (1967).

[20] Tandon, S. P., and Gupta, J. P., Phys. stat. solid! 37, 43

(1970).

[21] Dahl, J. P., and Switendick, A. C., J. Phys. Chem. Solids

27, 931 (1966).

[22] Smirnov, V. P., Sov. Phys.—Solid State (English Transl.)

7, 2312 (1966); 8, 2020 (1967).

[23] Gross, E. F., Sov. Phys. Uspekhi (English Transl.) 5, 195 (1962).

[24] Deiss, J. L., Daunois, A., and Nikitine, S., Solid State Commun. 7, 1417 (1969); 8, 521 (1970).

[25] Forman, R. A., Brower, W. S., Jr., and Parker, H. S., Phys. Letters 36A, 395 (1971).

[26] Elliot, R. J., Phys. Rev. 108, 1384 (1957); 124, 340

(1961).

[27] Tunell, G., Posnjak, E., and Ksanda, C. J., Z. Krist.

0 90, 120 (1935).

[28] Asbrink, S., and Norrby, L.-J., Acta Cryst, 26B, 8 (1970).

[29] Minomura, S„ and Dnckamer, H. G., J. Appl. Phys. 34,

3043 (1963).

[30] Zuev, K. P., and Dojgintsev, V, D., Izv. Akad. Nauk

SSSR, Neorg. Mater. 4, 1498 (1968).

[31] Schmahl, N. G., and Minzl, E., Z. Physik. Chem. (N.F.)

47, 142 (1965).

[32] Schmahl, N. G., and Eikerling, G. F-, Z. Physik. Chem.

(N.F.) 62, 268 (1968).

[33] Muller, O., and Roy, R., J, Less-Comm. Metals 19,

209 (1969).

[34] Hallberg, J., and Hanson, R. C., Phys. stat. solidi 42,

305 (1970).

1. 10. Zinc Oxides

ZnO is the stable oxide of zinc and it has the

wurtzite (hexagonal) structure at room temperature.

It is piezoelectric. No phase transformations are known in ZnO in the range 4.2 to 890 K. At high

pressures, it transforms to the rock salt structure at room temperature.

Yannerberg [1] has reported a cubic form of zinc

peroxide, Zn02 (a = 4.871 ±0.006 A), but apparently no other studies on this material are available in

the literature.

Because of the technical importance, single crystals of ZnO have been grown by a variety of techniques including hydrothermal, flux growth, traveling solvent, vapor phase reaction and oxidation

methods [2-7]. Lattice parameters and thermal ex- pansion coefficients have been determined by various

workers in the range 4.2 to 890 K [7-13]. ZnO is a semiconductor and the electrical transport properties

and optical characteristics have been examined in

detail in the literature [6, 14-18],

High pressure x-ray diffraction [19, 20] and optical

studies [21] indicated that ZnO transforms from

hexagonal to cubic structure at pressures '--'100-125

kbar at 300 K (Bates et al. give a value 670 K).

Studies of Bates and co-workers [20] indicate that

NH4 C1 acts as a catalyst in the formation of the

high pressure phase and the transformation is sluggish. The cubic phase does not revert back to the

wurtzite structure even after prolonged standing

(several weeks) at room temperature; however, at ^390 K, it reverts within three weeks time.

Zinc oxides

Oxide and description of the study

Data Remarks and inferences References

ZnO

Crystal structure and

x-ray studies. Range, 4.2-890 K; Hexagonal;

space group, P6amc; Z = 2; 6 d =

370 K. T = 297 K: a = 3.2497 dh 0.0009 A; c = 5.206 ±0.0009 A. TEC (300-890 K; °/C)i || a -

6.05X 1O“8 +2.20X 10=

® J+2.29X

10=18 *»; 1 1® = 3.53 X 10 =e H=2.38 X

10=# t+9.24X10=ls

t

1 (t in § C).

Ibach [12] and Reeber [9] find that in the range 4.2=300 K, a

goes through a minimum at 93 K.

Reeber [9] has summarized all the

earlier crystal data on ZnO.

Slight anisotropy in TEC values

is evident.

[9, 11, 12].

81
Zinc oxides—Continued

Oxide and description

of the study

Data Remarks and inferences References

Pressure transition High pressure x-ray study shows Tt to be 673 or 300 K at pressures ~105-110 kbars. Cubic phase; space group, Fm3m; Z = 4; a =

4.28 A. AH = 785 cal/mol; AS =

-2.5 e.u.; dP/dT = 42.5 X10~3 bar/°C; AV = —2.55 cm3 /mol.

High pressure optical absorption

indicates transition at 125 kbars

at 300 K. Absorption edge is at 3.14 eV ( = E0, at 1 bar);

d(AEg )/dP = 0.6-1.9X10-" eV/

bar (wurtzite); = 1.9X1O-0 eV/bar (rock salt).

The 4:4 coordination of the

wurtzite phase of ZnO changes to 6:6 coordination in the rock salt phase; the latter phase is denser.

[19-21].

Electrical properties. Pure undoped crystals have p~l100 ficm but doping increases the

p by a factor '"-•'10. The p of

sintered and unsintered speci- mens behave differently with

respect to temperature. Interstitial metal atoms appear to play a dominant part.

Eg is ~3 eV but donor levels of ~0.3 eV below the conduction

band always create charge carriers thermally and contribute to

to the conductivity.

[14-18].

Hutson [16] obtained the following

transport parameters at 300 K:

a«1.7 mV/K; mh = 180 cm2 /V s; nc =2 X 1016 cm-3

; m* =0.5 m;

trip* = 0.27 m; a?* = 0.85; «o = 8.5; e» = 3.73; 0 d =92O K.

In the range 150-900 K, T de- pendence of mh has been ex- plained by the optical mode

scattering; however, below 150 K,

ionized impurity scattering and

phonon -drag effects play a

significant role.

References

[1] Vannerberg, N. G., Arkiv Kemi 14, 119 (1959).

[2] Weaver, E. A., J. Cryst. Growth 1, 320 (1967).

[3] Kleber, W., and Mlodoch, R., Krist. Tech. 1, 245 (1966);

Chem. Abstr. 69, 46561m (1968).

[4] Hirose, M., and Kubo, I., Japan J. Appl. Phys 8, 402

(i969).

[5] Cleland, J. W., in Mass Transport in Oxides, Nat. Bur.

Stand. (U.S.), Spec. Publ. 296, (1968), p. 195.

[6] Nielsen, K. F., J. Cryst. Growth 3-4, 141 (1968).

[7] Heller, R. B., McGannon, J., and Weber, A. H., J. Appl. Phys. 21, 1283 (1950).

[8] Abrahams, S. C., and Beinstein, J. L., Acta Cryst. 25B,

1233 (1969).

[9] Reeber, R. R., J. Appl. Phys. 41, 5063 (1970).

[10]

Sirdeshmukh, D. Ef., and Deshpande, V. T., Curr. Sci. (India) 36, 630 (1967).

[11] Khan, A. A., Acta Cryst. 24A, 403 (1968).

[12] Ibach, H., Phys. Stat. solidi 33, 257 (1969).

[13] Kirchner, H. P., J. Am. Ceram. Soc. 52, 379 (1969).

[14] Hahn, E. E., J. Appl. Phys. 22, 855 (1951).

[15] Hutson, A. R., Phys. Rev. 108, 222 (1957).

[16] Hutson, A. R., in Semiconductors, Ed. N. B. Hannay

(Reinhold Publ. Corp., New York, 1959), p. 541.

[17] Seitz, M. A., and Whitmore, D. H., J. Phys. Chem.

Solids 29, 1033 (1968).

[18] Hideo, H., and Kasae, K., Nippon Kagaku Zasshi 90, 112 (1969); Chem. Abstr. 70, n821z (1969).

[19] Class, W., Iannucci, A., and Nesor, H., Norelco Reporter

13, 87 (1966); 13, 94 (1966).

[20] Bates, C. H., White, W. B., and Roy, R., Science 137,

993 (1962).

[21] Edwards, A. L., Slykhouse, T. E., and Drickamer, H. G.,

J. Phys. Chem. Solids 11, 140 (1959).

82
II. Oxides of 4d Transition Elements

j

II. 1. Yttrium Oxides

The fully oxidized form, Y2O3, is the only solid

oxide of yttrium [1-3]; substoichiometric yttrium

oxide, Y0i. 5_ x, has, however, been reported by a few workers [4-6]; with x = 0.009, the oxide possesses

intense green color and is paramagnetic in nature,

but the crystal structure is the same as that of

stoichiometric Y2O3. Miller and Daane [5] report a higher lattice parameter and a lower density for

YO1.491 compared to YOi.soo* A monoxide, YO, was

reported [7] as an impurity in Y metal on the basis

of x-ray pattern, but it is likely to have been a sur- face film; the existence of YO as a bulk phase has

not been reported.

Y2O3: Y2 03 has a cubic C-type rare-earth oxide

structure at room temperature and 1 atmospheric

pressure and can be described as a modified fluorite

phase having one anion in four missing to balance

the tri valent cationic charge with a slight readjustment of the positions of the remaining ions. The

resulting unoccupied anion sites may be considered

as forming nonintersecting strings along the four

(111) directions of the cubic cell [1-3].

A hexagonal (H) form of Y2O3 at high temperatures

(~2650 K) has been reported by Foex and Traverse

[8]. This H-form of Y2O3 is closely related to the

A-type rare-earth oxide structure. The C—>H trans- formation has been found to be reversible [8] and the

two modifications are usually observed to coexist over a remarkably extended range of temperatures.

Noguchi et al. [9] report a crystal structure transition (probably C—>H) at ~2550 K. Mehrotra et al. [10] have reported a high temperature transformation

in Y2O3 which is reversible and occurs over the range

670 to 1170 K. The high temperature phase has been

indexed on a hexagonal basis but a monoclinic symmetry could not be ruled out. The C-type structure of Y2O3 is relatively loose

and is of lower density compared to the B- (monoclinic) and A- (hexagonal) type structures (usually

adopted by rare earth sesquioxides). One would,

therefore, expect that application of pressure should

favor the denser structures. Accordingly, Hoekstra

and Gingerich [11], Hoekstra [12] and Prewitt et al. [13] noticed that C—*B conversion occurs in Y2O3 at ^25 kbar and at ~T270 K. The high pressure phase can be quenched and the reversibility of the phase

transformation has been confirmed by heating the

B-form in air at ^1270 K or annealing in air at ambient pressure and ^1170 K for several hours

[11, 12]; from the observed pressure -temperature

data, the enthalpy and entropy changes accompanying the C—>B transformation in Y2O3 have been

estimated [1, 14].

Electrical properties of Y2O3 have been examined

by a few workers [15-22], Y2O3 exhibits p-type semiconduction with considerable ionic contribution to the total conductivity. The ionic conductivity has

been ascribed to the anionic migration [17, 18, 20-

24] but the contribution is smaller the larger the

temperature and is less than 1 percent above 1500 K;

impurities in slight amounts seem to play a dominant

role in ionic conduction [18, 24].

Y2O3 forms solid solutions with oxides of the type

MO (M = Sr [25]), M2 03 (M = B [26], Al [27], Sc

[28, 29], La [30] and Nd [30]) and M02 (M = Ti

[31, 32], Zr [9, 31, 33-36], Hf [37-42], Ce [23],

Th [43, 44] and Re [45, 46]) and their properties

have been examined extensively. Some of the TI1O2-

Y2O3 solid solutions exhibit 100 percent ionic conductivity and are used as solid electrolytes [47, 48].

83
Yttrium oxides

Oxide and description of the study

Data Remarks and inferences References

Y2O3

Crystal structure (including

high temperature high

pressure x-ray studies.

T = 300 K, 1 atm pressure: Cubic

(C-type); space group, Ia3; Z =

16; a = 10.604 A. High temperature phases. T = 1170 K: Hexagonal (A-type?); a =9.80 A;

c = 10.17 A. T = 2603 K: Hexagonal (H-type); a =3.805 A; c =

6.085 A. Cubic-monoclinic (Btype) transition at ~25 kbar

and at ~1270 K. High pressure phase is monoclinic; space

group, C2/m; Z = 6; a = 13.91 A±

0.01 A; b = 3.483 ±0.003 A; c =

8.593±0.008 A; /3 = 100.15±

0.05 A. AH (estimated) ~2 kcal/

mol; AS (estimated) ~1.5 e.u. Melting point = 2959 ±20 K

[49].

The cubic structure of Y2O3 is the usual structure type adopted by

the heavy rare earth oxides. It is remarkable that the structure change on heating is to that of a lower symmetry and is due to the changes in coordination and

increase in density. The pressure

transition does not go through

the corundum phase and the detailed mechanism has been dis- cussed by Prewitt et al. [13].

[1-3, 8, 10-12].

Electrical properties

(300-1500 K).

Semiconductor; p (970 K)~107 J2cm; Ea ~1 eV; a (900 K)~2

mV/°C; p-type to n-type change

at low oxygen partial pressures. Considerable ionic conductivity

below 1000 K; above 1500 K,

pure electronic conduction.

Y2O3 at low temperatures can be

considered to be almost an in- sulator having a very high re- resistivity. Impurities exert a lot of influence in the ionic conductivity of the material; purest

sample has a very low ionic contribution. The p- to n-type

change in Y2O3 may be due to

loss of oxygen and creation of anion vacancies at high

temperatures.

[15-24].

Dielectric and infrared

studies. eo=9.77; e«, = 3.58. Several bands

in the region 300-560 cm-1 are noted; weak bands in the region

800-1070 cm-1 also exist. The

bands are attributed to metaloxygen vibrations.

[50-53].

Heat capacity studies (300-1800 K).

Cp (1300 K) =31.17±19 cal/deg

mol. An anomaly has been noted

at 1330 K with a AH = 310 cal/

mol; AH (estimated, by extra- polation) = 249 ±66 cal/mol;

AS =0.19±0.05 e.u.

The cause of the Cp anomaly at 1330 K is not known.

[54, 55].

References

[1] Brauer, G., Progr. in the Science and Technology of Rare Earths Ed. L. Eyring (Pergamon, Oxford), 1, 152 (1964); 2, 312 (1966); 3, 434 (1968).

[2] Fert, A., Bull. Soc. Franc. Mineral Crist. 85, 267 (1962).

[3] Paton, M. G., and Maslen, E. N., Acta Cryst. 19, 307

(1965).

[4] Haefling, J. A., Schmidt, F. A., and Carlson, O. N.,

J. Less-Comm. Metals 7, 433 (1964).

[5] Miller, A. E., and Daane, A. H., J. Inorg. Nucl. Chem.

27, 1955 (1965).

84
6] Miller, A. E., Thesis, Iowa State Univ. of Sci. and Tech.

1964; Nucl. Sci. Abstr. 20, 880 (1966).

[7] Huber, E. J., Jr., Head, E. L., and Holley, C. E., Jr.,

J. Phys. Chem. 61, 497 (1957).

[8] Foex, M., and Traverse, J.-P., Compt. Rend. (Paris)

261, 2490 (1965).

[9] Noguchi, T., Mizuno, M., and Yamada, T., Bull. Chem.

Soc. Japan 43, 2614 (1970).

[10] Mehrotra, P. N., Chandrashekar, G. V., Rao, C. N. R., and Subbarao, E. C., Trans. Faraday Soc. 62, 3586

(1966).

[11] Hoekstra, H. R., and Gingerich, K. A., Science 146,

1163 (1964).

[12] Hoekstra, H. R., Inorg. Chem. 5, 754 (1966).

[13] Prewitt, C. T., Shannon, R. D., Rogers, D. B., and

Sleight, A. W., Inorg. Chem. 8, 1985 (1969).

[14] Westrum, E. F., Jr., Progr. in the Science and Technology of Rare Earths, Ed. L. Eyring (Pergamon,

Oxford) 3, 459 (1968).

[15] Noddack, W., and Walch, H., Z. Elektrochem. 63, 269

(1959); Z. Physik. Chem. (Leipzig) 211, 194 (1959).

[16] Neuimin, A. D., and Pal’guev, S. F., Chem. Abstr. 59, 941 7g (1963).

[17] Tare, V. B., and Schmalzried, H., Z. Physik. Chem.

(NF) 43, 30 (1964).

[18] Tallan, N. M., and Vest, R. W., J. Am. Ceram. Soc. 49, 401 (1966).

[19] Zyrin, A. V., Dubok, V. A., and Tresvyatskii, S. G., Chem. Abstr. 69, 62562t (1968).

[20] Chandrashekar, G. V., Mehrotra, P. N., Subba Rao,

G. V., Subbarao, E. C., and Rao, C. N. R., Trans. Faraday Soc. 63, 1295 (1967).

[21] Subba Rao, G. V., Ph.D. Thesis, Indian Institute of Technology, Kanpur, India, 1969.

[22] Subba Rao, G. V., Ramdas, S., Mehrotra, P. N., and

Rao, C. N. R., J. Solid State Chem. 2, 377 (1970).

[23] Subba Rao, G. V., Ramdas, S., Tomar, M. S., and

Rao, C. N. R., Ind. J. Chem. 9, 242 (1971) and refer- enccs therein.

[24] Etsell, T. H., and Flengas, S. N., Chem. Rev. 70, 339

(1970).

[25] Lopato, L. M., Yaremenko, Z. A., and Tresvyatskii,

S. G., Dopovidi Akad. Nauk Ukr RSR, 1493 (1965).

[26] Levin, E. M., Phys. Chem. Glasses 7, 90 (1966).

[27] Bondar, I. A., and Toropov, N. A., Izv. Akad. Nauk

SSSR, Ser. Khim. 212 (1966).

[28] Hajek, B., Petru, F., Kalalova, E., and Dolezalova, J., Z. Chem. 6, 268 (1966).

[29] Trezebiatowski, W., and Horyn, R., Bull, Acad. Polon.

Sci., Ser. Sci. Chim. 13, 311 (1965).

[30] Andreeva, A. B., and Keler, E. K., Zh. Prikl. Khim. 39,

489 (1966).

[31] Collongues, M. R., Queyroux, F., Perez, M., Jorba, Y.,

and Gilles, J.-C., Bull. Soc. Chim. France 4, 1141

(1965).

[32] Ault, J. D., and Welch, A. J. E., Acta Cryst. 20, 410

(1966).

[33] Rouanet, A., Compt. Rend. (Paris) 267C, 1581 (1968).

[34] Takahashi, T., and Suzuki, Y., Deuxiemes Jour. Int. des

piles a Combustible, Bruxelles 378 (1967).

[35] Strickler, D. W., and Carlson, W. G., J. Am. Ceram.

Soc. 48, 286 (1965).

[36] Duwez, P., Odell, F., and Brown, F. N., J. Electrochem.

Soc. 98, 356 (1951).

[37] Volchenkova, Z. S., and Pal’guev, S. F., Trans. Inst. Elektrokhim. Akad. Nauk SSSR, Ural. Flilial 5, 133

(1964).

[38] Robert, G., Deportes, C., and Besson, J., Deuxiemes

Jour. Int. des piles a Combustible, Bruxelles 368

(1967).

[39] Komissarova, L. N., Wang, C. S., and Spitsyn, V. I., Izv. Akad. Nauk SSSR Ser. Khim. 1, 3 (1965).

[40] Caillet, M., Deportes, C., Robert, G., and Vitter, G., Rev. int. Hautes Temp. Refract. 4, 269 (1967).

[41] Besson, J., Deportes, C., and Robert, G., Compt. Rend.

(Paris) 262, 527 (1966).

[42] Duclot, M., Vicat, J., and Deportes, C., J. Solid State Chem. 2, 236 (1970).

[43] Wimmer, J. M., Bidwell, L. R., and Tallan, N. M.,

Nucl. Sci. Abstr. 19, 42896 (1965).

[44] Subbarao, E. C., Sutter, P. H., and Hrizo, J., J. Am.

Ceram. Soc. 48, 443 (1965).

[45] Roy, R., McCarthy, G. J., Muller, O., and White, W. B., Nucl. Sci. Abstr. 20, 36784 (1966).

[46] Roy, R., Kachi, S., McCarthy, G. J., Muller, O., and

White, W. B., Nucl. Sci. Abstr. 20, 13260 (1966).

[47] Wimmer, J. M., Bidwell, L. R., and Tallan, N. M.,

J. Am. Ceram. Soc. 50, 198 (1967).

[48] Patterson, J. W., Bogren, E. C., and Rapp, R. A.,

J. Electrochem. Soc. 114, 752 (1967).

[49] Noguchi, T., and Kozuka, T., Solar Energy 10, 203

(1966).

[50] McDevitt, N. T., and Davidson, A. D., J. Opt. Soc. Amer. 56, 636 (1966).

[51] Goldsmith, J. A., and Ross, S. D., Spectrochim. Acta

23A, 1909 (1967).

[52] Subba Rao, G. V., Rao, C. N. R., and Ferraro, J. R., Appl. Spec. 24, 436 (1970).

[53] Nigara, I., Ishigame, M., and Sakurai, T., J. Phys. Soc. Japan 30, 453 (1971).

[54] Pankratz, L. B., King, E. G., and Kelley, K. K., U.S. Bur. Mines, Rept. Invest. No. 6033, 1962.

[55] Holley, C. E., Jr., Huber, E. J., Jr. and Baker, F. B., Progr. in the Science and Technology of Rare Earths

Ed. L. Evring (Pergamon, Oxford), 3, 343 (1968).

II. 2. Zirconium Oxides

The solubility of oxygen in a-zirconium (hexagonal, a = 3.231 A, c= 5.148 A) is appreciable and with

increasing oxygen content, the c-axis of the metal

elongates, the change being marked around ZrOo.3 [1]; the a axis passes through a maximum around

ZrOo.25- The Zr30 phase (a = 5.629 A, c = 5.197 A)

is completely ordered, the compositions richer than

Zr30 being analogous to Zr30 [1]. Electrical and

mechanical properties of the solid solutions of oxygen

in Zr have been measured and Zr60 phase with a decomposition temperature of 1200 K has been identified in addition to the more stable Zr30 [2]; both

Zr60 and Zr30 are reported to be semiconductors.

Both the Zr60 and Zr30 phases have been examined

by electron microprobe and electron microscopic

techniques and the ordered nature of the latter phase

confirmed [3]. The structures of the solid solutions in

the composition range 11-28.5 atomic percent oxygen have been examined by x-ray, neutron and elec- tron diffraction techniques [1, 4, 5]. The compositions

in the range 15-25 atomic percent oxygen can be described in terms of a one-dimensional long-period

superstructure with a periodicity which increases

with oxygen content [4]. The superstructure can be

expressed in terms of the stacking sequence of octa- hedral interstitial sites in the lattice of a-Zr with

ABCABC ... in ZrO* (0.25 <x< 0.33) and ABAB . .

in ZrO„ (0.33 <j<0.40) respectively; Zr30 exhibits an intermediate structure with ABC BC AC AB

stacking with 9 layers period [5]. An examination of chemically thinned single crystals of zirconium by electron diffraction during an

isobaric treatment with O2 at 5 X 10-4 torr at 670 K

85
revealed superstructure patterns due to Zr20 [6]. Zr2 0 crystallizes in the hexagonal system with a =

16.16 A and c= 5.148 A. Six oxygens form a ring and

a total of four such rings form the supercell.

The most stable oxide of zirconium is the important ceramic dioxide, Zr02 which normally crystallizes in the monoclinic structure (P2i/c) possessing

4 Zr02 units in the unit cell [7-9]. The monoclinic

structure reversibly transforms to a tetragonal struc- ture around 1420 K and to a fluorite type cubic

form at very high temperatures around 2500 K

[10-14]. Zr02 melts at 2995±20 K [15].

Oxygen equilibria over nonstoichiometric Zr02 (with oxygen deficiency) have been examined by

Aronson and others [16] in the 1100 to 1400 K range

at 10-12 -10~20 atm pressures. The compositions of

the oxide in the 1670 to 2170 K range at 1-10-6 atm

have been investigated exhaustively by Carniglia et

al. [17] who find the ZrOo-^-Zr boundary to be x =

0.014 at 2070 K and 3.5 XlO-6 atm of oxygen. A

black form of Zr02 has been prepared by heating

hydrated Zr02 in argon or vacuum; this has a tetra- gonal structure [18, 19]. Amorphous hydrated Zr02 prepared by precipitation methods has been shown

to possess short range order [19].

The phase transitions of Zr02 have been investigated extensively by several workers. Although there were some inconsistencies and uncertainties in the

earlier literature [20] recent studies have clearly

established the transition temperatures [21] and

there appears to be no doubt regarding the monoclinic-tetragonal and tetragonal-cubic transitions

mentioned earlier [10-14]. The monoclinic-tetragonal

transition shows appreciable hysteresis similar to martensitic phase transitions [22] and is predominantly athermal [22, 23]; the formation of the cubic

form is reversible [22]. Anomalous intensity changes

in the x-ray pattern are noticed prior to the monoclinic-tetragonal transition (in the 1200 to 1370 K

range) and hybrid crystal formation has been noticed [24]; orientation relationship between the monoclinic and tetragonal phases has been established.

The tetragonal-cubic transition around 2550 K (observed by x-ray studies in vacuum), is undoubtedly

accompanied by partial reduction of Zr02 and the

phase boundary between the cubic and tetragonal

forms is affected by the stoichiometry or oxygen

deficiency [17, 25]. The lattice dimensions of monoclinic and tetragonal Zr02 decrease with increase in

oxygen deficiency, but the phase does not appear to be significantly affected.

Pressure induced monoclinic-tetragonal transition

of Zr02 has been reported [26-28]. Polymorphic behaviour of evaporated films of Zr02 has been investigated by electron microscopy and diffraction

[29]. As mentioned earlier, the mechanism of the

monoclinic-tetragonal transition of Zr02 has been

examined by several workers [10-14, 21-24]. The

kinetics of this transition with particular reference to

its martensitic nature have been studied by Suk-j

harevskii et al. [30]. The mechanism of the transition

has also been studied with single crystals [31].

Preparation of finely-divided samples of the metastable phases of Zr02 has been described by several

workers [10, 11, 21, 32-34]. Zr02 prepared by decomposition of alkoxides is cubic at room temperature and transforms to the metastable tetragonal

phase around 570 K which then transforms to the

monoclinic phase in the range 580 to 670 K [32].

Occurrence of the metastable tetragonal phase in

high surface area. (small crystallite size) samples of

Zr02 has been examined by Garvie [33] and the

kinetics of transformation of the metastable phase

to the monoclinic phase have been investigated by

Dow Whitney [34] and Surkarevskii et al. [30]. The

low temperature metastable cubic and tetragonal

phases possess lattice parameters identical to those

of the corresponding high temperature phases [11,

21]. Thin evaporated films of Zr02 also exhibit the

cubic structure at room temperature as shown by

electron diffraction [29]; the cubic form transforms

to the tetragonal and monoclinic forms on heating.

Electrical conductivity measurements of Vest and

co-workers [35, 36] have clearly shown that Zr02 is a semiconductor, the sign of the majority carriers depending on the oxygen partial pressure and temperature; some evidence of ionic conduction in tetragonal

Zr02 has been presented. Electrical conductivity of

the tetragonal phase has been measured by other

workers [37, 38] as well. Electrical conductivity

measurements have been employed to study the

monoclinic-tetragonal transformation under pressure

[26].

Phase equilibria, phase transitions, defect chem- ! istry, electrical properties and other aspects of solid

solutions of Zr02 with a variety of oxides have been

reported in the literature [39]. Solid solutions like

Zr02 -CaO [39, 40] are well-known solid electrolytes.

Typical of the solid solutions investigated are Zr02

- MgO [41, 42], Zr02 -SrO [43], Zr02 -MnO [44], Zr02 - I R2 03 where R = Sc, Y, or a rare earth [10, 39, 45-

54], Zr02 -Ce02 [55], Zr02 -Ge02 [56], Zr02 -Ti02 [57-

59] and Zr02 -Hf02 [60-62]. The monoclinic tetragonal

transition of Zr02 is markedly affected in such solid

solutions; as can be expected the cubic phase is I stabilized in many of the solid solutions.

86
Zirconium oxides

Oxide and description of the study

Data Remarks and inferences References

rO

Crystal structures of the The stable room temperature mono- The monoclinic structure can be [7-9]. monoclinic, tetragonal and clinic phase has the P2i/C space considered to be a distorted cubic phases of Zr02 by group with 4 Z1O2 in unit cell: fluorite structure. Detailed

x-ray diffraction. a = 5.1454 ±0.0005 A; b = atomic positions in the mono5.2075 ±0.0005 A; c = 5.3107±

0.0005 A and /3 =99°14'±0°05\

The Zr-0 distances vary from

2.05 to 2.28 A (for the near

neighbors) and there is a strong

tendency to twin in the (100) direction and polymorphism can be understood in terms of the

(fluorite type) layers parallel to

(100).

The tetragonal phase (D4®-P42/

clinic phase as well as structural rearrangements in polymorphic

transitions to tetragonal and

cubic phases have been dis- cussed. The structures of ZrC>2 and HfC>2 have been compared.

The lattice dimension of the cubic [11-13].

Phase transitions by

nmc) has dimensions: a = 3.64 A

and c = 5.27 A at 1520 K with 2 molecules in the cell. The

fluorite type cubic phase has

o*5.26 A at the transition temperature.

The phase transition temperatures

phase will be a function of stoi- chiometry which in turn will depend on the temperature and

oxygen partial pressure.

The transitions of HfC>2 and ZrC>2 [11-14, x-ray diffraction. reported by different workers are similar. 21-25]. vary to some extent. The

ranges are: monoclinic—tetragonal, 1320-1490 K; tetragonal—*

cubic, ~2550-2650 K.

There is considerable thermal The transition is martensitic in [14, 22-24].

hysteresis in the monoclinictetragonal transition just like in the transition of HfC>2 . For

example, the tetragonal—monoclinic transition (on cooling) has

been reported in the range 1320-1170 K.

Coexistence of monoclinic and

character.

The thermal hysteresis in the [24].

Stoichiometry and its

tetragonal phases as well as hybrid crystal formation have been

observed in the range 1370-1490

K. The orientation relationship

between the two phases is found

in terms of the parallelism of the

(100) and (010) planes of the

tetragonal phase; the b axis of the monoclinic phase is similar to the

c axis of the tetragonal phase.

The tetragonal-cubic transition

monoclinic-tetragonal transition

arises from the differences be- tween the forward and reverse

transitions (i.e., heating versus cooling) The pretransformation

region is between 1200 and

1370 K.

At ~2670 K and 10-8 torr of oxy- [25]. relation to the tetragonal- temperature found by x-ray gen, the composition will be

cubic transition. diffraction (in vacuum) varies ZrOi.96o*o. oo 2 » At ~2670 K and

widely due to the changes in the stoichiometry of Z1O2 . For a 2 X 10-3 torr of oxygen, the composition will be ZrOi.gge.

87
Zirconium oxides—Continued

Oxide and description of the study

Data Remarks and inferences References

composition of ZrOi,97 , the

tetragonal-cubic transformation

temperature corresponds to cross- ing of the (tetragonal Z1O2 +

cubic ZrOj)-(cubic ZrOj) phase

boundary.

The stoichiometry of ZrOi as a function of temperature and

oxygen partial pressure has been

investigated and the cubic- tetragonal transition temperature

is significantly lowered in samples containing a-Zr and Zr02. ZrC>2 is oxygen-deficient over the

entire temperature range of 1370-

2170 K at /?oj<1 atm. In the 1670-2170 K range, oxygen deficiency is given by log x »

- 0.890[ (0.400 Xl0-«)/T]-

[ (logP)/6] where x is in ZrC> 2 -*, T is K and p is in atmospheres

of oxygen.

ZrC>2 is nearly stoichiometric at 1270 K and 1 atm poa

. The

Zr02_*-Zr phase boundary is at x =0.018 at pot »3.5X10“# atm

and T«2070 K.

[16, 17, 25]

Lattice thermal expansion is not

affected by the oxygen deficiency

although the dimensions of the monoclinic and tetragonal phases

decrease; the monoclinic-tetra- gonal transition is not significantly affected. No change in Young’s modulus and strength of ZrC>2 is noticed due to oxygen

defciency in sintered samples.

Metallographic observations. The monoclinic-tetragonal phase

transition has been examined on

a vacuum hot -stage microscope

(up to 1570 K). In the 1320-

1420 K range, the platelet substructure within ZrC>2 grains

forms rapidly. Photomicrographs

and motion pictures were taken

through the transformation.

Based on these observations the transformation has been shown

to be diffusionless and

athermal as in martensite

transformations.

[23].

High pressure studies.

Kinetics of monoclinic

tetragonal transformation.

Monoclinic Z1O2 reversibly trans- forms to the tetragonal form at 298 K on application of pressures

greater than 37 kbar. The tetra- gonal phase cannot be retained at ambient conditions. Martensite (monoclinic^tetragonal)

transformations in ZrC>2 show no

principle difference between

athermal and isothermal kinetics. At a certain supercooling, the transformation velocity increases sharply and the transformation

becomes athermal. In the tetra- gonal-monoclinic transition, a change in the kinetics of dif- fusionless transition is observed. This phenomenon has also been

studied with the metastahle tetra- gonal phase prepared by decomposition of the oxychloride.

dT/dP of the transition has been

reported to be — 0.01°/bar in

[27] and -0.003°/bar in [26].

Transformation velocity decreases with decreasing temperature in isothermal runs; the energy of activation is 150 kcal/mol.

[26-28].

[30].

88
Zirconium oxides—Continued

Oxide and description of the study

Data Remarks and inferences References

Thermal expansion. Thermal expansion of the lattice parameters up to 1370 K have

been reported. The monoclinictetragonal transition occurs around 1370° and the reverse

transition occurs at much lower

temperatures (980-1250 K).

There is a critical Zr-0 distance

above which the monoclinic

structure is no longer stable.

[14, 63].

Thermal expansion data for the

tetragonal phase are: a =3.5882 +

4.50 X10-5

1 within O.OOI3 A;

c = 5.1882 +7.57X10-5

t within

0.0022 A where t is in degrees

centigrade between 1150 and

1700 °C.

The data are probably applicable

up to 2270 K. At 1420 K, a =

3.639 A and c = 5.275 A and at 1970 K, a=3.678 A and c =

5.339 A.

[64].

Thermal expansion data for monoclinic ZrC>2 have been measured

in detail along the three axes and

found to be highly anisotropic.

The monoclinic -tetragonal transi- tion is accompanied by a contraction of 3.42%.

[65].

Heat of transition. The monoclinic -tetragonal transi- tion is associated with an

enthalpy change of 1420 cal/mol.

[66].

Superplasticity Enhanced plasticity during the monoclinic -tetragonal transition has been studied.

[67].

Infrared spectra. Cubic phase gives one band at 490 cm-1

. Monoclinic phase shows

several bands as expected.

[52, 68].

Neutron induced The monoclinic -cubic transforma- Impurities stabilize the high [69]. transformation. tion by neutrons depends on the method of preparation.

symmetry phase.

Metastable cubic and Finely divided samples of Z1O2 Cubic phase lattice parameter from [10, 11, 21,

tetragonal phases and their prepared by the decomposition of electron diffraction is ~ 5.07 A. 29, 32, 33,

phase transitions. alkoxides, oxychloride and other Thin evaporated films of Z1O2 52, 70]. salts give rise to the metastable

cubic and tetragonal phases. The

metastable cubic form prepared

from alkoxides transforms to the metastable tetragonal form

(~570 K), and then to the monoclinic form (580-670 K). The

temperatures of transformation seem to vary with the method of sample preparation and some

workers have reported cubic- monoclinic transitions. The lat- tice parameters of these low temperature cubic and tetragonal

forms agree with those of the high temperature phases. The

metastable tetragonal phase pre-

(as shown by electron diffrac- tion) also exist in cubic form and

the transformation to tetragonal

and monoclinic form is affected by annealing, oxygen pressure

etc. The cubic lattice parameters

are around 5 A, the actual value

varying with annealing.

89
Zirconium oxides—Continued

Oxide and description of the studyData Remarks and inferences References

pared by precipitation of alkaline aqueous solution or low-temperature calcination of zirconium

nitrate has been found to be

stable at room temperature due to the small crystallite size (or large surface area).

The kinetics and mechanism of No induction period was seen in the [34].

Electrical properties.

transformation of the metastable

tetragonal phase to the monoclinic phase have been studied

and the kinetic data have been

interpreted in terms of Avrami’s

equation and order -disorder theory.

The cubic-tetragonal transformation

in thin films is monotropic while

the tetragonal-monoclinic trans- formation exhibits the nucleation

and growth mechanism.

Electrical properties have been rekinetics.

There is considerable disagreement

[29].

[35, 36, 39].|

ported by several workers. Monoclinic Zr02 is an amphoteric

semiconductor at 1270 K and at high oxygen pressures (1-10-6 atm), the predominant defects are

ionized Zr vacancies which give

rise to low mobility holes. The

defect structure of tetragonal

ZrC>2 has been examined by

measuring the temperature and

oxygen pressure dependence of conductivity and the electronic

regarding the ionic transport

numbers in monoclinic and tetra- gonal Z1O2 and the available data

have been nicely reviewed by

Etsell and Flengers [39]. There is no doubt that ZrC>2 is a mixed

conductor. The conductivity

minima have been reported at IO

-9 and 10~16 atm at 1270 K

for the monoclinic phase. For the

tetragonal phase the minima are reported at 10-4

, 10-6 and 10-7 transference number. At high

temperatures and low po2 , ZrC>2 is an n-type semiconductor (due

to ionized oxygen vacancies).

Tetragonal ZrC>2 is a mixed elec- tronic and ionic conductor. The

conductivity change accompanying the monoclinic-tetragonal

transition is isothermal and the

rate of change depends on

thermal and sample history.

atm at 1570 K. The minima are

at higher oxygen pressures at higher temperatures.

90
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Invest No. 6619, 19 (1965), CA, 63, 5027A.

[32]

Mazdiyasni, K. S., Lynch, C. T., and Smith, J. S.,

J. Am. Ceram. Soc. 48, 372 (1965); ibid. 49, 286 (1966).

[33]

Garvie, R. C., J. Phys. Chem. 69, 1238 (1965).

[34]

Dow-Whitney, E., Trans. Faraday Soc. 61, 1991 (1965).

[35]

Vest, R. W., and Tallan, N. M., and Tripp, W. C.,

J. Am. Ceram. Soc. 47, 635 (1964).

[36] Vest, R. W., and Tallan, N. M., J. Am. Ceram. Soc. 48, 472 (1965).

[37] Anthony, A. M., Guillot, A., and Nicolau, P., Compt.

Rend., Ser. A 262B, 896 (1966).

[38] McClaine, L. A., and Coppel, C. P., J. Electrochem. Soc. 113, 80 (1966).

[39] Etsell, T. H., and Flengers, S. N., Chem. Revs. 70, 339

(1970).

[40] Subbarao, E. C., in Non-Stoichiometric Compounds, Ed.

L. Mandelcorn (Academic Press, New York, 1964).

[41] Grain, C. F., J. Am. Ceram. Soc. 50, 288 (1967).

[42] Viechnicki, D., and Stubican, U.S., J. Am. Chem. Soc.

48, 292 (1965).

[43] Noguchi, T., Okubo, T., and Yonemochi, O., J. Am.

Ceram. Soc. 52, 178 (1969).

[44] Bayer, G., J. Am. Ceram. Soc. 53, 294 (1970).

[45] Thornber, M. R., Bevan, D. J. M., and Graham, J., Acta Cryst. 24B, 1183 (1968).

[46] Spiridonov, F. M., Popova, L. N., and Popilskii, R. Y.,

J. Solid State Chem. 2, 430 (1970).

[47] Isupova, E. N., Glushkova, V. B., and Keler, E. V.,

Izv. Akad. Nauk SSSR Neorg Mater. 4, 399 (1968).

[48] Collongues, R., Queyroux, F., Jorba, M. P. Y., and

Gilles, J. C., Bull. Soc. Chim. France 1141 (1965).

[49] Rouanet, A., Compt. Rend. 267C, 1581 (1968).

[50j Takahashi, T., and Suzuki, Y., Deuxiemes Jour. Int. des Piles a Combustible, Bruxelles, 378 (1967).

[51] Mazdiyasni, K. S., Lynch, C. T., and Smith, J. S.,

J. Am. Ceram. Soc. 50, 532 (1967).

[52] Glushkova, V. B., and Koehler, E. K., Mat. Res. Bull.

2, 503 (1967); see also, Sci. Ceram. 4, 233 (1967),

CA, 70, 60405r.

[53] Davtyan, I. A., Glushkova, V. B., and Keler, E. K.,

Izv. Akad Nauk SSSR Neorg Mater. 2, 890 (1966).

[54] Strickler, D. W., and Carlson, W. G., J. Am. Ceram.

Soc. 48, 286 (1965).

[55] Casselton, R. E. W., Phys. stat. solidi (a) 1, 787 (1970).

[56] Lefevre, J., and Collongues, R., Bull. Soc. Chim. France

1959 (1966).

[57] Ksendzov, Y. M., and Prokhvatilov, V. G., Zhur Fiz. Khim. 31, 321 (1957).

[58] Keler, E. K., and Andreeva, A. B., Ognewpory 25, 320

(1960); CA, 51, 15237d; CA, 48, 6798h.

[59] Shakhtin, D. M., Levintovich, E. V., Pivovar, T. L., and Eliseeva, G. G., Izv. Akad. Nauk SSSR Neorg. Mater. 4, 1603 (1968).

[60] Gavrish, A. M., Susharevskii, B. Y., Krivoruchko, P. P., and Zoz, E. I., Izv. Akad. Nauk SSSR Neorg. Mater.

5, 547 (1969).

[61] Ruh, R., Garret, H. J., Domagala, R. F., and Tallan, N. M., J. Am. Ceram. Soc. 51, 23 (1968).

[62] Stansfield, O. M., J. Am. Ceram. Soc. 48, 436 (1965).

[63] Grain, C. F., and Campbell, W. J., U.S. Bur. Mines

Rept. Invest. 5982 (1962).

[64] Lang, S. M., Sci. Tech. Aerospace Rept. 2, 2065 (1964).

[65] Filatov, S. K., and Frank-Kamenetskii, V. A., Soviet

Phys. -Cryst. 14, 696 (1970).

[66] Coughlin, J. P., and King, E. G., J. Am. Chem. Soc. 72,

2262 (1950).

[67] Hart, J. L., and Chakladar, A. C. D., Mat. Res. Bull.

2, 521 (1967).

[68] White, W. B„ Mat. Res. Bull. 2, 381 (1967).

[69] Adam, J., and Cox, B., J. Nucl. Energy 17, 435 (1963).

[70] Mazdiyasni, K. S., and Brown, L. M., J. Am. Ceram.

Soc. 53, 43 (1970).
II. 3. Niobium Oxides

Phase relations in the Nb-0 system have been

examined and reviewed over the past vears by many

workers [1-12]. In addition to the well-established

monoxide, dioxide and pentoxide, several suboxides

as well as a homologous series of oxides of the

formula, Nb3n+i08n_2 (n = 5 to 8), are known. Niobium sesquioxide, Nb2 03, is rot known, but the data

of Nakayama et al. [13] seem to indicate the formation of this material as a hexagonal-tvpe oxide.

Suboxides: The suboxides NbOx (Nb60), NbOy (Nb40) and NbO z (NboO) form only by the oxidation of niobium metal or by ageing niobium which

is supersaturated with oxygen in definite ranges of

temperature fnd oxygen pressure. NbC)* and Nb0 2 are tetragonal while NbOy is orthorhombic [11].

Detailed physical properties of these suboxides are not known.

NbO: Niobium monoxide is easily made by the

oxidation of the metal and has a narrow range of

homogeneity (0.982^x^1.008 in NbOx) [1, 11, 14-

17]. It has a cubic rock salt structure with ~25

percent vacancies in the lattice which are ordered

[1]. No phase transformations are known in this

material; high temperature, high-pressure studies

[18, 19] indicate no change in the vacancy concentration up to 77 kbar and at 1920 K. It appears that

the nature of bonding in the ordered NbO structure

is significantly different from that in the disordered

vacancy structures of TiO and VO. Detailed re- sistivity studies by many workers [14-16, 20-22]

indicate the metallic behavior of NbO in the range

2 to 900 K. The material becomes superconducting

at ~1.5 K [20]. Solid solution formation and the

solubility of other monoxides in NbO have been

examined by Gel’d et al. [23].

Nb02 : Niobium dioxide has a distorted rutile struc- ture with a narrow homogeneity range at room

temperature [11, 12, 15, 24-29] and exhibits a first order transition at ~1070 K transforming to a

(perfect) rutile phase [26-29]. Discontinuities in x and p have been noted at Tt , but the material is paramagnetic with no magnetic ordering in the range

300 to 1200 K [30]. Reports of the electrical properties are conflicting: Janninck and Whitmore [31]

and Roberson and Rapp [21] reported an apparent

semiconductor-to-metal transition at T<, but Sakata

[27] finds semiconductor behavior throughout the

range 300 to 1270 K with a ten-fold jump in a at Tt . Metal-metal bonding in the low temperature phase

of Nb02 has been established [26, 27] and it may be

argued [32, 33] that this oxide should exhibit a semiconductor-metal transition anologous to V02 . The available data are all on polycrystalline samples

and may not be reliable. Rao et al. [28] have studied

the transitions in Nb02 and its solid solutions with

V02 , Nbi_x Fx02 (0<x^0.05), and clearly established that the transitions are from semiconducting

to metallic state; vibration mode softening and c-axis

Nb-Nb pairing have been shown to be important

factors in the mechanism of the transitions. Detailed studies on single crystal samples are, however,

necessary to draw definitive conclusions.

Solid solutions of Nb02 with Ti02 [30, 34, 35],

V02 [28, 29, 34, 36, 37], Cr02 [34], and Mo02 [34,

37] have been examined in the literature to elucidate

the metal-metal bonding and the nature of the

transition. It may be noted that in some of these

solid solutions, there would be change in cation

valencies.

Homologous series, Nb3n+i08n_2 (5^n^8): Studies by Norin and Magneli [38] indicate that in the

composition range between Nb02 and Nb2 05 there

exist several nonstoichiometric oxides shown by

Gatehouse and Wadsley [39] (and in many later

papers) to belong to the general homologous series

of the formula Nb3n+i08ra_2 (rc = 9 would be Nb2Os if

it formed, but H-Nb2 05 actually has a slightly differ- ent structure). In addition, oxides Nbi2 02 g (Nb02 . 4 n)

and Nb470ii6 (Nb02 464 ) have also been reported

[40, 41]. All these oxides have structures similar to but different from, the basic high temperature

(monoclinic) structure of H-Nb2 05 [42]. Physical i properties of these systems have not been investigated in detail.

Nb2 05

: Niobium pentoxide, Nb205 , has a narrow

homogeneity range [1, 43] and the stable phase is the high-temperature (H) form. The polymorphic

behavior of this oxide has been examined by various

workers over the past many years; however, a detailed picture is far from clear at the present stage

and several metastable phases of Nb205 are being

reported from time to time and several reports are contradictory in nature.

There are at present eight crystalline modifications of Nb205 known and many of them can be

grown by vapor trar sport methods. Schafer et al.

|| [44] have reviewed various preparation methods of

the Nb2 Os polymorphs while Wadsley and Andersson

[42] elucidated the structural relationships involved

and pointed out that many new modifications are

feasible and may be discovered in future.

92
The low-temperature form, TT-Nb2 05 , first re- ported by Frevel and Rinn [45]; is hexagonal and

can be best prepared by the oxidation of Nb02 in

air at 590 K for 24 h [44]. Holtzberg et al. [46] and

Shafer and Roy [47] obtained this (5) phase by carefully heating the niobic acid hydroxide gel or amorphous Nb2 05 at 770 to 870 K or at low temperatures

for prolonged periods of time. This metastable form

has also been realized by vapor transport reactions

with Nb2 05 [48] and by rapid-quenching of the melt

[49]

. The TT form transforms continuously into

T-Nb2 05 on heating at 870 to 1073 K [50]; annealing

at 1670 K results in the high temperature (H)

polymorph [49]. The orthorhombic T-Nb2 05 has been obtained by

Brauer [1], Holtzberg et al. [46] (y-form) and Shafer

and Roy [47] (Ill-form), by heating the hydroxide

gels or amorphous Nb2 05 at 870 to 1073 K. It is also produced by the oxidation of Nb or Nb alloys

below 1270 K [51] or Nb02 at 870 to 1070 K in air

[50]

. Under hydrothermal conditions (170 atm, 600-

650 K), amorphous niobic acid also changes to TNb205 [47]. Single crystals of the T-form can be

obtained either by vapor transport or quenching the

supercooled melt [52].

The T-form changes to the B-form on heating by

an exothermic irreversible process and DTA scan

indicates a Tt of 1090 K; heating to higher temperatures, however, leads to the formation of M and H

forms of Nb205. However, Goldschmidt [53] con- cluded from his x-ray data that T-M3 conversion

is time -dependent and that the T form transforms

slowly to H modification even at room temperature.

The detailed crystal structure of the T-Nb205 is not known at present but it appears that TT and T

modifications are closely related since TT—>T process

is continuous and TT may be a less ordered form of

T-Nb205 and it is likely that substitution of F

-

or (OH)-

for O

2-

is necessary for their stability [42]. On the other hand, these two phases may well be

members of a family with complex compositions

and large unit cells and not true polymorphs of

Nb205 [54]. Schafer and co-workers noticed the

'memory’ effect in T-Nb2 Os [42]. Thus, the sample

prepared by heating Nb02 in air at 770 to 870 K is converted to the B-form sharply at 1090 K whereas

the T-Nb205 obtained by reaction of Nb02 with Cl2 (9-10 h, 540 to 570 K and further heating at 770 K

for 15 min in air) exhibited a slow T—»B transition

and not to completion even after heating for 165 h

at 1120 K. On the other hand, T-Nb205 obtained by

wet-precipitation-and-ignition methods transformed

to a mixture of M and H forms after heating for 16 h

at 1070 K without first going into the B-form. The

detailed mechanism is not known but probably impurities play a significant role. B-Nb2 05 is the densest of all the polymorphs of

niobium pentoxide [55-57] and obtained as characteristic plate-like crystals having monoclinic symmetry by vapor transport methods [44]. Polycrystalline material can be obtained easily by the oxidation

of Nb02 in air (24 h heat treatment at 590 and at 1120 K) [44]. This polymorph is depicted as the

high-pressure phase by Wadsley and Andersson [42]

and thermal studies by Schafer et al. [44] indicate

it to be thermodynamically stable at low temperatures. DTA studies indicate a B—>H endothermic conversion at 1230 K but isothermal experiments in

NbOCh and Cl2 atmospheres reveal that the transition is reversible and occurs at lower temperatures

(T/=1020 K; Tr — 920 K) with AH~2 kcal/mol.

The tetragonal M-Nb2Os has been first reported

by Brauer [1] and can be obtained by heating either

the amorphous or T form at 1170 to 1220 K for some hours or for shorter periods at higher temperatures [46, 58]. Single crystals have been grown by

vapor transport [59] which indicate the true symmetry to be tetragonal rather than monoclinic [57]. This phase transforms to H-Nb205 continuously on

heating at 1170 K.

Monoclinic N-Nb2Os can be prepared by vapor

transport and also by the thermal decomposition

of Nb02 F [48, 60-62]. It has a needle-like habit

and F

-

ion appears to stabilize the structure. Transformation to H-Nb2 05 takes place slowly on heating

to 1170 to 1270 K [44].

P-Nb205 has been obtained by Schafer et al. [48]

and Lavers et al. [56] by vapor transport in the range

1020-1120 K. Water vapor and chloride ions seem

to have stabilizing effect on this phase [44]. This

tetragonal modification has an idealized V2 05 struc- ture and transforms to H-Nb205 on heating at 1120

K; the process is endothermic and irreversible [44].

Gruehn [63] has reported the formation of R-Nb2Os

by vapor transport. It has a monoclinic symmetry

and is supposed to have a simple structure compared

to all other polymorphs. Thermochemical behavior

is not known at present.

The stable H-Nb2C>5 is easy to obtain and all other forms of the niobium pentoxide transform to

this polymorph on heating in air to ~1370 K. The

same result is obtained on heating the amorphous

forms of Nb205 or the oxidation of Nb, Nb alloys or Nb02 or slow cooling of the molten Nb2C>5 [64] or under hydrothermal conditions (170 atm, 1350

93
K) [47]. The detailed crystal structure of this mono- Schafer and co-workers [44] reported the formation

clinic modification has been worked out by Gate- of several modifications of Nb205 under nonequilibhouse and ^ adsley [39, 42]. rium conditions from the NbOx phases (Ox i tc , Several metastable phases of Nb2 05 have been Oxvi); the powder x-ray data are almost identical

reported in the literature whose identity is not very to the initial NbO^ phases from which they are ob- well established. Reisman and Holtzberg [64] ob- tained but differ from one another and from all served a high-temperature e form which crystallizes other polymorphs of Nb205 . Detailed structural in-

" without supercooling from the molten NboOs at 1708 formation is lacking at present and it is not known

K. The e form transforms on cooling irreversibly whether they can be classified as the true polymorphs

(without evolution of much heat) to the H-form at of Nb2 05

. Ox i to Ox vi change to H-Nb205 above

1670 to 1470 K; e-Nb205 melts at 1708 K whereas 1370 K.

the equilibrium melting point of H-Nb2 05 is 1764 K. Electrical properties of Nb2 Os have been examined

X-ray and DTA studies by Shafer and Roy [47]

h >' Greener and co-workers [66, 67], The stoichio

indicated the existence of a metastable H' form of

metric Nb2°5 is a semiconductor and the defect

tvt, ^ ,

. , tt tvti /-x i structure is interpreted to be due to the oxyger

JSboOs obtained on heating the H-JNboOs above 1560 . . . . r

,

, . T _ T _ T

T

/ . i ,

. . vacancies; highly reduced material appears to bti,, K; H—>11 is an endothermic and reversible transi- •, ^ t-.. , . , ,

J ® degenerate. Dielectric properties have been reported

tion. Recently, Jonejan and Wilkins [65] reported a by a few workers in the i iterature [68> 69]. ; high-temperature phase transition in Nb205 taking Systems Nb2 05-PbO [70], -B2 Os [71], -Ln2 0: place at 1756 K; the true melting point of Nb205 (Ln=Rare earth) [72], -Ge02 [73], -Ti02 [65], obtained is 1780 K as evidenced by DTA and hot- — P2 05 [74], — V205 [75], — Ta2 05 [76, 77], — WO,

stage microscopic data. [78] have been examined in the literature.

Niobium oxides

Oxide and description Data Remarks and inferences References

of the study

Suboxides

Crystal structures. NbOx (Nb60): forms at 570-620 K; These suboxides form only by oxi- [11, 12]. tetragonal; a = 3.387 A; c = 3.27 dation of metal or by ageing of A. NbOy (NluO): forms at 620- Nb that is supersaturated with

670 K; orthorhombic or tetra- oxygen in definite ranges of

gonal; details not known. NbOx temperature and oxygen pres- (Nb20): forms at 670-970 K; sures. They are metallic in tetragonal; a =6.645 A; c = nature but the detailed structure 4.805 A. and properties are not known.

NbO

Crystal structure and Narrow homogeneity range; cubic; No crystal structure transforma- [15-19, 79,

physical properties. space group, Fm3m; Z = 4; a = tions are known in this material. 80]. 4.2101 A. Contains 25% cation The low value of k and the magand anion vacancies. TEC (°/K): nitude of 70 indicate that NbO is 4.8 Xl0-« at ~297 K; k (cm2 / metallic in nature. Banus and

dyn): 4.2 ±0.06 X10~13

; Cv (erg/ Reed [18] suggest that NbO can i 1 mol, deg at 297 K): 41.06X107

; be considered as a simple cubic 1 \ 7G = 1.26. Melting point = 2218 oxide (with Z = 3 and Nb and O

K. Pressure and temperature do in four-fold coordination and not

not produce any change in the octahedral type) with no vacanconcentration of vacancies indi- cies in the structure which would

eating that these are either explain the results of pressure

greatly ordered or form an in- experiments.

herent part of the structure and

are not true lattice defects in NbO.

94
Niobium oxides—Continued

Oxide and description Data Remarks and inferences References

of the study

Electrical and optical Metallic behavior in the range Detailed band picture and vacancy [14-16, 20-

properties. 2-900 K with low resistivity; p (77 K)~10-« ficm; p (300 K)~

10-8 ficm. Seebeck coefficient and

Hall data not available. Exhibits

superconductivity below 1.5 K.

Plasma edge from reflectance data = 4.3 eV; IR frequency =

1080 ±10 cm-1.

ordering effects not known.

Available superconductivity data

are qualitative and need rein- vestigation.

22, 81].

Nb02

Crystal structure, x-ray and DTA studies. Rutile structure; space group,

I4i/a; Z = 32; a = 13.690 ±0.001

A; c = 5.9871 ±0.0003 A. Superstructure and metal-metal bonding below Tt

. Tt »1070 K;

thermal hysteresis ~10°; AH=

600 cal/mol. The Tt obtained by

x-ray data is higher (~50°) than

the DTA value. Melting point =

2190 K.

The first order nature of the

transition is established. Superstructure lines of the a sublattice of rutile structure and metalmetal bonding disappears above

Tt

.

[15, 24-29].

Magnetic and electrical Paramagnetic in the range 100- Paramagnetic semiconducting [21, 27, 28,

properties. 1300 K; xm s»20X10-8 emu/mol;

XM exhibits jump at Tt

. Semiconductor behavior below 7\;

P'-’-'lO 5 ficm (300 K; polycrystalline material); p~14 12cm (300 K,

single crystal). Ea «0.26 eV (both

samples). Drop in p by a factor of 10 at Tf. Temperature inde- pendent p observed by Janninck

and Whitmore [31] above Tt

while Sakata [27] noted semiconductor behavior with Ea ~

0.15 eV in the range 1100-

1270 K.

characteristics are confirmed below

T

and above TtNb02 appears

to be metallic. There is also discrepancy in the absolute magnitude of p for polycrystalline

and single crystal samples of Nb02 .

30, 31, 37].

Infrared studies. Bands at 715 and 1080 cm-1 are noted and are correlated with the metal-oxygen vibrations. — [81].

Homologous series,

Nbsn+lOgn-l

Crystal structure. All phases are monoclinic; n =5;

Nb02 > 3 7 ; o = 21.2 A; 6 = 3.82 A;

c = 12.0 A; 0 = 132.2°. n=6;

Nb02 ,«; o=20.73 A; 6=3.835 A; c- 15.67 A; 0 - 112.93°. n= 7; Nb08 ,4«; 0=21.19 A; 6-3.822 A; c- 15.75 A; 0-124.51°. n -8;

NhOj.u; o -21.20 A; 6-3.824 A;

c -29.97 A; 0-95.07°.

95

These homologous series have

structures somewhat to the high -temperature structure of Nb2Os even though n =9 actually

corresponds to this oxide.

[11].
Niobium oxides—Continued

Oxide and description of the studyRemarks and inferences References

NbOj^nCNbuOig)

Crystal structure.

NbO*.464 (NR47O116 )

Crystal structure.

Nl^Os

Crystal structures and

transformations

.

Appears to exist in orthorhombic

and monoclinic forms. Orthorhombic form: space group. Amma; Z = 4; a =28.99 A; 6 =

3.825 A; c = 20.72 A. Physical

properties not known.

[40].

Monoclinic: a = 57.74 A; 6 = 3.823 A; c =21.18 A; 0 = 105.32°. The

oxide has been prepared using

NbCla/NbOCl3 at 1520 K and the

structure is probably a combination of Nb220a4 and Nb2aOe2 of the homologous series. Physical

properties are not known.

[41].

Eight modifications known.

TT-Nb2Oa: best prepared by the oxidation of Nb02 in air at 590 K

for 24 h; pseudohexagonal; Z =

0.5; a =3.607 A; c = 3.925 A: Transforms continuously to T-Nb2Oa on heating at 870-1073

K [50]; annealing at 1670 K re- sults in H-Nb206 [49]. T-Nb206: best prepared by heating the amorphous oxide at 870-1073 K

[46, 47] or oxidation of Nb02 at 670 K for 4 to 5 h. Two forms

are reported. Orthorhombic: a =

6.19 A; 6 = 3.65 A; c = 3.94 A; Z = 12. Monoclinic: a = 7.13 A;

6 = 15.72 A; c = 10.75 A; 0 =

120.7°; Z = 12. Changes to B-Nb206; Tt = 1090 K; exo- thermic and irreversible; heating

to higher temperatures produces

H-Nb206. TT and T modifications

appear to be structurally related. B-Nb2Os: Best prepared by the

oxidation of Nb02 in air (24 h at 590 and at 1120 K) or by vapor

transport. Monoclinic; space

group, C2/c; Z = 4; a = 12.73 A; 6=4.88 A; c = 5.56 A; 0 = 105.1°; density = 5.29 g/cm3

. Densest

of all the modifications; depicted

as the high-pressure form;

thermal studies indicate it to be

The polymorphic behavior of Nb2C>6 is complicated. Wadsley

and Andersson [42] have been

able to explain the stability and

structure of the stable polymorphs using the principle of

crystallographic shear and even

predicted the existence of

hitherto unknown forms: However, various metastable and

nonequilibrium phases need ex- planation. Hydrothermal behavior, various impurity effects and 'memory’ of Nb2Os warrant

careful and comprehensive

investigation.

[39, 42,

44-49, 54-

57, 59-63].

96
Niobium oxide8=Gontinued

Oxid© and description ©I the study

Data Bemarke end inferences

tb© thermodynamically stable form at low temperatures, DTA

studies show endothermic B—»H

transition at 1230 K; however,

isothermal experiments in NbOCb and Cla atmospheres . indicate the transition to be re- versible and lower T< with ^//~2

keal/mel.

M-NbiOi: Pure phase is diffi- cult to obtain and partial trans- formation to H-form always occurs; prepared by heating oxide

gels or amorphous form to 1170=

1220 K or by vapor transport of NbgOg with NbOGls or NbQBr®

(7\ = 1120=1170 K and 7V>7\ =

100 K). Tetragonal; space group, 14/mmm; a =20,01 A; c = 3.84 A. Transforms to H-NbaOs on heating at 1170 K; no thermal effects noticeable,

N-NbaQs: Best prepared by

heating NbQaF at 1270 K in vacuum for some hours; F

-

ion appears to stabilize the structure. Monoclinic; space group, C2/m;

a = 28.51 A; 6 = 3.83 A; c = 17.48 A; /3 = 120.8°. Structures of M

and N forms appear to be

basically related [42]. N-NbaOe

transforms to H-NbaCb slowly on heating at 1170-1270 K.

P-NbgOs: Prepared by vapor

transport (Cla ; Ti = 1020 K;

7V-7W00 K). Cl= ions and

water vapor appear to have

stabilizing effect. Tetragonal:

space group, 14i/22; Z=4; a =

3.896 A; 6=23.43 A. P=>H &

1120 K; endothermic and

irreversible. B-NbiOiS Prepared by vapor trans- port but usually associated with

other polymorphs, MonoeUnie;

space group, G2/m; a = 12.79 A;

6 = 3,826 A; e = 3.983 A;| =

90,75 §. Thermal behavior not known.

H-NbgOii Easy to obtain; any other

form heated in air at 1370 K

produces this form; also under

hydrothermal conditions. Monoclinic; space group, P2; Z-14;

a-21.16 A; 6-3.822 A; c-19.35

97
Niobium oxides—Continued

Oxide and description

of the study

Data Remarks and inferences References

A; /3 = 119.83°. Most stable form;

structure based on the crystallographic shear has been worked

out by Gatehouse and Wadsley

[39, 42]. The melting points of Nb206 reported are inconclusive

and the values quoted are 1758,

1764, and 1780 K [46, 65, 78].

Electrical properties. Semiconductor; <r~10“4 ftcm at ^1000 K; E0 “1.65 eV. Reduced

NbjOj tends to behave as a de- generate semiconductor. Defect

structure is interpreted to be due

to anion vacancies.

[66, 67].

IR studies. Bands are noted at 790 and 1010 cm-1 for H-NbjOj and are inter- preted in terms of metal-oxygen

vibrations.

[81].

References

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46, 397 (1940); Z. anorg. allgem. Chem. 248, 1 (1941).

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[7] Taylor, A., and Doyle, N. J., J. Less-Comm. Metals 13,

313 (1967).

[8] Fromm, E., and Jehn, H., J. Less-Comm. Metals 15, 242

(1968)

[9] Schafer, H., Bergner, D., and Gruehen, R., Z. anorg.

allgem. Chem. 365, 31 (1969). 10 Terao, N., Japan J. Appl. Phys. 2, 156 (1963).

11 Niebuhr, J., J. Less Comm. Metals 11, 191 (1966). 12 Chang, L. L. Y., and Phillips, B., J. Am. Ceram. Soc.

52, 527 (1969).

[13] Nakayama, T., Osaka, T., and Kitada, A., J. Less- Comm. Metals 19, 291 (1969).

[14] Pollard, E. R., Ph.D. Thesis, MIT, USA (1968).

[15] Subbarao, G. V., Ph.D. Thesis, Indian Institute of Technology, Kanpur, India, 1969.

[16] Rao, C. N. R., Wahnsiedler, W. E., and Honig, J. M.,

J. Solid State Chem. 2, 315 (1970).

[17] Taylor, A., and Doyle, N. J., J. Appl. Cryst. 4, 103

(1971).

[18] Banus, M. D., and Reed, T. B., in The Chemistry of Extended Defects in Non-metallic Solids, Eds. L. Eyring and M. O’Keeffe (North Holland Publ. Co.,

Amsterdam, 1970), p. 488.

[19] Taylor, A., and Doyle, N. J., in The Chemistry of Extended Defects in Non-metallic Solids, Eds. L. Eyring and M. O’Keeffe (North Holland Publ. Co.,

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Chandrashekar, G. V., Moyo, J., and Honig, J. M.,

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Gel’d, P. V., Shveikin, G. P., Alyamovskii, S. I., and

Tskhai, V. A., Russ. J. Inorg. Chem. 12, 1053 (1967).

Marinder, B.-O., Arkiv Kemi 19, 435 (1962).

Rogers, D. B., Shannon, R. D., Sleight, A. W., and

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Rao, C. N. R., Rama Rao, G., and Subba Rao, G. V.,

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Rao, C. N. R., Natarajan, M., Subba Rao, G. V., and

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M., J. Am. Chem. Soc. 79, 2039 (1957).

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A., Mh. Chem. 91, 975 (1960).

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21, 615 (1967).

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i (Phys. and Chem.), No. 1, 11-16 (Jan.-Feb. 1966).

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(U.S.), 69A (Phys. and Chem.), No. 2, 119-129 (Mar.-

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(U.S.), 70A (Phys. and Chem.), No. 4, 281-303 (JulyAug. 1966).

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. (1971).

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Otdel. Tekhn. Nauk Met. i Toplivo 2, 79 (1960).

1 [81] Alyamovskii, S. I., Shveikin, G. P., and Gel’d, P. V., Russ. J. Inorg. Chem. 12, 915 (1967).

II. 4. Molybdenum Oxides

Phase studies on the Mo-0 system have been

carried out by various workers [1-6]. In addition to the well-known dioxide and trioxide, many mixed

valence phases (between Mo02 and M0O3) have been

reported [3-7]; of these, Mo4On and Mo9026 seem

to be stable [1, 5, 6] whereas M017O47, M05O14 and

Mo802 3 seem to be metastable and can be stabilized

by incorporation of other elements like Ti, V, and

W [8]. These mixed valence oxides can not be considered to be truly nonstoichiometric, since they

have narrow ranges of homogeneity; some of them

have tunnel and layer structures. No oxide of molybdenum in the range Mo-Mo02 is known and the

reported existence of Mo30 [2] is in doubt [1, 5, 6, 9]. M0O2 : Molybdenum dioxide, Mo02 , has a monoclinic structure [1, 7, 10-12] and exhibits metallic

behavior in the range 4 to 300 K [12, 13]. No phase

transitions are known in this material. Other properties are not known.

Mo02 forms solid solutions with Ti02 [14], Nb02 [14] and V02 [15-17] which have rutile structures. Mixed Valence Phases: M04O11 has a monoclinic

structure (stable below 890 K) and assumes an

orthorhombic symmetry when synthesized in the

range 890 to 950 K; both the forms are closely re- lated [3, 7, 18] in structure and Tt appears to be

^900 K. M04O11 appears to be metallic [3] with no

significant change in p at Tt

. M017O47 has an or- thorhombic symmetry [3, 18] whereas M06O14 is tetragonal [8]; Mo802 3 is monoclinic [3, 7, 18-20],

M09O26 is dimorphic and can assume a triclinic or monoclinic structure depending on the method of

preparation [5, 9, 18, 19]. Phillips and Chang [5] report a phase transformation in Mo902 e at 1040 K;

the low-temperature form is monoclinic, but the

structure of the high-temperature form is not ex- actly known. M017O47 exhibits low resistivity and

appears to be metallic [3]. M04O11 and Mo902 6 melt

at high temperature (>1000 K) and decompose to lower oxides [5]. Detailed physical properties are not known for these mixed valence phases.

M0O3: Molybdenum trioxide, M0O3 , has an or- thorhombic symmetry and consists of a layered

(or chain) structure [1, 5, 7, 21]. Detailed studies [5] indicate no phase transitions in this material up to the melting point (^1055 K). MoOs is a semiconductor [ 22]. Molybdenum bronzes are formed from M0O3 and

other metallic oxides; these interesting materials

have been discussed in detail in the literature [23]

99
Molybdenum oxides

Oxide and description of the study

Remarks and inferences References

Mo02

Crystal structure and

electrical properties.

Mixed Valence Phases

M04O11

Crystal structure and

electrical properties.

M017O47

Crystal structure and

electrical properties.

MosOu

Crystal structure.

Mo»02j

Crystal structure.

MogOw (M018O62)

Crystal structure and

x-ray studies.

Monoclinic; space group, P2i/c;

Z = 4; a = 5.6109 ±0.0008 A; 6 =

4.8562 ±0.0006 A; c = 5.6285 ±

0.0007 A; 0 = 120.95°. No phase

transitions are known. Metallic

behavior; p (4.2 K)»5.4X10~7 (2cm; p (300 K) *8.8X10"8 12cm.

Monoclinic (<900 K); a =24.54 A;

6 = 5.439 A; c = 6.701 A; 0 =

94.28°. Orthorhombic (>900 K);

a =24.49 A; 6 = 5.459 A; c =

6.752 A. Tt »900 K. Monoclinic

and orthorhombic forms can be

obtained at ordinary temperatures depending on the method of

preparation. Both forms show

metallic behavior; p (300 K)~0.2

12cm; p does not show much

change at T<.

Orthorhombic; space group, Pba2;

Z-2; 0-21.61 A; 6-19.63 A; c- 3.951 A. Metallic behavior;

p (300 K)~5X10“* (2cm. Detailed

data are lacking.

Tetragonal; a = 11.50 A; c = 3.937 A. Metastable form and de- composes on prolonged heat

treatment; Ti, V, and W seem to

stabilize the phase.

Monoclinic; a = 16.88 A; 6 > A; c = 13.39 A; 0 = 106.19°. Properties not known.

= 4.052

Dimorphic: Triclinic phase; a —

8.145 A; 6 = 11.89 A; c = 21.23 A; a = 102.67°; 0=67.82°; 7 =

The structure is closely related to that of rutile; the metallic properties have been interpreted

in terms of a modified Goodenough’s model.

Both the modifications are closely related structurally and are

built up of M0O4 tetrahedra and

distorted MoOe octahedra; the

difference lies in the relative orientation of the substructures

in nieghboring slabs, being

parallel in the monoclinic and

alternating in the orthorhombic

form. Detailed data on the

physical properties are lacking.

This has a tunnel structure which

may be considered as built up

from MoO« octahedra and M0O7 pentagonal bipyramids. Metalmetal bonding is noted and

probably accounts for the low

measured resistivity.

The structure is related to that of M017O47 . Detailed data are

lacking.

The structure is that of distorted ReOs-type in which recurrent

dislocations of atoms occur along

the shear planes.

The triclinic phase has a basic M0O3 structure. According to Kihlborg [24], MoigOs2 is the

[1, 7, 10-

13].

[3, 7, 18].

[3, 18].

[ 8].

[3, 7, 18- 20].

[5, 9, 18,

19].

100
Molybdenum oxides—Continued

Oxide and description of the study

Data Remarks and inferences References

109.97°. Monoclinic phase; a =

16.74 A; b =4.019 A; c = 14.53 A;

0 =95.45°. Both forms can be

prepared at room temperature.

Monoclinic phase shows a transi- tion at 1040 K; the structure of the high -temperature phase is not known in detail. Physical properties are not known.

first member (m = 1 ) of a homologous series, Mon03n-m+i* The existence of oxides M012O33 and M026O76 is suggested.

M0O3

Crystal structure and

properties.

Orthorhombic; space group, Pbnm;

Z = 4; a =3.966 A; 6 = 13.88 A;

c = 3.703 A. No phase transformations exist up to the melting

point (1055 K). Semiconductor;

p (450 K)~1010 Stem; p (900 K)

~10

5

Stem; lnp — 1/T plot shows a break at ~650 K; Eo (<650 K)

«0.56 eV; Ea (>650 K)«1.83

eV.

It has a chain structure. The break

in lnp — 1/T plot at 650 K may

correspond to a change in the mechanism of conduction.

[1, 5, 7, 21,

22].

References

[1] Hagg, G., and Magneli, A., Arkiv Kemi, Mineral Geol. 19A, 1 (1945).

[2] Schonberg, N., Acta Chem. Scand. 8, 221 (1954); 8, 617

(1954).

[3] Kihlborg, L., Acta Chem. Scand. 13, 954 (1959); Adv.

Chem. Ser. 39, 3745 (1963).

[4] Rode, E. Ya., and Lysanova, G. V., Dokl. Akad. Nauk

SSSR 145, 351 (1962).

[5] Phillips, B., and Chang, L. L. Y., Trans. AIME 233,

1433 (1965).

[6] Chang, L. L. Y., and Phillips, B., J. Am. Ceram. Soc.

52, 527 (1969).

[7] Magneli, A., Andersson, G., Holmberg, B., and Kihlborg,

L., Anal. Chem. 24, 1998 (1952).

[8] Kihlborg, L., Acta Chem. Scand. 23, 1834 (1969).

[9] Kihlborg, L., Acta Chem. Scand. 16, 2458 (1962).

[10] Magneli, A., Arkiv Kemi, Mineral Geol. 24A, 11 (1946).

[11] Brandt, B. G., and Skapski, A. C., Acta Chem. Scand.

21, 661 (1967).

[12] Rogers, D. B., Shannon, R. D., Sleight, A. W., and

Gillson, J. L., Inorg. Chem. 8, 841 (1969).

[13] Perloff, D. S., and Wold, A., Crystal Growth, Proc. Int. Conf. on Cryst. Growth, Ed. H. S. Peiser (Pergamon,

London, 1967).

[14] Marinder, B.-O., Dorm, E., and Seleborg, M., Acta Chem. Scand. 16, 293 (1962).

[15] Marinder, B.-O., and Magneli, A., Acta Chem. Scand.

11, 1635 (1957); 12, 1345 (1958).

[16] Israelson, M., and Kihlborg, L., Mat. Res. Bull. 5, 19

(1970).

[17] Rao, C. N. R., Natarajan, M., Subba Rao, G. V., and

Loehman, R. E., J. Phys. Chem. Solids 32, 1147

(1971).

[18] Kihlborg, L., Arkiv Kemi 21, 471 (1963).

[19] Magneli, A., Acta Chem. Scand. 2, 501 (1948).

[20] Kihlborg, L., Arkiv Kemi 21, 461 (1963).

[21] Kihlborg, L., Arkiv Kemi 21, 357 (1963).

[22] Deb, S. K., and Chopoorian, J. A., J. Appl. Phys. 37,

4818 (1966).

[23] Rao, C. N. R., and Subba Rao, G. V., Phys. Stat. Solidi

(a) 1, 597 (1970).

[24] Kihlborg, L., Arkiv Kemi 21, 443 (1963).

101
II. 5. Technitium Oxides

Studies by various workers [1-3] have shown that

in the system Tc-O, Tc02 is the only stable oxide at high temperatures. A covalent heptoxide, Tc207 , is also known. Tc02 has a monoclinic, Mo02 -type struc- ture [1, 3] and according to Marinder and Magneli

[5], metal-metal bonds exist in this material. A different monoclinic cell was suggested by Zachariasen

[6]; it has been pointed out [3] that Tc02 may exhibit

polymorphism like Re02

. Tc207 is orthorhombic and the structure consists

of an arrangement of isolated Tc207 molecules with

tetrahedral coordination of Tc atoms; the structure

of Tc207 is more closely related to Cr03 , Ru04 and

0s04 than to Re207. Physical properties are not

known at present.

Technitium oxides

Oxide and description of the study

Data Remarks and inferences References

Tc02

Crystal structure. Monoclinic; space group, P2 i/C;

Z = 4; a = 5.55±0.01 A; 6 =

4.85 ±0.01 A; c = 5.62 ±0.01 A;

/3 = 121.9±0.1°.

Rutile based structure with metalmetal bonds.

[1, 3, 5].

TC2O7

Crystal structure. Orthorhombic; space group,- Pbca;

Z = 4; a = 13.756 A; 6 = 7.439 A;

c = 5.617 A.

The structure consists of isolated TC2O7 molecules.

[4].

References

[1] Muller, O., White, W. B., and Roy, R., J. Inorg. Nucl.

Chem. 26, 2075 (1964).

[2] Rulfs, C. L., Pacer, R. A., and Hirsch, R. F., J. Inorg.

Nucl. Chem. 29, 681 (1967).

[3] Rogers, D. B., Shannon, R. D., Sleight, A. W., and

Gillson, J. L., Inorg. Chem. 8 , 841 (1969).

[4] Krebs, B., Angew. Chem. Int. Ed. (Engl.) 8, 381 (1969);

Z. anorg. allgem. Chem. 380, 146 (1971).

[5] Marinder, B.-O., and Magneli, A., Acta Chem. Scand.

11, 1635 (1957).

[6] Zachariasen, W. H., ACA Program and Abstr. of Winter

Meeting, F-4 (1951).
II. 6. Ruthenium Oxides

Phase relations in the Ru-0 system have been

reported in the literature [1,2]. In the temperature

range 1070 to 1770 K, Ru02 is the only stable oxide

(in air); Ru03 and Ru04 exist as vapor species.

Attempts to prepare the stable nonstoichiometric

oxide, Ru02 ±x, have not been successful [2]. Ru02 dissociates in air at ^1680 K to the metal.

Several workers have studied the crystal growth

of Ru02 by chemical transport [3-5] and other

methods [6, 7]. Ru02 has a tetragonal rutile structure

[2, 3, 5-9] at room temperature and no phase transitions are known in the temperature range 4.2-1270 K

[5, 8, 10-12]. It has a low and almost temperature

independent magnetic susceptibility [6, 13]; the resistivity is very low and exhibits metallic behavior

[3, 5-7, 11-14].

Fletcher and co-workers [13] have shown that the

oxidation state of Ru in Ru02 is more than 4. The

de Hass-van Alphen effect [15], magnetoresistance

[16] and Azbel-Kaner cyclotron resonance [17] studies on Ru02 suggest that the simple band picture

proposed by Rogers and co-workers [7] is inadequate

to explain the observed properties. Ryden et al. [12]

have found that electron-phonon and electron-elec- tron interband scattering mechanism accounts

quantitatively for the observed resistivity behavior

of Ru02 in the range 10 to 1000 K.

McDaniel and Schneider [2] report that Ru02 and

Ir02 can form complete solid solutions of rutile structure in the entire composition range.

Ruthenium oxides

Oxide and description of the study

Data Remarks and inferences References

Ru02

Crystal structure and TEC. T = 298 K: Tetragonal; space

group, P42 /mnm; Z = 2; a =

4.4910 ±0.0003 A; c = 3.1064±

0.0002 A. TEC (XlO-6/°C:

|| c = —1.4; ||- = +7.0.

The c lattice parameter decreases with rise in T; slight anisotropy

is evident.

[5, 10].

Magnetic and electrical properties.

Xm=2.03X10-4 cgs units at 298 K.

p (4.2 K)~l.0X10-8 ftcm; p (100

K)~1X10-6 ftcm; p (298 K)~

4.0X10-5 ftcm. At 77 K, Rh =

—0.79, mh=61 cm2 /V s; at 300 K, Rn = —1.10, Mh=3.1 cm2 /V s. 0d (calc.) =900±5 K.

l/x = d/T+aT" (for T<1.5 Tm) where /3 = 18.1 cm K2 /W, a =

8.42X10'4 cm K^/W, n =1.66

and Tn is the temperature at which K is a maximum. Lorentz

ratio =2.6 X10_8W ft/

2

.

The metallic behavior of Ru02 is evident; the Lorentz number is close to the theoretical value for typical metals.

[3, 6, 7, 11, 12, 18].

References

[1] Bell, W. E., and Tagami, M., J. Phys. Chem. 67, 2432

(1963).

[2] McDaniel, C. L., and Schneider, S. J., J. Res. Nat. Bur.

Stand. (U.S.), 73A (Phys. and Chem.), No. 2, 213-219

(Mar.-Apr. 1969).

[3] Schafer, H., Schneidereit, G., and Gerhardt, W., Z. anorg.

allgem. Chem. 319, 327 (1963).

[4] Schafer, H., Chemical Transport Reactions (Academic

Press, New York, 1964), p. 48.

[5] Butler, S. R., and Gillson, J. L., Mat. Res. Bull. 6, 81 U97D.

[6] Cotton, F. A., and Mague, J. T., Inorg. Chem. 5, 317

(1.966).

[7] Rogers, D. B., Shannon, R. D., Sleight, A. W., and

Gillson, J. L., Inorg. Chem. 8, 841 (1969).

[8] Krishna Rao, K. V., and Iyenger, L., Acta Cryst. 25A,

302 (1969).

[9]

Boman, C. E., Acta Chem. Scand. 24, 116 (1970).

[10] Hazony, Y., and Perkins, H. K., J. Appl. Phys. 41, 5130

(1970).

[11] Osburn, C. M., and Vest, R. W., Bull. Am. Ceram. Soc.

47, 354 (1968).

[12] Ryden, W. D., Lawson, A. W., and Sartain, C. C., Phys. Rev. B 1, 1494 (1970).

[13] Fletcher, J. M., Gardner, W. E., Greenfield, B. F., Holdoway, M. J., and Rand, M. H., J. Chem. Soc. A

653 (1968).

[14] Ryden, W. D., Lawson, A. W., and Sartain, C. C., Phys. Letters 26A, 209 (1968).

[15] Marcus, S. M., and Butler, S. R., Phys. Letters 26A, 518

(1968).

[16] Marcus, S. M., Phys. Letters 28A, 191 (1968).

[17] Slivka, R. T., and Langenberg, D. N., Phys. Letters 28A, 169 (1968).

[18] Millstein, J., J. Phys. Chem. Solids 31, 886 (1970).

103
II. 7. Rhodium Oxides

In the Rh-0 system, the only well-established

oxides are Rh2 Os and RI1O2 , even though there are numerous claims and counter claims regarding the

existence of oxides such as Rh20 and RhO [1-5].

Rh2Os exhibits polymorphism; the low-temperature

low-pressure form belongs to the corundum struc- ture and the high-temperature low-pressure form has

the orthorhombic perovskite structure [1, 5], According to Wold and co-workers [1], the transition

temperature is '"-'1020 K and is sluggish; Tt also depends on the starting materials used. The hightemperature low-pressure form can be prepared

from Rh metal at 1270 K [1], Shannon and Prewitt

[6] have prepared a high-temperature high-pressure

form of Rh2Og (at 65 kbar pressure and 1470 K)

which has an orthorhombic symmetry but closely

related to the corundum structure. The high-temperature form of Wold et al., and the high-pressure

form of Shannon and Prewitt appear to be closely

related [6]. The latter material appears to be semiconducting at room temperature [6]. Rh02 has the rutile structure [2-4]; it has a low

resistivity and exhibits metallic behavior [3, 4], Other

Physical properties are not known.

Rhodium oxides

Oxide and description of the studyData Remarks and inferences References

RI12O3

Crystal structures and

electrical properties.

Low-temperature low-pressure

form: Hexagonal (Corundum

structure); space group, R3C;

Z = 6; a =5.108 A; c = 13.810 A. Tt is ~1020 K. High temperature form: Orthorhombic; space

group, Pbca; a = 5.1477 A; 6 =

5.4425 A; c = 14.6977 A. Hightemperature high-pressure form:

Orthorhombic; space group, Pbna; Z = 4; 0 = 5.1686 ±0.0003

A; b = 5.3814±0.0004 A; c =

7.2486 ±0.0004 A. p (300 K)~

130 Ocm; Ea “0.16 eV.

The low-temperature—^high-temperature transformation is sluggish

and depends on the starting materials employed; the high temperature form can be prepared

directly from Rh metal at 1270 K.

The orthorhombic structures are

related to the corundum structure and may be described as containing layers of the corundum struc- ture cut parallel to (lOll) and

stacked together.

[1, la, 6, 7].

[ 6].

RhOj

Crystal structure and

electrical properties.

Rutile structure; space group

P42 /mnm; Z = 2 ; a=4.4862±

0.0005 A; c = 3.0884 ±0.0005 A.

p (4.2 K)~2X10-6 ftcm; p (300

K)~lXlO-4 ficm. RI1 O3 de- composes to RhOs at ~1120 K

followed by decomposition to the metal and oxygen at ~1320 K.

Rutile structure characteristic of many transition metal dioxides. Typical metallic behavior.[2-4].

References

[1] Wold, A., Arnott, R. J., and Croft, W. J., Inorg. Chem.

2, 972 (1963).

[la] Bieaterboa, J. W. M., and Horuatia, J., J. Le»>.Common

Metalt, 30, 121 (1973).

[2] Muller, O., and Roy, R., J. Leaa-Common Metala 16, 129

[3] Shannon, R. D., Solid State Commun. 6, 139 (1968).

[4] Rogers, D. B., Shannon, R. D., Sleight, A. W., and

Gillaon, J. L., Inorg. Chem. 8, 841 (1969).

[5] Prewitt, C. T., Shannon, R. D., Rogera, D. B., and

Sleight, A. W., Inorg. Chem. 8, 1985 (1969).

[6] Shannon, R. D., and Prewitt, C. T., J. Solid State Chem.

2, 134 (1970).

[7] Coey, J. M. D., Aota Cryat. 26B, 1876 (1970).

104
1 1. 8. Palladium Oxide

Palladium monoxide is the only stable oxide known

in the Pd-0 system. Attempts to prepare PdsCX

have not been successful [1]. PdO has a tetragonal

structure [2-4], Studies by Rogers et al. [4] on PdO

single crystals indicate that the material is a p-type

semiconductor. No phase transformations are known

in this material; it decomposes in air at ~1070 K

[5].PdO forms solid solutions with CuO [6, 7], PbO

[8] and rare-earth sesquioxides [5] and the phase

relations have been described in detail.

Palladium oxide

Oxide and description

of the study

Data Remarks and inferences References

PdO

Crystal structure and Tetragonal; space group, P42/ The material is a p-type extrinsic [2-5]. properties. mmc; Z = 2; a = 3.0434 ±0.0002

A; c = 5.3363 ±0.0004 A. Semiconductor (p-type); p (300 K)~

10-1000 ficm; Ea =0.04-0.10 eV. Decomposes to the metal and

oxygen in air at ~1070 K; the

process of decomposition is re- versible. No phase transitions are known.

semiconductor.

References

[1] Muller, 0., and Roy, R., J. Less-Comm. Metals 16, 129

(1968).

[2] Moore, W. J., Jr. and Pauling, L., J. Am. Chem. Soc. 63,

1392 (1941).

[3] Wasor, J., Levy, H. A., and Peterson, S. W., Acta Cryst.

6, 661 (1953).

[4] Rogers, D. B., Shannon, R. D., and Gillson, J. L., J. solid State Chem. 3, 314 (1971).

[5] McDaniel, C. L., and Schneider, S. J., J. Res. Nat. Bur.

Stand. (U.S.), 72A (Phys. and Chem.), No. 1, 27-37

(Jan.-Feb. 1968).

[6] Schmahl, N. G., and Minzyl, E., Z. Phys. Chem. (N.F.)

47, 142 (1965).

[7] Schmahl, N. G., and Eikerling, G. F., Z. Phys. Chem.

(N.F.) 62, 268 (1968).

[8] Shaplygin, I. S., Bromberg, A. V., and Sokol, V. A., Russ. J. Inorg. Chem. (English Transl.) 15, 1195 (1970).

105
II. 9. Silver Oxides

Even though the literature contains many refer- ences [1-8] where silver oxides have been studied, it appears that only Ag2 0 and AgO (and probably

Ag2 03 ) are the only well-characterized oxides of

silver. Silver sesquioxide, Ag2 03 , is prepared at low

temperature in aqueous solutions [8]; it appears to have cubic structure [9] and decomposes to AgO at higher temperatures.

Ag2 0: The stability range of Ag2 0 is limited [8, 10-13] and decomposes to the metal above ~500 K.

Ag2 0 has a cubic structure [11] and exhibits semiconducting behavior [14, 15], A new high pressure

modification of Ag20 has been described by Kabalkina et al. [16]; this appears to have a layered Cdl2 structure possesing metal-metal bonds. These authors report that it may be a degenerate semiconductor or a semiconductor with a low energy gap.

AgO: Silver monoxide, AgO, has a monoclinic struc- ture [3, 17, 18]; the oxygen content seems to vary

depending on the method of preparation [3], but no other crystalline modifications of AgO are known.

The material is diamagnetic and has a very low room

temperature resistivity (^TOficm), but p shows negative temperature coefficient in the range 230 to 290 K

[1, 18]; these properties are explained as due to the

existance of monovalent and trivalent Ag ions in

AgO and confirmed by x-ray structure analysis [17].

Silver oxides

Oxide and description of the study

Data Remarks and inferences References

AgjO

Crystal structure and

x-ray studies. Cubic; space group, Pn3m; Z — 2; a *4.720 ±0.004 A. TEC~2X

10“«/°C (not very accurate

value). Decomposes above <~500

K at atmospheric pressure. Highpressure form (prepared at 115-

125 kbar and 1670±200 K):

Hexagonal; a =3.072 ±0.003 A;

c = 4.941 ±0.004 A. The high

pressure phase is denser by

~30%.

Ag20 has the same structure as Cu20 with considerable coval- ency; the high-pressure modification has a layer -structure probably with metal-metal bonding.

[11, 16].

Thermal properties

(10-470 K).

Heat capacity data have been re- ported in the 10-500 K range. DTA studies have been carried out in the 300-470 K range.

Anomalies have been noted in the heat capacity curves around 30

and 420 K. The low temperature

anomaly is difficult to understand in terms of particle size of

surface area effects alone. The

high temperature anomaly is probably associated with annealing of crystal defects or

crystalization.

[19-21].

Electrical properties

measured in oxygen

atmospheres (300 and 450 bar)

Semiconductor; p-type behavior

with p «po*-1/4

. p (500 K)~10* —

10< Qcm; p (620 K)~10l -10a ficm; E„ *0.64 eV.

a data lacking; the conduction

mechanism is interpreted as due

to positive holes excited from

traps (cation vacancies).

[14, 15].

106
Silver oxides—Continued

Oxide and description

of the study

Data Remarks and inferences References

AgO

Crystal structure, magnetic

and electrical properties.

Monoclinic; space group, P2i/c;

Z = 4; a=5.85±0.02 A; 6 = 3.47 ±

0.08 A; c = 5.49 ±0.05 A; 0 =

107.5°. Structure analysis indi* cates that Ag+ and Ag8+ ions

exist; two different Ag-0 dis- tances are encountered. Diamagnetic in the range 90-370 K;

Xm = — 19.1X10-6 emu; Unusually

low room temperature resistivity

(p~10 flem); p decreases with

rise in T in the range 230-290 K.

AgO and CuO are isomorphous; the unusual physical properties of AgO compared to CuO are ex- plained as due to the coexistence of mono- and trivalent Ag. p-T

behavior indicates either near degeneracy or a semiconductor

with a low energy gap. Detailed

studies on AgO are urgently

needed.

[1, 3, 17,

18].

References

f l] Neiding, A. B., and Kazarnovskii, I. A., Dokl. Akad.

Nauk SSSR 78, 713 (1951).

[2] Jones, P., and Thirsk, H. R., Trans. Faraday Soc. 50,

732 (1954).

[3] Graff, W. S., and Stadelmair, H. H., J. Electrochem. Soc. 105, 446 (1958).

[4] Ylach, J., and Stehlik, B., Collec. Czech. Chem. Commun.

25, 676 (1960).

[5] Amlie, R. F., and Riietschi, P., J. Electrochem. Soc. 108,

813 (1961).

[6] Karpov, A. A., Borisova, T. I., and Veselovskii, V. I., Zh. Fiz. Khim. 36, 1426 (1962).

[7] Allen, J. A., Proc. Australian Conf. Electrochem., Sydney,

Hobart, Australia, 72, (1963) Publ. 1965.

[8] Nagy, G. D., Moroz, W. J., and Casey, E. J., Proc. Am.

Power Sources Conf. 19, 80 (1965).

[9] Stehlik, B., Weidenthaler, P., and Vlach, J., Chem. Listy

52, 2230 (1958).

[10] Garner, W. E., and Reeves, L. W., Trans. Faraday Soc.

50, 254 (1954).

[11] Suzuki, T., J. Phys. Soc. Japan 15, 2018 (1960).

[12] Weidenthaler, P., Collec. Czech. Chem. Commun. 28,

137 (1963).

[13] Otto, E. M., J. Electrochem. Soc. 113, 643 (1966).

[14] Cahan, B. D., Ockerman, J. B., Amlie, R. F., and

Riietschi, P., J. Electrochem. Soc. 107, 725 (1960).

[15] Talukdar, M. I., and Baker, E. H., Solid State Commun.

7, 309 (1969).

[16] Kabalkina, S. S., Popova, S. V., Serebryanaya, N. R., and Vereshchagin, L. F., Dokl. Akaa. Nauk SSSR

152, 853 (1963).

[17] Scatturin, V., Bellon, P. L., and Zannetti, R., J. Inorg. Nucl. Chem. 8, 462 (1958).

[18] Scatturin, V., Bellon, P. L., and Salkind, A. J., J. Electro- chem. Soc. 108, 819 (1961).

[19] Pitzer, K. S., and Smith, W. V., J. Am. Chem. Soc. 59,

2633 (1937).

[20] Kobayashi, K., Sci. Rep. Tohoku Univ. 35, 173 (1951).

[21] Pitzer, K. S., Gerkin, JR. E., Gregor, L. V., and Rao,

C. N. R., Pure and Applied Chem. (J.IUPAC) 2, 211

(1961).

II. 10. Cadmium Oxides

Cadmium monoxide, CdO, is the well known oxide

in the Cd-0 system [1-4]. Hoffman et al. [5] re- ported the preparation and structure of the peroxide

Cd02 , but detailed properties are not known.

CdO: CdO has the cubic rock salt structure and is diamagnetic [6]. No phase transitions are known in

this oxide. The substance is usually nonstoichiometrie (Cd excess) with pronounced cation interstitials or anion vacancies [2, 7-9], resulting in a free electron

concentration of ^1019-1021 cm-3

. Such high elec- tron concentration induces degenerate behavior; at high temperatures, CdO is either nondegenerate or degenerate n-type semiconductor depending on the

history of the sample, i.e., sintering time and temperature, carrier concentration and oxygen pressure

[1]. Donor impurities which contribute conduction

electrons give rise to an impurity level a few tenths

of an electron volt below the conduction band [10].

Detailed resistivity, Seebeck coefficient and Hall

coefficient measurements [1, 2, 9-14] as well as optical [15, 16], NMR [17, 18] and Faraday rotation

[19] studies indicate that depending on the carrier

concentration, the impurity band may overlap with

the conduction band in CdO giving rise to quasimetallic or metallic properties.

107
Cadmium oxides

Oxide and description of the study

Data Remarks and inferences References

CdO

Crystal structure and

magnetic properties.

Cubic; space group, Fm3m; Z = 4; a=4.6949 A. Slight variation of a with the stoichiometry. Diamagnetic; x = —48.74 X 10-8 emu/g.

CdO exhibits pronounced nonstoichiometry which is attributed

to either Cd interstitials or oxy- gen vacancies.

[3, 6, 7].

Electrical properties. Degenerate semiconductor behavior; p (300 K)~10-2-10-3 flcm; n~1018 -1021 cm-3 depending on the sample history but

independent of temperature; a (300 K)~20 mV/°C; Mh (81 K)

^700-1100 cm

2

/V s; m* = 0.15—

0.45 m.

Nonstoichiometry gives rise to

large number of charge carriers; n can be controlled by sample

treatment. The impurity level belonging to the ionized donors

merges into the conduction band

giving rise to quasi -metallic behavior in some samples. Detailed conduction mechanism

treated.

[1, 2, 9-

14].

Optical properties. Band gap = 2.3 eV; a possible in- direct gap noted at ~1.2 eV; m*

(optical) = 1.4 m. Plasma edge

is ~0.2-0.5 eV depending on n.

eo = 18.1; Coo = 5.6.

Free carrier absorption is noted in the optical spectra due to quasifree electrons. The results are interpreted in terms of the band

structure of CdO [20].

[15, 16, 19].

113 Cd NMR studies (1.4-300 K).

Results indicate that the nuclei

interact strongly with the de- generate conduction electrons and

the electrons are in the host

lattice conduction band, r «

n~il3 T~i; k « n

113

.

[17, 18].

cao2

Crystal structure Cubic; space group, Pa3; Z = 4; a = 5.313 ±0.003 A. Decomposes

violently at ~450-470 K to CdO

and oxygen. Other properties not known.

[5].

References

[1] Hogarth, C. A., Proc. Phys. Soc. (Lond.) 64B, 691 (1951).

[2] Wright, R. W., and Bastin, J. A., Proc. Phys. Soc. (Lond.) 71, 109 (1958).

[3] van Houten, S., Nature (Lond.) 195, 484 (1962).

[4] Hoschl, P., Konak, C., and Prosser, V., Mat. Res. Bull.

4, 87 (1969).

[5] Hoffman, C. W. W., Ropp, R. C., and Mooney, R. W.,

J. Am. Chem. Soc. 81, 3830 (1959).

[6] Mookherji, T., J. Electrochem. Soc. 117, 1201 (1970).

[7] Cimino, A., and Merezio, M., J. Phys. Chem. Solids 17, 57 (1960).

[8] Haul, R., and Just, D., J. Appl. Phys. 33, 487 (1962).

[9 Koffyberg, F. P., Phys. Letters 30A, 37 (1969).

[10]

Lamb, E. F., and Tompkins, F. C., Trans. Faraday Soc.

58, 1424 (1962).

[11] Bastin, J. A., and Wright, R. W., Proc. Phys. Soc. (Lond.) 68A, 312 (1955).

[12] Konak, C., Hoschl, P., Dillinger, J., and Prosser, V., Crystal Growth, Suppl. J. Phys. Chem. Solids 341

(1967).

[13] Wada, M., Japan J. Appl. Phys. 9, 327 (1970).

[14] Koffeyberg, F. P., J. Solid State Chem. 2, 176 (1970);

Canad. J. Phys. 49, 435 (1971).

[15] Finkenrath, H., and von Ortenberg, M., Z. angew. Phys.

23, 323 (1967).

[16] Altwein, M., Finkenrath, H., Konak, C., Stuke, J., and

Zimmerer, G., Phys. Stat. Solidi 29, 203 (1968).

[17] Look, D. C., Phys. Rev. 184, 705 (1969).

[18] Benedict, R. P., and Look, D. C., Phys. Rev. B 2, 4949

(1970).

[19] Zvara, M., Kocka, J., Konak, C., and Hoschl, P., Phys.

Stat. Solidi 42, K5 (1970).

[20] Maschke, K., and Rossler, U., Phys. Stat. Solidi 28, 577

(1968).

108
III. Oxides of 5d Transition Elements

III.l. Lanthanum Oxides

The common oxide of lanthanum is the sesquioxide, La203 [1]. Warf and Korst [2] have reported

the monoxide, LaO, (probably a surface film), but

the material has not been prepared in bulk form.

Rare-earth sesquioxides (of which La203 is the

first member) generally exist in the cubic (C), hexagonal (A) or the monoclinic [1, 3] modifications;

the C-form is a defect structure of fluorite type

where the metal ion has the MO0V2 coordination,

where V stands for a vacancy along the body diagonal. The A- and B-forms are essentially close-packed

structures with MO7 coordination. All the oxides

have the C-type structure at low temperature. The

sesquioxides of heavier rare earths (Dy to Lu) as well as Sc203 and Y203 retain the C structure even up to very high temperatures (^2300 K); however,

sesquioxides from La to Nd revert to the A-type

structure depending on the temperature and method

of preparation of the sample etc. High-temperature,

high-pressure transitions are exhibited by almost all the rare-earth sesquioxides and these have been discussed in detail in the literature [1, 3]. La2 03 : The usual form of La2 03 that is encountered

is the hexagonal (A) modification [1, 4]. Detailed

studies by Foex and co-workers [5, 6] indicate that

the A-form transforms on heating to another hexagonal form (H) at 2310 K which is followed by an

additional transformation at 2380 K to an unidentified structure (X) before melting (at 2573 K); A—>H

and H—>X transformations are reversible. The A<p*H

transition does not involve a change in the crystal

symmetry, but discontinuities are noted in the a and c parameters as well as in the c/a ratio at Tt

. The presence of the X-modification is indicated by

the appearance of one or two characteristic fines in

the x-ray diffraction patterns. The X-form appears

to be denser than the H-form, but the detailed structure has not yet been established; preliminary

data seem to indicate [7, 8] a cubic structure differ- ent from the C modification.

The C-form of La203 can be prepared by decomposing the precipitated hydroxide under vacuum

and slow heating to 670 K [9]. DTA and resistivity

studies indicate the C—»A transition at ~870 K

which is irreversible and apparently sluggish; moisture seems to affect the Tt considerably. According

to Foex et al. [5, 6, 10], the A«=±H transition (marked

by small thermal effects and lack of hysteresis) is of

the displacive type [11] whereas the H«=*X transition

(marked by strong thermal effects) belongs to the

reconstructive type. During the course of the latter

transition, the two corresponding modifications are usually observed to coexist over a remarkably ex- tended range of temperatures; the energy barrier

between the corresponding modifications is relatively

high. No high pressure transitions are known in

La203. Daire and Wilier have recently reported the

existence of the B-modification of La203 [12, 13];

thus, in this oxide it is possible to realize all the

different modifications that are generally exhibited

by the rare-earth oxides.

The magnetic behavior of La203 has been examined

in the range 290 to 1070 K by Smol’kov and

Dobrovol’skaya [14] who found that the substance is diamagnetic. Electrical properties have been ex- amined in detail by various workers in the literature

[9, 15-21]. La203 is a p-type semiconductor and

shows considerable ionic conductivity at relatively

low temperatures (<800 K). Impurities seem to play a significant role in determining the contribution

to ionic conductivity [22]; pure electronic conduction

exists at high temperatures.

La203 forms solid solutions with oxides of the type

MO (M = Be [23], Mg [24], Ca [20, 25], Pb [26], and

Pd [27]); M203 (M = B [28], Al [29, 30], Cr [29], Fe

[29], Ga [29], Pr [31], and Y [29]); M02 (M = Ti

[29, 32], Zr [29, 32-35], Hf [36], Ce [18, 29], and Sn

[29]); M206 (M = Pa [37]) and M03 (M = W [38],

and U [39]). The crystal structures and electrical

properties of these solid solutions have been ex- amined in detail in the literature with particular

reference to ionic conduction.

109
Lanthanum oxides

Oxide and description of the study

La203

Remarks and inferences References

Crystal structure and

high temperature x-ray

studies.

C-type: Cubic; space group, Ia3; Z = 16; o = 11.39±0.03 A. C-form

is metastable; C—*A transition at ~870 K; energy of activation

~20 Kcal/mol.

A -type: Hexagonal; space group, P3ml; Z = 1; at T = 293 K, a =

3.937 A; c = 6.13 A; at T = 2213

K, o=4.028 A; c = 6.385 A. A-+H transition at 2310 K.

Depending on the method of [1, 4-9, 12,

preparation of the sample, differ- 13]. ent types of polymorphs of La203 can be realized. C—+A transition

is irreversible; A—*H and H—*X

transitions are reversible but

differ in the mechanism of the

process.

H-type: Hexagonal; at 2393 K, a =

4.063 A; c = 6.43 A. if-*X transi- tion at (or above) 2380 K.

Magnetic and electrical properties.

Infrared studies.

Heat capacity studies (5-1800 K).

X-type: Most probably cubic; detailed structure not known.

Melting point ~2570 K.

B-type: Monoclinic; space group, C2/m; Z = 6; a = 14.60 A; b =

3.717 A; c = 9.275 A; 0=99.77°.

Diamagnetic (range 290-1070 K);

X= —0.23 X10-6 emu/g.

Semiconductor; p (700 K)~106-

10® Ocm; Ea (C-form) ~1.1 eV; Ea (A-form)~1.2 eV. C—*A transi- tion is shown as a change in the

slope of lnp — 1/T plot; moisture seems to affect Tt considerably,

p-type behavior; p

oc po2 _1/4

; seems to show p- to n-transition at low po2 and high temperature.

Considerable ionic conductivity

at low enough temperatures and

probably controlled mostly by

impurities.

Bands are noted in the range 280-

430 cm-1 and are ascribed to the metal-oxygen vibrations.

C„ (300 K) =25.92 ±0.08 cal/mol,

K. There are no anomalies in the measured temperature range.

The magnetic behavior is understandable because there are no 4f

electrons to contribute to para- magnetism. Electrical data are lacking especially at high temperatures where crystal structure

transitions occur. The conductivity behavior of highly pure and single crystalline material is urgently needed to elucidate the

detailed mechanism of conduction

and the contribution of the ionic conductivity.

[9, 14-21].

[40].

[41].

References

[1] Brauer, G., Progr. in the Science and Technology of Rare

Earths Ed. L. Eyring (Pergamon, Oxford) 1, 152

(1964); 2, 312 (1966); 3, 434 (1968).

110

[2] Warf, J., and Korst, W., U.S. At. Energy Comm. NP6078, 1956.

[3] Eyring, L., and Holmberg, B., in Non-stoichiometric

Compounds (Adv. in Chem. Series) 39, 46 (1963).

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371, 263 (1969).
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[7] Foex, M., and Pierre, J., Rev. Inst. Hautes Temp.

Refract. 3, 429 (1966).

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Abstr. 70, 60406s (1969).

[9] Mehrotra, P. N., Chandrashekar, G. V., Rao, C. N. R., Subbarao, E. C., Trans. Faraday Soc. 62, 3586 (1966).

[10] Foex, M., and Traverse, J.-P., Compt. Rend. (Paris)

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[11] Buerger, M. J., in Phase Transformations in Solids Ed.

R. Smoluchowski (John Wiley, New York, 1951).

[12] Daire, M., and Wilier, B., Compt. Rend. (Paris) 266C,

548 (1968).

[13] Wilier, B., and Daire, M., Bull. Soc. Franc. Mineral

Crist. 92, 33 (1969).

[14] Smol’kov, N. A., and Dobrovol’skaya, N. V., Izv. Akad.

Nauk SSSR, Neorg. Mater. 1, 1564 (1965).

[15] Foex, M., Compt. Rend. (Paris) 220, 359 (1945).

[16] Rudolph, J., Z. Naturforsch. 14a, 727 (1959).

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(1959); Z. Phys., Chem. (Leipzig) 211, 194 (1959).

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Rao, C. N. R., J. Solid State Chem. 2, 377 (1970).

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(1970).

[23] Kuo, C.-K., and Yen, T.-S., Chem. Abstr. 63, 10743e

(1965).

[24] Tresvyatskii, S. G., and Lopato, L. M., Vpor. Teorii i Prim. Redkozem. Metal. Akad. Nauk SSSR 155 (1964).

[25] Barry, T. L., Nucl. Sci. Abstr. 20, 36784 (1966).

[26] Cassedanne, J., Anais Acad. Brasil. Cienc. 36, 413 (1964).

[27] McDaniel, C. L., and Schneider, S. J., J. Res. Nat. Bur.

Stand. (U.S.), 72A (Phys. and Chem.), No. 1, 27-37

(Jan.-Feb. 1968).

[28] Levin, E. M., Phys. Chem. Glasses 7, 90 (1966).

[29] Andreeva, A. B., and Keler, E. K., Zh. Prikl. Khim. 39,

489 (1966).

[30] Bonder, I. A., and Toropov, N. A., Izv. Akad. Nauk

SSSR, Ser. Khim. 212 (1966).

[31] Brauer, G., and Pfeiffer, B., Paper presented in Fifth Rare Earth Conference’, Chem. Session C, 1965.

[32] Collongues, M. R., Queyroux, F., Perez, M., Zorba, Y., and Gilles, J.-C., Bull. Soc. Chim. France 4, 1141

(1965).

[33] Lin, T.-H., and Yu, H.-C., Kuei Suan Yen Hsueh Pao

3, 159 (1964).

[34] Fehrenbacher, L. L., Jacobson, L. A., and Lynch, C. T., Proc. Conf. Rare Earth Res., 4th, Phoenix, Arizona,

1964, p. 687 (Publ. 1965).

[35] Strickler, D. W., and Carlson, W. G., J. Am. Ceram. Soc.

48, 286 (1965).

[36] Komissarova, L. N., Wang, K. S., Spitsyn, V. I., and

Simanov, Yu. P., Russ. J. Inorg. Chem. (English

Transl.) 9, 385 (1964).

[37] Keller, C., J. Inorg. Nucl. Chem. 27, 797 (1965).

[38] Tyushevskaya, G. I., Afonskii, N. S., and Spitsyn, V. I., Dokl. Akad. Nauk. SSSR 170, 859 (1966).

[39] Koshcheev, G. G., and Kovba, L. M., Izv. Akad. Nauk

SSSR, Neorg. Mater. 2, 1254 (1966).

[40] Subba Rao, G. V., Rao, C. N. R., and Ferraro, J. R., Appl. Spec. 24, 436 (1970) and references therein.

[41] Holley, C. E., Jr., Huber, E. J., Jr. and Baker, F. B.» Progr. in the Science and Technology of Rare Earths,

Ed. L. Eyring, (Pergamon, Oxford) 3, 343 (1968) and

references therein.

III. 2. Hafnium Oxides

Phase studies on the Hf-0 system [1-3] have shown

that the dioxide, Hf02 , is the only stable oxide. Hf02 exhibits polymorphism and has been investigated in

detail by many workers.

The stable room temperature modification of Hf02 is monoclinic [4, 5] which transforms to the tetragonal

structure reversibly at ~1890 K [6, 7] with appreciable thermal hysteresis [8]. A reversible tetragonalcubic inversion in Hf02 has been reported at ~2970

K, which is close to the melting point [7]. Even

though the detailed mechanism of the monoclinictetragonal transition in Hf02 is not known, comparison with the analogous transition in Zr02 suggests

the first order, diffusionless and athermal nature of

the transformation in this oxide [5, 6, 8, 9]. A high pressure orthorhombic modification of Hf02 has been described by Bocquillon et al. [10]; this is obtained by heating the monoclinic Hf02 at ^1870

K under a minimum pressure of ~15 kbar; this

metastable phase reverts to the monoclinic form on

prolonged heating at ~570 K.

The tetragonal or cubic modification of Hf02 can- not be quenched nor can it be stabilized at room

temperature [7, 10] and efforts to obtain these forms

by precipitating Hf02 in finely divided form have

been unsuccessful [9]. Submicron Hf02 prepared by

the hydrolytic decomposition of alkoxides [9] is amor- phous and transforms directly to the monoclinic

form at ~600 K. However, the cubic form (and not

the tetragonal form) seems to be stabilized in thin

films of Hf02 and recently El-Shanshoury et al. [11]

have discussed their polymorphic behavior.

Electrical properties of Hf02 have been examined

in detail by Tallan, Tripp, and Vest [12]. The material is a p-type semiconductor and a is essentially

electronic in the range 1300-1800 K (po2 from 10-18 to 1 atm). At low oxygen partial pressures, a small

contribution due to the ionic conductivity has been

noticed [12-14]. Measurements on polycrystalline

Hf02 by Tallan et al. [12] did not show a discontinuity in the ln<r — 1/T plot at the monoclinictetragonal phase transition.

Hf02 forms a complete series of solid solutions with

Zr02 (8, 15-17] and generally, the monoclinictetragonal transition temperature is decreased with

increasing concentration of Zr02

. A decrease in Tt is also observed in the solid solutions of Hf02 with

rare-earth sesquioxides [18]. Systems Hf02 -MO (M = Mg [19], Ca [19-22]), and Hf02-M203 (M = Sc [23,

24], Y [20, 25-27], La [25, 28], Nd [25, 29], Sm [30],

and Gd [31]) have been investigated in the literature.

Ill
Hafnium oxides

Oxide and description

of the study

Data Remarks and inferences References

HfOg

Crystal structure and

x-ray studies. fe 298 K: Monoellnie; space

group, P2i/c; Z = 4; a=»5.1156±

0.0005 A; b = 5.1722 ±0.0005 A;

c = 5.2948 ±0.0005 A; 0= 99.02 ±

0.08 A. TEC (470-670 K): [100], 5.9X10-®; [001], 11.9 X10-®.

TEC slightly increases with temperature up to 1470 K. Monoclinic—tetragonal transition at 1890 K; thermal hysteresis, 30 K.

The transition appears to be

endothermic associated with a volume contraction, but detailed data are not available. Boganov

et al. [7] report the Tj to be

~2170-2270 K for this transition. At 2770 K, a = 5.21 A, c = 5.35 A;

this tetragonal phase is stable up

to ~2970 K and then transforms

to cubic fee modification with

a = 5.300 A. The tetragonalcubic inversion is reversible; however detailed data are lacking.

High-pressure form: obtained by

heating at 1870 K under 60 kbar

pressure and quenching; ortho- rhombic; a =5.008 ±0.006 A; b =

5.062 ±0.006 A; c = 5.223 ±0.006

A. This metastable phase reverts

to the monoclinic form on pro- longed heating at 570 K.

Amorphous Hf02 obtained by the thermal decomposition of the metal alkoxide goes to the monoclinic variety on heating at ~600

K; the intermediate cubic and

tetragonal forms are not realiz- able, neither they can be

quenched to room temperature

from the respective temperature

ranges of existence. Melting

point of HfOj is 3070 K.

In many respects, HfOg resembles

Zr02 even though there are subtle

differences in behavior. The se- quence (and the mechanism )of the transformations from the low

symmetry to the high symmetry

in the crystal structure are the same in Hf02 and Zr02 except

that the T< for Hf02 are higher

and the thermal hysteresis values

are lower. The high temperature

modifications of HfOg cannot be

quenched nor can they be

stabilized at room temperature by

solid solution formation.

[4-8, 10,

32],

Electrical properties. p-type semiconductor in the range 1270-1770 K; <r (1570 K)~10-»

B"1 cm"1 ; E. is 0.7 eV ( <1570 K)

and 0.2 eV ( >1570 K); a « poj 1/# (poi>10“® atm); md = 0.3-1.6X

10“® cm2 /V s. At low po*(<10-1 atm) small contribution due to

ionic conductivity seems to be

present. The defect structure is interpreted in terms of holes and

fully ionized cation vacancies.

The electrical behavior is as ex- pected and similar to Zr02 which

also behaves as an extrinsic p- type semiconductor. Apparently, no discontinuity in the ln<r — 1/T

plot has been noticed at the monoclinic -tetragonal transition

in Hf02 .

[12].

112
References

[1] Rud^, E., and Steeher, P., J. Lesa-Comm. Metals 5, 78

[2] Bomagala, R. F., and Ruh, R., Trans. Am. Soe. Met. 58,

164 (1965).

[3] Karnilov, I. L, Glazova, V. V., and Ruda, I. G., Izv. Akad. Nauk SSSR, Neorg. Mater. 4, 2106 (1968).

[4] Adam, J., and Rogers, M. D., Acta Cryst. 12, 951 (1959).

[5] Ruh, R., and Corfield, P. W. R., J. Am. Ceram. Soc. 53,

126 (1970).

[6] Wolten, G. M., J. Am. Ceram. Soc. 46, 418 (1963).

[7] Boganov, A. G., Rudenko, V. S., and Makarov, L. P., Dokl. Akad. Nauk SSSR 160, 1065 (1965).

[8] Ruh, R., Garret, H. J., Domagala, R. F., and Tallan, N. M., J. Am. Ceram. Soc. 51, 23 (1968).

[9] Mazdiyasni, K. S., and Brown, L. M., J. Am. Ceram.

Soc. 53, 43 (1970).

[10] Bocquillon, G., Susse, C., and Vodar, B., Rev. Int. Hautes, Temp. Refract. 5, 247 (1968).

[11] El-Shanshoury, I. A., Rudenko, V. A., and Ibrahim, I. A.,

J. Am. Ceram. Soc. 53, 264 (1970).

[12] Tallan, N. M., Tripp, W. C., and Vest, R. W., J. Am.

Ceram. Soc. 50, 279 (1967).

[13] Robert, G., Deportee, C., and Besson, J., J. Chim. Phys.

64, 1275 (1967).

[14] Etsell, T. H., and Flengas, S. N., Chern. Rev. 70, 339

(1970).

[15] Mazdiyasni, K. S., Lynch, C. T., and Smith, J. S.,

J. Am. Ceram. Soc. 48, 372 (1965).

[16] Stansfield, O. M., J. Am. Ceram. Soc. 48, 436 (1965).

[17] Gavrish, A. M., Sukharevskii, B. Ya., Krivornchko, P. P., and Zoz, E. I., Ivz. Akad. Nauk SSSR, Neorg. Mater.

5, 547 (1969).

[18] Koehler, E. K., and Glushkova, V. B., Sci. Ceramics 4, 233 (1967, publ. 1968).

[19] Strekalovskii, V. N., Volchenkova, Z. S., and Pal’guev,

S. F., Izv. Akad. Nauk SSSR, Neorg. Mater. 2, 1230

(1966).

[20] Volchenkova, Z. S., and Pal’guev, S. F., Trans. Inst. Elektrokhim Akad. Nauk SSSR, Ural. Filial 5, 133

(1964).

[21] Johansen, H. A., and Cleary, J. G., J. Electrochem. Soc. Ill, 100 (1964).

[22] Delamarre, C., Silicates Ind. 32, 345 (1967).

[23] Komissarova, L. N., and Spiridinov, F. M., Dokl. Akad.

Nauk SSSR 182, 834 (1968).

[24] Kalinovskaya, C. A., Spiridinov, F. M., and Komissarova,

L. N., J. Less-Comm. Metals 17, 151 (1969).

[25] Komissarova, L. N., Wang, C. S., and Spitsyn, V. I., Izv. Akad. Nauk SSSR, Ser. Khim. 1, 3 (1965).

[26] Besson, J., Deportee, C., and Robert, G., Compt. Rend.

(Paris) 262,327 (1966).

[27] Duclot, M., Vicat, J., and Deportee, C., J. Solid State Chem. 2, 236 (1970).

[28] Komissarova, L. N., Wang, C. S., Spitsyn, V. I., and

Simanov, Yu. P., Russ. J. Inorg. Chem. 9, 385 (1964).

[29] Komissarova, L. N., Spitsyn, V. I., and Wang, C. S., Dokl. Akad. Nauk SSSR 150, 816 (1963).

[30] Isupova, E. N., Glushkova, V. B., and Keler, E. V.,

Izv. Akad. Nauk SSSR, Neorg. Mater. 4, 399 (1968).

[31] Spiridinov, F. M., Stepanov, J. A., Komissarova, L. N.,

and Spitsyn, V. I., J. Less-Comm. Metals 14, 435

(1968).

[32] Filatov, S. K., and Frank-Kamenetskii, V. A., Sov. Phys. -Crystallogr. (English Transl.) 14, 696 (1970).

III. 3. Tantalum Oxides

The Ta-0 system has been investigated by many

workers [1-7] and the stability and phase relationships of the various oxides have been reviewed and

discussed by Steeb and Renner [8], Niebuhr [9],

and Chang and Phillips [10]. In addition to the suboxides, TaO* (Ta@0), TaOy (Ta 40) and TaO* (Ta20),

the monoxide TaO (whose existence is not proved

beyond doubt), dioxide, TaG2 and the pentoxide,

Ta206 are known; however, Ta2 06 is the only oxide

which is well-characterized and extensively investigated in recent years.

Suboxides: The suboxides form only by the oxidation of tantalum metal or tantalum compounds.

Ta60 (tetragonal) forms at ~570 K; Ta40 (orthorhombic) forms below 770 K and Ta20 (tetragonal)

forms between 620 to 1470 K. A complex suboxide

of unknown structure is formed above 1770 K. Electron diffraction studies of monocrystalline samples

above 1770 K. Electron diffraction studies of monocrystalline samples reveal superlattices [8, 9]. These

suboxides appear to be metallic, but detailed data

are lacking.

Monoxide, TaO: Lagergren and Magn61i [1] and

Schonberg [3] reported the existence of tantalum

monoxide with a narrow homogeneity range; it has

a cubic rock salt structure with a variable lattice

parameter. Physical properties are not known. The

existence of this monoxide has not been proved beyond doubt and systematic investigations are called

for. Ta02 : Rutile type Ta02 is known to be a stable

oxide [1, 3, 11], but detailed data on the physical

properties are lacking. Ta02 appears to form solid

solutions with Ti02 [3, 12], although there are changes in the cation valencies.

Ta206

: Tantalum pentoxide, Ta206 , is a stable oxide

with a very narrow homogeneity range [13] and has

been well-investigated by various workers. The exact

relationships of the various polymorphs of Ta2Oa are not clearly understood because of the complexity of

the structures of the phases, sluggish nature of the

transitions and the existence of various metastable

phases that can be stabilized by impurities.

It is fairly well established that Ta2C>6 exists in

two polymorphic forms, /3 and a, with a reversible

phase transition at ~1630 K [1, 14]; the transition

between them is sluggish and the low temperature /3 form can persist as a metastable phase in the stability field of the a form and can be melted at ~2060

K. The high temperature (a) form melts at ^2160 K

[14, 15].

The proper method of indexing the powder pattern

113
of the /3 form has puzzled crystallographers for a long time; the more intense diffraction lines can be

indexed on the basis of a simple orthorhombic subcell with a = 6.20, 6 = 3.66, and c = 3.89 A. However, numerous weak lines which also appear in the pattern have not been interpreted unambiguously mainly because the position of these super-structure lines

is strongly dependent upon both the nature of the

heat treatment and the amount of impurities [16,

17]. Consequently, different workers have suggested

different lattice parameters and symmetries to ex- plain the x-ray pattern of the /3 form of Ta2 C>6 [7, 18-25]. Systematic examination of the polymorphic

behavior of Ta2 06 by Roth and co-workers [15, 16,

26-28] has contributed significantly to the understanding of the structure of the low-temperature

phase. These workers found that many metallic

oxides, especially WOa, can stabilize the /3-form and

that this pure phase exists in two slightly different

modifications with 6-axis multiplicities of 11 and 14

at low temperatures and at ~1580 K respectively.

At intermediate temperatures, an infinite number of

at least partially ordered sequences of these two

modifications exist in equilibrium. The addition of

WO3 (or other impurities) causes the stabilization

of an infinite number of phases similar in structure

to the low-temperature form of Ta206

. The structure of the high-temperature (a) form

of Ta206 has been a subject of intensive study and

and tetragonal, hexagonal, orthorhombic, monoclinic, and triclinic symmetries have been suggested

by various workers [1, 14, 20, 22, 29]. Detailed

studies have, however, indicated [15, 29] that the a form actually undergoes several unquenchable

phase transitions upon cooling from high temperatures. The true high-temperature form is postulated

to be tetragonal in its field of stability and several

metastable phases occur in the stability field of the

low-temperature polymorph. Ta20B when quenched

from above 1630 K (T\) is triclinic and transforms

to a monoclinic form on heating to ~590 K and this

in turn reverts to the tetragonal form at ~1220 K.

Sarjeant and Roy [30] reported a metastable hightemperature hexagonal form of Ta20& obtainable by

rapid quenching techniques; this 5 form reverts to

the /3 form on annealing at ~1470 K. Impurities

like titanium lower the Tt of the /3—>a transition

and Sc8+ in low concentrations appears to stabilize

the high-temperature tetragonal phase [16, 28]; the

crystal structure consists of a-UOa-type blocks in

which Ta atom is surrounded by a pentagonal bipyramid of oxygen atoms. These blocks are infinite

in two directions and are separated from similar

blocks along a third direction by shear planes and in

the vicinity of these shear planes, the Ta atoms are surrounded by distorted octahedral coordination

polyhedra.

Semiconducting behavior of Ta20B has been ex- amined by Kofstad [13] who finds that the material

is p- or n-type depending on the po2 . Differences in

the dielectric behavior of the polymorphs of Ta206

have been reported by Pavlovic [31]. Systems K20-

Ta206 [32] and Nb206-Ta20B [33, 34] have been investigated in the literature.

Tantalum oxides

Oxide and description of the study

Data Remarks and inferences References

Suboxidea

Crystal structure. Ta#0: Tetragonal; a *=3.36 A; c =

3.25 A. Ta40: Orthorhombic; o =

3.61 A; b “3.27 A; c = 3.20 A. TajO: Tetragonal; a = 6.63 A;

c = 4.75 A. Detailed data on the

physical properties are lacking.

These oxides are formed from Ta by

the oxidation at various temperatures; some reveal superstructure

in electron diffraction patterns.

[It 2, 3, 5, 9, 10].

TaO

Crystal structure. Cubic; space group, Fm3m; 2 = 4; 0 = 4.422-4.429 A. Formed in the range 870-1770 K; physical properties not known in detail.

[1. 31.

114
Tantalum oxides—Continued

Oxide and description of the study

TaOj

Remarks and inferences References

Crystal structure.

TagOg

Tetragonal; space group, P42/mnm;

Z = 2; 0 = 4.709 A; c = 3.065 A.

Physical properties are not known

in detail.

[1. 31-

Crystal structures and

x-ray studies. Low-temperature 03) form: Subcell: orthorhombic; Z= 1; a —

6.20 A; 6 = 3.66 A; c = 3.89 A.

True-cell: orthorhombic; Z = ll; o=6.198 A; 6=40.29 A; c = 3.888 A; 6-axis multiplicity = 11. 0-form at a given temperature

consists of an infinite number of

partially ordered sequences of two simple modifications with

multiplicities 11 and 14 and these sequences are affected both by

heat treatments and by impurities thus explaining considerable

ambiguity encountered by various

workers in the interpretation of the powder patterns. T<~1630 K;

sluggish but completely reversible

transition. High-temperature (a) form: tetragonal; space group;

I4j/amd; Z = 6; a = 3.81 A; c*

36.09 A. This phase is un- quenchable in pure TagOg; however, Sc,+ appears to stabilize this form. The structure consists of a-UOj type blocks in two direc- tions and separated in the third

direction by shear planes. Sc8+ doping seems to introduce ran- dom shear planes (Wadsley defects) which stabilize the structure. TagOg quenched from

temperatures above T, is tri- clinic: a =3.801 A; 6 = 3.785 A;

c = 35.74 A; <*=90.91®; /3 =90.19®;

yss*906

. This triclinic phase

transforms to monoclinic phase on heating to ~590 K; ameao - V2 Ot,i„; 6meao “ y/% bttlei Csnooo®*

cuio. Monoclinic phase reverts to the tetragonal phase on heating

to '>-'1220 K. Rapid quenching of TagOg from high temperatures

appears to produce a hexagonal

modification; a =3.874 A; c =

Systematic examination of the detailed phase relationships by

Roth and coworkers has revealed

a proper understanding of the

structures of TagOg modifications

and cleared the ambiguities of the earlier workers.

[15, 16, 26-28].

115
Tantalum oxides—Continued

Oxide and description of the study

Data Remarks and inferences References

3.62 A. This 6 form reverts to the

/3 form on annealing at ~1470 K.

Melting pointcii2160±20 K.

Electrical properties. Semiconductor in the range 1000-

1670 K. cr (1000 K)~10"5 0"1 cm"1 ; E0 = 1.79 eV; a (1300 K;

P02 ~l atm)~l mV/K. p-type

behaviour at poj^l atm and re- type at lower po*. p to re transi- tion takes place at lower po* with

decreasing T.

The defect structure is interpreted

in terms of oxygen interstitials (p-type) and oxygen vacancies

(re-type). No change in the slope of bnr-l/T plot noticed at T<; the reason is not clearly understood.

[13].

Infrared spectra. Bands are noted at 720 and 860

cm-1 for a-Ta206 and interpreted

in terms of the metal-oxygen

vibrations.

• [34],

[ 1 ]

[23B

[si

(ioj

[11]

[ 12 ] 1314

[15]

[16]

[17]

[18]

References

Lagergren, S., and Magneli, A., Acta Chem. Scand. 6,

\\\ (1952).

Wasilewski, R. J., J. Am. Chem. Soc. 75, 1001 (1953).

Schonberg, N., Acta Chem. Scand. 8, 240 (1954).

Pawel, R. E., Cathcart, J. V., and Campbell, J. J., Acta

Met. 10, 149 (1962).

Norman, N., J. Less-Comm. Metals 4, 52 (1962).

Kofstad, P„ J. Inst. Metals 90, 253 (1962); 91, 209

(1963).

Terao, N., Japan J. Appl. Phys. 6, 21 (1967).

Steeb, S., ana Renner, J., J. Less-Comm. Metals 10, 246

(1966).

Niebuhr, J., J. Less-Comm. Metals 10, 312 (1966).

Chang, L. L. Y., and Phillips, B., J. Am. Ceram. Soc.

52, 527 (1969).

Rogers, D. B., Shannon, R. D., Sleight, A. W., and

Gillson, J. L., Inorg. Chem. 8, 841 (1969).

Riidorff, W., and Luginsland, H. H., Z. anorg. allgem.

Chem. 334, 125 (1964).

Kofstad, P., J. Electrochem. Soc. 109, 776 (1962).

Reisman, A., Holtzberg, F., Berkenblit, M., and Berry,

M., J. Am. Chem. Soc. 78, 4514 (1956).

Waring, J. L., and Roth, R. S., J. Res. Nat. Bur. Stand.

(U.S.), 72A (Phys. and Chem.) No. 2, 175-186 (Mar.-

Apr. 1968).

Rotn, R. S., Waring, J. L., and Brower, W. S., Jr.,

J. Res. Nat. Bur. Stand. (U.S.), 74A (Phys. and

Chem.), No. 4, 485-493 (July-Aug. 1970).

Jahnberg, J., and Andersson, S., Acta Chem. Scand. 21,

615 (1967).

Frevel, L., and Rim, H., Anal. Chem. 27, 1329 (1955).

[19]

[20]

[ 21 ]

[ 22] 23

24

[25]

[26]

[27]

[28]

[29]

[30]

[31

[32

[33]

[34]

Simanov, Yu. P., Lapitskii, A. V., and Artamanova,

E. P., Vetn. Mosk. Univ. 9, Ser. Fiz. Mat. i Estestven Nauk (6), 109 (1954).

Zaslavskii, A. I., Zvinshuk, R. A., and Tutov, A. G.,

Dokl. Akad. Nauk SSSR 104, 409 (1955).

Lapitskii, A. V., Simanov, Yu. P., Semenen, K. N., and

Yarembash, E. I., Vetn. Mosk. Univ. 9, Ser. Fiz. Mat.

i Estestven Nauk, (2), 85 (1959).

Harvey, J., and Wilman, H., Acta Cryst. 14, 1278 (1961).

Lehovec, K., J. Less-Comm. Metals 7, 397 (1964).

Turnock, A. C., J. Am. Ceram. Soc. 48, 258 (1965);

49, 382 (1966).

Wolten, G. M., and Chase, A. B., Z. Krist. 129, 365

(1969).

Roth, R. S., and Stephenson, N. C., in The Chemistry

of Extended Defects in non-Metallic Solids, Eds. L. Eyring and M. O’Keeffe, (North Holland Publ. Co.,

Amsterdam, 1970).

Roth, R. S., Waring, J. L., and Parker, H. S., J. Solid State Chem. 2, 445 (1970).

Stephenson, N. C., and Roth, R. S., J. Solid State Chem.

3, 145 (1971).

Laves, F., and Petter, W., Helv. Phys. Acta 37, 617

(1964).

Sarjeant, P. T., and Roy, R., J. Am. Ceram. Soc. 50, 500

(1967).

Pavlovic, A. S., J. Chem. Phys. 40, 951 (1964).

Holtzberg, F., and Reisman, A., J. Phys. Chem. 65, 1192

(1961).

Mohanty, G. P., Fiegel, L. J., and Healy, J. H., J. Phys.

Chem. 68, 208 (1964).

Alyamovskii, S. I., Shveikin, G. P., and Gel’d, P. V., Russ. J. Inorg. Chem. 12, 915 (1967).

116
III.4. Tungsten Oxides

Tungsten dioxide and trioxide are the well-known

oxides in the W»0 system [1=3]. Two mixed valence

phases, Wi§04§ (WOg,?g) and WggOig (WOg.go) also

exist [1, 4=7] in the range WOg-WO§ and substoiehiometrie phases of WO§, viz., WOg,g§ (WioOiu) and

WOg.ii (WioOiw) have been characterized in the

literature [ 8]. Even though earlier workers [9, 10]

have claimed the existence of an oxide W3O, recent

studies [7, 11] indicate that the oxide is not a stable

phase.

WO2: The range of homogeneity of W02 appears to

be small (1.94-2.025) [12] and the material is stable

up to 1800 K after which it decomposes. It has a monoclinic structure [2, 5, 13, 14] and exhibits metallic behavior [13]; no phase transitions are known in

W02 .

. W02 forms solid solutions with Ti02 [15] and

V02 [14, 16] giving rise to rutile based structures Wi8049: This oxide is monoclinic [4, 5] and is stable

above 860 K; below this temperature, it decomposes

to W02 and W20O58. Wi8C>49 is the most refractory

oxide (stable up to 1970 K) of the W-0 system [7]. Physical properties of this oxide are not known.

W20O68 { This monoclinic oxide seems to have a nar- row range of homogeneity [4, 5, 17, 18] and decomposes below 760 K to W02 and WOa [7]. Measurements of resistivity and Seebeck coefficient indicate

metallic behavior [18].

Substoichiometric phases : WB oOi48 and W40O119

are monoclinic and according to Gebert and Ackerman [8], a high-temperature polymorph of W6 oOi48 exists. Detailed data are lacking.

W08

: Tungsten trioxide is a stable phase and is usually associated with slight nonstoichiometry [1, 7, 18]; it is polymorphic and not less than five

distinct crystallographic modifications are known (fig.

III.l). The phase transitions among the various

forms of WO3 have been examined using a variety of

techniques.

At room temperature, WOa is monoclinic [1, 5, 7,

12, 19-22] and this phase is stable in the range 290

to 583 K [23]. On cooling to 290 K, the room temperature polymorph transforms to a triclinic form;

this reversible change is associated with some thermal

hysteresis (3-13° depending on the sample), AH

and AS and discontinuities in p, a, 1?#, and p# [18,

23-25]. Roth and Waring [26] noted that the monoelinie WO§ ean be partially transformed to th© tri- elinie phas© by grinding. Th© latter phai© ii itabl©

in th© rang© 233 to 290 K. On cooling to 233 K, th©

trielinie phai© of pur© WO§ changes to a monoclinic

phas© with observable temperature hysteresis, TEC,

AH and AS changes, discontinuities in p, g, H#, and

Hu [18, 23=23, 27=29] and changes in the domain

structure [29]. This monoelinie form seems to be

stable in the range 77 to 233 K and has been found

to be ferroelectric [28, 30, 31]. Roth and Waring [26]

obtained this phase at room temperature by quenching a soid solution of 2 percent Nb205 in WO3 from

1500 to 1660 K; however, this solid solution does

not exhibit ferroelectricity whereas specimens of sintered W08 containing 2-4 percent Ta2Os have been

reported to be ferroelectric at room temperature

[32], Nonstoichiometry in W03 , by way of removal

of oxygen from the lattice, apparently decreases the

Tt of the two low-temperature transitions and brings

about changes in the electrical properties [18]. The room-temperature monoclinic modification of

WO3 transforms to an orthorhombic structure re- versibly at '"‘-'580 K [24, 33-37]; DTA peak [31]

and resistivity discontinuity have been noted at this

temperature [24]. Roth and Waring [26] obtained the

orthorhombic form at room temperature on heating

the stabilized monoclinic W03 (containing 2%

Nb2Oe) to -—'1170 K and rapid cooling. The orthorhombic—^tetragonal transition in WO3 is well established and investigated by many workers [26, 33, 34,

36, 38-41]; x-ray diffraction, DTA and resistivity

measurements indicate the Tt to be 1010 K (accompanied by thermal hysteresis). The tetragonal phase

is related to the ReOa structure and doping with

Nb lowers the Tt considerably [26]; according to Roth and Waring [26], the (Nb) stabilized monoclinic phase of WOa transforms to the tetragonal

modification (without going through the triclinic,

monoclinic and orthorhombic structures as in the

case of pure WO3) at ^1010 K and this monoclinic—

tetragonal transition has been found to be reversible

below 1025 K.

DTA [35, 38], heat capacity and resistivity [39]

studies indicate transitions at ~1170 and ~1500 K

in pure WOa, but no structural changes have been

observed (fig. III.l) [26, 35, 38]; the high temperature

phases remain tetragonal. A transformation from

tetragonal to cubic structure of the ideal ReOa type

(which seems quite logical) has not been observed

in pure WOa below the melting point (1700 K).

Attempts to prepare the cubic phase by doping have

not been successful.

WO3 is diamagnetic [18, 42] in the range 77 to 292

117
250 K 293-303 K 600 K Monoctinlc Triclinic (77-233 K) 230 K (233-290 K) 290 K

(Ferroelectric) AH = 39cal/mole AH=20 cal/moie

Tetragonal ~1500 K

Monoclinic (290-583 K)

Tetragonal AH=120 cal/moie

560 K AH=330cal/mole

1170 K

~t170 K AH = 2 80 cal/moie

Orthorhombic (580-1010 K)

Tetragonal

(1010-1700 K)

(Antiferroelectric ?)

Figure III.l. Phase relations in WD8.

K; feeable paramagnetism has been noted at -~1.3 K

in the oxygen deficient WO3 [18]. It is semiconducting at low temperatures, but exhibits semimetallic

behavior at higher temperatures. As mentioned earlier, electrical characteristics show anomalous behavior at the transition points. The mechanism of

electrical conduction in stoichiometric and oxygen

deficient WO3 has been investigated in detail by

Sienko and co-workers [18, 24, 25],

W03 in combination with other metal oxides form

very interesting mixed oxides called Tungsten bronzes.’ The physical properties of these bronzes have

been examined in great detail and discussed in the

literature [43-46].

Tungsten oxides

Oxide and description of the study

Data Remarks and inferences References

wo2

Crystal structure and

electrical properties.

Monoclinic; space group, P2i/c;

Z = 4; a = 5.5607 ±0.0005 A; b =

4.9006 ±0.0005 A; c = 5.6631 ±

0.0005 A; j8 = 120.44°. Metallic

behavior; p (4.2 K)=2.0X10-4 12cm; p (300 K) = 2.9 XlO-3 12cm.

The structure is that of a dis- torted rutile; metal-metal bonding seems to exist and the electrical properties are consistent

with this assumption.

[2, 5, 13,

14].

w18o49

Crystal structure. Monoclinic; a = 18.28 A; 6=3.775

A; c = 13.98 A; 0 = 115.14°. Decomposes below 860 K but stable up to 1970 K. Detailed data are

lacking.

[4, 5, 7].

Crystal structure and

electrical properties.

Monoclinic; a = 16.74 A; 6=4.019

A; c = 14.53 A; 0 =95.45°.

Decomposes below 760 K but

stable up to 1820 K. Metallic

behavior; p (80 K)~5X10~4 12cm; p-T plot exhibits a maximum at 270 K. a (90 K)~ — 15 pV/°C; a (240-320 K) 30

pV/°C. n«4XlO*Vcm3

.

The structure appears to be re- lated to WO2 and metal-metal

bonding seems to be present, p anomaly at ~270 K is not ex- plainable at present.

[4, 5, 7, 17,

18].

118
Tungsten oxides—Continued

Oxide and description of the study

Data Remarks and inferences References

W50O148 (W02.9#)

Crystal structure Monoclinic; space group, P2/c; a =

11.90 ±0.02 A; 6 = 3.826±0.012

A; c = 59.64 ±0.06 A; 0=98.4°. A

high -temperature polymorph with

a unit cell having the formula

W26O74 is found to be stable at ~1520 K. Detailed data are

lacking.

The structure appears to be re- lated to that of WO3 .

[ 8].

W40O119 (WOi.m)

Crystal structure Monoclinic; space group, P2i/m;

Z = 4; a = 7.354 ±0.005 A; 6 =

7.569 ±0.005 A; c = 3.854 ±0.005

A; /3=90.6°. Detailed properties not known.

[8].

wo8

Crystal structure and Range 77-233 K: Monoclinic; a = WO3 is the only oxide which shows [1, 5, 7, 12, x-ray studies. 5.27 A; 6 = 5.16 A; c = 7.67 A; 0 = a wide variety of crystallographic 19-22, 26, 91.72°. This phase can be stabil- transitions in the easily attain- 27-29, 33-

ized at room temperature by

quenching a solid solution of 2 %

Nb2C>6 in W08 from 1500-1660 K.

Monoclinic to triclinic transition

at 251 K (heating); hysteresis

~15°. The stabilized solid solu- tion transforms to a tetragonal

form at ~1010 K; transition re- versible if the sample is heated

able ranges of temperatures;

many of the transitions are of

first order. Surprisingly, the tetra- gonal—»cubic transition has not been noticed either in the pure or doped or nonstoichiometric samples; neither it has been possible to stabilize the cubic phase at any temperature. The effect of

41].

below 1015 K and if heated to ~1170 K, tetragonal phase

transforms irreversibly to an orthorhombic modification. Range 233-290 K: Triclinic; a = 7.30 A; 6 = 7.52 A; c = 7.69 A; a = 88.83°; 0=90.92°; 7=90.93°. Triclinic phase not encountered in the

solid solutions of WO3 with

Nb20s at any temperature. Triclinic to monoclinic transition at ~293-303 K (heating); hysteresis, ~3-13°. Reverse transition (mono.—»tric.) noted on grinding

the specimen at room temperature. Removal of oxygen from the

lattice of pure WO* reduces the Tt of the two low -temperature

transitions. Range 290-583 K: Monoclinic;

space group, P2i/n; Z = 8 ; a =

7.306 ±0.001 A; 6 = 7.540 ±

0.001 A; c = 7.692 ±0.001 A;

nonstoichiometry is pronounced

in the case of low-temperature

transitions. Data is lacking for the high -temperature transitions and also the effect of pressure on

the transitions.

119
Tungsten oxides—Continued

Oxide and description of the study

Data Remarks and inferences References

/3 =90.881 ±0.005°. Solid solu- tion with Nb206 retains the

monoclinic structure but slightly

changes the lattice parameters.

Monoclinic to orthorhombic

transition at ~600 K (heating);

thermal hysteresis ~40-50°.

Range 580-1010 K: Orthorhombic;

the lattice parameters remain

the same as in the monoclinic

phase (except for the effect of thermal expansion) and /3 ap- proaches 90° at « Tt

. According

to Rosen et al. [35], the b parameter shows an anomaly at Tt in WO3

. The orthorhombic form can

be stabilized at room temperature by heating the 2 % Nb206

solid solution to 1170 K and

rapid cooling. Orthorhombic to tetragonal transition at ~1010 K;

hysteresis ~10°. The solid solu- tion (of 2 % Nb206) transforms to tetragonal form at a lower temperature.

Range 1010-1700 K: Tetragonal;

space group, P4/nmm;

Z =2; a = 5.25 A; c = 3.91 A. A

transition takes place at ~1170

K in WOs but x-ray studies do not indicate any change

in the crystal symmetry. Roth

and Waring [26] point out that

it is possible that the tetra- gonal unit cell has a doubled c

axis below 1170 K and only

above this temperature does the

powder pattern yields the correct unit cell. Melting point of WO3 is 1700 K and it melts under its own equilibrium oxygen partial pressure [12, 41, 47].

Magnetic properties. Diamagnetic in the range 77-300 K;

X m (=300 K) = -21.0±1.7X

10_a. Below 77 K, oxygen defi- cient WO* shows decrease in xm

eventually switching to a feeble paramagnetism (xm = 10 X 10“# cgs u.) at 1.28 A.

Both stoichiometric and nonstoichiometric samples exhibit similar magnetic behavior.

[18, 42].

120
Tungsten oxides—Continued

Oxide and description of the study

Data Remarks and inferences

Electrical properties. Semiconductor behavior; p (200 K)

~103 12cm; p (400 K)~5 !2cm;

a (300 K) 0.5 mV/°C; pH

(400 K)~10 cm2 /V s. Discontinuities in p, a and ph are en- countered near the transition temperatures of the various

phases encountered with the

associated hysteresis. In the range 400-900 K, p is very small

( 0. 1-0.5 Ocm) and shows

slight increase with rise in temperature (quasi-metallic behavior). Sawada [31] has reported

a resistivity anomaly at ~1500 K

in WO3 ; this may correspond to a change in the mechanism of con- duction or to a new modification.

Details are not known. The

mechanism of electrical conduction in the range 100-500 K is interpreted in terms of various

polaron theories.

The electrical properties show complex behavior; the actual values

of p, a and ph vary from sample

to sample and depend on the method of preparation, stoichiometry etc. However, the qualitative

features are reproducible and p-T

behavior provides a good tool for indicating the phase transitions. Electrical data on the doped WO3

(where various high- and low- temperature phases have been

stabilized) are lacking.

Optical and dielectric properties.Absorption edge is at 2.7 eV. Re- moval of oxygen from the lattice causes an increase in absorption

at all wave lengths. Absorption

edge shows an anomalous shift at the ~250 K transition. In the range 270-970 K, absorption

edge shows a red shift (~10-4 eV/K); near 1010 K transition, a sudden red shift is noted for a

polarized light. The static dielectric constant at room temperature is ~103-104 (at 1 MHz) and

decreases to ~102 [50] at -~250

K. According to Matthias and

Wood [28, 30], the low-temperature monoclinic phase (in the range 77-233 K) is ferroelectric. On the basis of structural studies,

Kehl et al. [38] concluded that

the high -temperature tetragonal

may be antiferroelectric. Levine

et al. [32] reported that WO3 containing 2-4% TajOs exhibits

ferroelectricity at room temperature whereas Roth and Waring

[26] do not find this behavior in the 2% NI^Ob doped sample.

Accurate dielectric constant data

and the study of ferroelectric and

antiferroelectric properties could not be carried out because of the high conductivity effects in pure W03

.

References

[18, 23-25,

31, 48].

[18, 28, 30,

38, 49, 50].

121
Tungsten oxides—Continued

Oxide and description

of the study

Data Remarks and inferences References

DTA studies. All the crystallographic transitions are associated with DTA anomalies; in addition, transitions are

also indicated at ~1170 and

~1500 K (fig. III.l). Hysteresis

is noted in all cases. AH (cal/ mol): Monocl. (low -temperature)

—triclinic, 39; tricl.—»monocl., 20; monocl.—»orthorh., 330; orthorh. —tetra., 450; tetra.—»tetra., 280; 1500 K transition, 120. Removal

of oxygen from the lattice decreases Tt as well as the AH of the two low-temperature transitions. Data are not available for other transitions.

The nature of the high -temperature

transitions is not known in detail; the transitions seem to be

first order.

[18, 31].

References

[1] Hagg, G., and Magneli, A., Arkiv Kemi Mineral. Geol. 19A, No. 2 (1944).

[2] Magneli, A., Arkiv Kemi Mineral. Geol. 24A, No. 2

(1946).

[3] H., Braekken, Z. Krist. 78, 484 (1931).

[4] Magneli, A., Arkiv Kemi 1, 223 (1949); 1, 513 (1950).

[5] Magneli, A., Andersson, G., Blomberg, B., and Kihlborg,

L., Anal. Chem. 24, 1998 (1952).

[6] Glemser, O., and Sauer, H., Z. anorg. allgem. Chem.

252, 144 (1943).

[7] Phillips, B., and Chang, L. L. Y., Trans. AIME 230,

1203 (1964).

[8] Gebart, E., and Ackermann, R. J., Inorg. Chem. 5, 136

(1966).

[9] Neuberger, M. C., Z. Krist. 85, 232 (1933).

[10] Hagg, G., and Schonberg, N., Acta Cryst. 7, 351 (1954).

[11] St. Pierre, G. R., Ebinara, W. T., Pool, M. J., and

Speiser, R., Trans. AIME 224, 259 (1962).

[12] Chang, L. L. Y., and Phillips, B., J. Am. Ceram. Soc.

52, 527 (1969).

[13] Rogers, D. B., Shannon, R. D., Sleight, A. W., and

Gillson, J. L., Inorg. Chem. 8, 841 (1969).

[14] Israelsson, M., and Kihlborg, L., Mat. Res. Bull. 5, 19

(1970).

[15] Chang, L. L. Y., Scroger, M. G., and Phillips, B., J. Less-Comm. Metals 12, 51 (1967).

[16] Nygren, M.t and Israelsson, M., Mat. Res. Bull. 4, 881

(1969).

[17] Magn61i, A., Nature (Lond.) 165, 356 (1950).

[18] Berak, J. M., and Sienko, M. J., J. Solid State Chem. 2,

109 (1970).

Andersson, G., Acta Chem. Scand. 7, 154 (1953).

Tanisaki, S., J. Phys. Soc. Japan 15, 573 (1960).

Loopstra, B. 0., and Boldrini, P., Acta Cryst. 21, 158

(1966).

[22] Loopstra, B. O., and Rietveld, H. M., Acta Cryst. 25B,

1420 (1969).

[23] Tanisaki, S., J. Phys. Soc. Japan 15, 566 (1960).

[24] Crowder, B. L., and Sienko, M. J., J. Chem. Phys. 38,

1576 (1963).

[25] Crowder, B. L., and Sienko, M. J., Inorg. Chem. 4, 73

(1965).

[19 20

21

[26] Roth, R. S., and Waring, J. L., J. Res. Nat. Bur. Stand.

(U.S.), 70A (Phys. and Chem.), No. 4, 281-303 (JulyAug. 1966).

[27] Foex, M., Compt. Rend. (Paris) 220, 917 (1945); 228,

1335 (1949).

[28] Matthias, B. T., and Wood, E. A., Phys. Rev. 84, 1255

(1951).

[29] Hirakawa, K., J. Phys. Soc. Japan 7, 331 (1952).

[30] Matthias, B. T., Phys. Rev. 76, 430 (1949).

[31] Sawada, S., J. Phys. Soc. Japan 11, 1237 (1956).

[32] Levine, S., Corwin, R., and Blood, H. L., Bull. Am. Phys.

Soc. Ser. II, 1, 255 (1956).

[33] Wyart, J., and Foex, M., Compt. Rend. (Paris) 232,

2459 (1951).

[34] Ueda, R., and Ichinokawa, T., Phys. Rev. 82, 563 (1951).

[35] Rosen, C., Banks, E., and Post, B., Acta Cryst. 9, 475

(1956).

[36] Perri, J. A., Banks, E., and Post, B., J. Appl. Phys. 28, 1272 (1957).

[37] Gado, P., Magy Fiz. Folyoirat 10, 347 (1962).

[38] Kehl, W. L., Hay, R. G., and Wahl, D., J. Appl. Phys.

23, 212 (1952).

[39] Sawada, S., Phys. Rev. 91, 1010 (1953).

[40] Fiegel, L. J., Mohanty, G. P., and Healy, H., J. Chem.

and Engg. Data 9, 365 (1964).

[41] Chang, L. L. Y., Schroger, M. G., and Phillips, B.,

J. Am. Ceram. Soc. 49, 385 (1966).

[42] Sienko, M. J., and Banerjee, B., J. Am. Chem. Soc. 83,

4149 (1961).

[43] Sienko, M. J., in Non-stoichiometric Compounds, Adv.

Chem. Ser. 39, 224 (1963).

[44] Wadsley, A. D., in Non-stoichiometric Compounds, Ed.

L. Mandelcorn, (Academic Press, New York, 1964).

[45] Dickens, P. G., and Whittingham, M. S., Quart. Rev.

(Lond.) 22, 30 (1968).

[46] Rao, C. N. R., and Subba Rao, G. V., Phys. stat. Solidi

(a) 1, 597 (1970).

[47] Levin, E. M., J. Am. Ceram. Soc. 48, 491 (1965).

[48] Sawada, S., and Danielson, G. C., Phys. Rev. 113, 803

(1959).

[49] Iwai, T., J. Phys. Soc. Japan 15, 1596 (1960).

[50] Hirakawa, K., and Ueda, I., Mem. Fac. Sci. Kyusyu

Univ. Bl, 112 (1954).

122
III. 5. Rhenium Oxides

In the Re-0 system, ReQ2 , ReOs, and Re2t)7 are the oxides that are well characterized and examined

in detail. A tetragonal Re2 Q6 has been reported by

Trabalat et al. [1]. The di- and sesquioxides are

stable ionic compounds whereas Re2C>7 is mostly

covalent and has a low melting point (~490 K).

Re02 is polymorphous and when synthesized below

570 K, the structural modification (a) is that of

monoclinic Mo02 type [2, 3]; above ~570 K, this

oxide transforms irreversibly to an orthorhombic

form (/3) with a structure characterized by zig-zag

chains of Re atoms propagating along the c axis

of the unit cell. It is also possible to directly synthesize the high temperature form [2, 3] and the material is stable in the range 570 to 1320 K. Magnetic

susceptibility studies [4] indicate that both the forms

of Re02 are Pauli paramagnetic. Resistivity data

indicate that /3-Re02 is metallic [3] and most probably the low temperature modification also is metallic. Monoclinic Re02 forms rutile type solid solutions with V02 and Mo02 [5]. Re2 05 is prepared by precipitation from aqueous

solutions [1] and has a tetragonal structure. It appears to be stable up to 470 K, but decomposes to Re2C>7 and Re02 in vacuum at ^ 520 K. Detailed

properties are not known at present.

Re(>3 is cubic and no phase transformations are known in the range 1 to 300 K [2, 6-8]; the material

disproportionates to Re02 and Re20? in the neighborhood of 670 K. Re03 has a red metallic luster and

magnetic susceptibility [6, 8], electrical [6, 8], optical [9], NMR [10], magneto-thermal oscillation [II],

and de Hass-van Alphen effect [12] studies confirm

the typical metallic behavior of ReOs.

The simple cubic structure of ReOa is important

from the view point of understanding the structures

of perovskites (see fig. III.2) [13]. Goodenough [ISIS] has explained the metallic behavior of ReOa by

constructing a one-electron energy band diagram.

Recent band structure calculations on ReOa have

proved that the essential features of the Goodenough’s model are correct [16, 17].

Re2C>7 is orthorhombic at room temperature [18-

20]. It is yellow in color and is supposed to have a polymeric structure in the solid state; in the liquid

and vapor states, monomeric species have been identified [19, 21]. Infrared and Raman spectra of this

material have been studied in detail [21, 22] but

other properties are not known.

Figure XII.2. Cubic ReOa structure. Each metal atom is at the center of an octahedran of oxygen atoms. This

structure is closely related to the perovskite structure which is obtained by

insertion of a large cation in the center of the cube shown.

Rhenium oxides

Oxide and description of the study

Data Remarks and inferences References

ReOj

Crystal structure and Low-temperature for (T <570 K): The low temperature form is [2, 23]. x-ray studies. Monoclinic: space group, P2i/c;

Z = 4; a =5.562 A; 6=4.838 A;

c — 5.561 A; /3 = 120.87°. High

temperature form (T>570 K):

orthorhombic; space group, Pbcn; Z = 4; a =4.810 A; 6 =

5.643 A; c = 4.601 A.

closely related to rutile structure.

123
Rhenium oxides—Continued

Oxide and description of the study

Data Remarks and inferences References

Electrical properties. /S-ReOj:

P (4.2 K)«1.2X10“6 ficm;

p (300 K) » 1.0 X 10-4 ficm.

Metallic behavior is indicated; data

for a-Re02 are not available.

[3].

RejOj

Crystal structure. Tetragonal; Z — 4; a = 5.80 A; c =

12.87 A. The substance has been obtained

from aqueous solutions; exact

purity and characterization doubtful; other properties not known.

[1].

ReOs

Crystal structure. Cubic, space group, Pm3m; Z = 1; a = 3.7474 ±0.0003 A. This simple cubic structure is closely related to the perovskite

structure since the latter is obtained by insertion of a large

cation in the center of the cube

of the ReOa structure (fig. 1).

[8].

Magnetic and electrical properties.

Xm (300 K) =74.5X10-6 emu.

p (100 K)«6X10~7 item;

p (300 K)«lXl0"6 item.

Weak paramagnetism is indicated

contrary to the earlier data [6].

[6, 8].

Optical and NMR studies. Absolute reflection measurements

on ReOa give plasma edge at 2.1 eV; interband transitions dominate the optical spectrum above

the plasma edge, m* (calc.)~

0.86 m0 . k - -(0.25 ±0.02)%.

The data confirm the metallic be- havior of ReOs in which the con- duction bands are predominantly

d like.

[9, 10].

Band structure of ReO». LCAO, APW, and Slater-Kostiner

methods give a band structure

in essential agreement with the

band diagram proposed by Goodenough [13-15] and in reasonable

agreement with the optical and

Fermi surface data. The conduction bands are predominantly d

like.

Honig et al. [17] point out that the

band structure scheme will also be

useful in the characterization of the band structure for perovskites

which have a closely related

structure.

[16, 17].

RejO? T = 300 K; Orthorhombic; space group P2i2i2i; Z = 8; a = 12.508 A; b = 15.196 A; c = 5.448 A.

The structure consists of strongly

distorted ReOe octahedra and

fairly regular ReO« tetrahedra which are connected through

corners to form polymeric double

layers in the ac plane. The

double layers have only van der Waals contacts to neighboring

ones. RejO? is one of the few

known examples where metal

atoms of the same oxidation state occur with the coordination numbers 4 and 6 in the same structure. In its structure and bond

[20].

124
Rhenium oxides—Continued

Oxide and description

of the study

Data Remarks and inferences References

properties, Re207 represents an

intermediate between the polymeric oxides MoOg and WOg and

the more covalently bonded OsO*,

which forms a molecular

structure.

IR of Re207 . Several bands in the region 400-

1010 cm-1 are noted and assigned

to various stretching and bending

vibrations.

[21].

References

[1] Tribalat, S., Dalafosse, D., and Piolet, C., Compt. Rend.

(Paris) 261, 1008 (1965).

[2] Magneli, A., Acta Cryst. 9, 1038 (1956); Acta Chem.

Scand. 11, 28 (1957).

[3] Rogers, D. B., Shannon, R. D., Sleight, A. W., and

Gillson, J. L., Inorg. Chem. 8, 841 (1969).

[4] Gibart, P., Bull. Soc. Chim. France 444 (1967).

[5] Marinder, B.-O., and Magneli, A., Acta Chem. Scand.

11, 1635 (1957).

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[7| Gukova, Yu. Ya., and Emolaev, M. I., Russian J. Inorg.

Chem. (English Transl.) 13, 777 (1968).

[8] Quinn, R. K., and Neiswander, P. G., Mat. Res. Bull.

5, 329 (1970).

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(1968).

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(1969).

[12] Marcus, S. M., Phys. Letters 27A, 584 (1968).

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(a) 1, 597 (1970).

[14] Goodenough, J. B., J. Appl. Phys. 37, 1415 (1966).

[15] Goodenough, J. B., Czech. J. Phys. 17B, 304 (1967).

[16] Mattheis, L. F., Phys. Rev. 181, 987 (1969).

[17] Honig, J. M., Dimmock, J. O., and Kleiner, W. H.,

J. Chem. Phys. 50, 5232 (1969).

[18] ^ ilhelmi, K.-A., Acta Chem. Scand. 8, 693 (1954).

[19] Krebs, B., and Muller, A., Z. Naturforsch. 23b, 415

(1968).

[20] Krebs, B., Muller, A., and Beyer, H. H., Inorg. Chem.

8, 436 (1969).

[21] Ulbricht, K., and Kriegsmann, H., Z. Chem. 7, 244

(1967).

[22] Beattie, I. R., and Ozin, G. A., J. Chem. Soc. (Lond.)

A2615 (1969).

[23] Magnlli, A., Andersson, G., Blomberg, B., and Kihlborg,

L., Anal. Chem. 24, 1998 (1952).

III. 6. Osmium Oxides

The dioxide, 0s02 and the tetroxide, Os04, have

been studied exhaustively. Os02 forms lustrous yellowish brown crystals which decompose at T>770

K. Os04 is a covalent solid and soluble in nonpolar

solvents; the crystals are colorless with a low melting

point (~310 K) and high vapor pressure.

Os02 has a tetragonal rutile structure [1-4] and

exhibits slight temperature dependent magnetic sus- ceptibility [3]. It has a very low room-temperature

resistivity, positive temperature coefficient of re- sistivity and low negative Seebeck coefficient [1, 3] indicative of metallic behavior. No phase transitions are known in Os02 in the temperature range 77 to 500 K.

Thiele and Woditsch [2] report two forms (black

and brown) of Os02 ; both have tetragonal symmetry, but the lattice parameters and x values differ significantly. The lattice parameters of the brown

form are identical with the data reported by other

workers on single crystal Os02 [3, 4]. It is possible

that the black form of Os02 reported by Thiele

and Woditsche is slightly nonstoichiometric or impure.

OsO* at room temperature is a solid and has a monoclinic symmetry [5]. It is a covalent compound

(mp, 310 K; bp, 374 K) and is soluble in polar

solvents. The molecule is tetrahedral in vapor state. Infrared and Raman studies of 0s04 is nonaqueous

solvents have been carried out by various workers

[6, 7]; other physical properties are not known.

125
Osmium oxides

Oxide and description of the study

Data Remarks and inferences References

Os02

Crystal structure. T = 300 K: Tetragonal; space

group, P42 /mnm; Z = 2; a =

4.5000 ±0.0003 A; c = 3.1830 ±

0.0009 A.

Recent single crystal study; the

values are in agreement with the data of earlier workers.

[4).

Magnetic properties. Xii (X10-6 emu) is 311, 220, and

204 at 90, 195, and 291 K re- spectively. Greedon et al. [1] report a temperature independent

susceptibility (77-500 K) ofxM«

120 X10-6 emu.

The values of Thiele and Woditsch

[2] have been corrected for the diamagnetic contribution. These

authors argue for the presence of

a magnetic ground state for Os4+ in Os02 .

[2].

Electrical properties. p (4.2 K) =3.2 X10"7 ficm; p (300

K) =6.0X10-5 (km. a (~300 K)

~1 pV/K (negative); no Hall

voltage could be measured.

The data indicate the typical metallic nature of Os02 ; estimated

carrier concentrations are ~1022-

10 23 /cm3

. p, (calc.) ~1.5-15 cm2 /V s; m* «8 mo. The model

proposed by Goodenough [8] and

Rogers et al. [3] seem to explain

the observed behavior.

[1. 3].

O8O4

Crystal structure T = 300 K: Monoclinic; space

group, C2/c; Z = 4; a =9.379

0.005 A; b =4.515 ±0.002 A; c =

8.632 ±0.003 A; /3 = 116±0.05°.

The structure can be described as cubic closest packing of oxygen

atoms, with osmium in tetra- hedral holes. The intermolecular

0-0 distances exceed 2.98 A and

are consistent with weak inter

molecular forces indicated by the

high vapor pressure and low

melting point.

[5].

Infrared studies on solid 0s04. Bands at 965 and 959.5 cm-1 are noted. IR of Os04 in CCh solution and

Raman studies are also consistent with the behavior in the

solid state.

[ 6, 71.

References

[1] Greedon, J. E., Willson, D. B., and Haas, T. E., Inorg.

Chem. 7, 2461 (1968).

[2] Thiele, G., and Woditsch, P., J. Less-Comm. Metals 17,

459 (1969).

[3] Rogers, D. B., Shannon, R. D., Sleight, A. W., and

Gillson, J. L., Inorg. Chem. 8, 841 (1969).

[4] Boman, C.-E., Acta Chem. Scand. 24, 123 (1970).

[5] Ueki, T., Zalkin, A., and Templeton, D. H., Acta Cryst.

19, 157 (1965).

[6] Krebs, B., and MuUer, A., Z. Chem. 7, 243 (1967).

[7] McDowell, R. S., and Goldblatt, M., Inorg. Chem. 10,

-625 (1971) and references cited therein.

[8] Goodenough, J. B., Bull. Soc. Chim. France 4, 1200

(1965).

126
III. 7. Iridium Oxides

The phase study of the Ir-0 system by Cordfunke

and Meyer [1] and McDaniel and Schneider [2, 3] indicate that Ir02 is the only condensed oxide phase

stable in an air environment. Ir02 has a tetragonal

rutile structure and no phase transformations are known in the range 4.2 to 1000 K [4-8].

Magnetic properties of Ir02 have not yet been

investigated. Electrical resistivity studies [5-8] confirm the metallic nature of the material; as pointed

out by Butler and Giilson [8], the simple one-band

model of Rogers and co-workers [7] may not explain

the observed behavior. Ryden et al. [6] have found

that electron-phonon and electron-electron interband scattering mechanism accounts for the observed temperature dependence of resistivity.

Ir02 forms complete solid solutions with Ru02 and to a limited extent with Ti02 and Sn02 [2, 3]; these have the rutile structure.

Iridium oxides

Oxide and description Data Remarks and inferences of the study

Ir02

Crystal structure and T = 298 K: Tetragonal; space TEC is positive in Ir02 ; anisotropy

electrical properties. group, P42 /mnm; Z = 2; a = is evident. The compound is 4.4980 ±0.0003 A; c = 3.1543± metallic and most probably Pauli

0.0002 A. TEC (X10“

6

/°C):

|| e

: 1.7; ||“: 3.8. p (4.2 K)~1.5X

10~8 Ocm; p (298 K)~3.0X10-5 Ocm. f?H (77 K) ~ —3.12; juh

( 77 K) =130 cm2 /V s; RH (300

K) ~ -2.60; mh (300 K)=7.5

cm*/V s.

paramagnetic.

References

[1] Cordfunke, E. H. P., and Meyer, G., Rec. Trav. Chim.

81, 495 (1962); 81, 670 (1962).

[2] McDaniel, C. L., and Schneider, S. J., J. Res. Nat. Bur.

Stand. (U.S.), 71A (Phys. and Chem.), No. 2, 119-123

(Mar.-Apr. 1967).

[3] McDaniel, C. L., and Schneider, S. J., J. Res. Nat. Bur.

Stand. (U.S.), 73A (Phys. and Chem.), No. 2, 213-219

(Mar.-Apr. 1969).

[4] Hazony, Y\, and Perkins, H. K., J. Appl. Phys. 41, 5130

(1970).

[5] Ryden, W. D., Lawson, A. W., and Sartain, C. C., Phys.

Letters 26A, 209 (1968).

[6] Ryden, W. D., Lawson, A. W., and Sartain, C. C., Phys. Rev. Bl, 1494 (1970).

[7] Rogers, D. B., Shannon, R. D., Sleight, A. W., and

Giilson, J. L., Inorg. Chem. 8, 841 (1969).

[8] Butler, S. R., and Giilson, J. L., Mat. Res. Bull. 6, 81

(1971).

III. 8. Platinum Oxides

Although the literature on the Pt-0 system is extensive due to the catalytic and electrochemical

applications of the platinum oxides, many of them

are poorly characterized. Recent careful study by

Muller and Roy [1] has revealed the existence of

a-Pt02 , /3-Pt02 , and Pt304 as the stable oxides in

the Pt-0 system. Contrary to earlier reports [2-6],

the existence of PtO and Pt303 are doubtful.

Pt304 is cubic [1]. a-Pt02 has a chain structure and is hexagonal; /3-Pt02 is orthorhombic and has a distorted rutile structure [1, 7, 8], a-Pt02 transforms

127
to 0-form either by heating to 970 K at 3 kbar

pressure in the presence of KCIO3 or by heating to 1370 K at 65 kbar pressure [7]. DTA, TGA, and

high temperature x-ray studies show that 0-PtO2 decomposes to the platinum metal and oxygen at ~860 to 920 K [7, 8]. jS-PtCL appears to be a semiconductor [7, 9]. The

physical properties of a-Pt02 and Pta04 are not

known. Muller and Roy [10] have found that solid

solutions of the form Cui_xPtxO possessing the

tetragonal structure can easily be formed. The ex- trapolated lattice parameters for the hypothetical

TtO’ do not quite agree with the values reported by

Moore and Pauling [2].

Platinum oxides

Oxide and description of the studyData Remarks and inferences References

Pt3 0«

Crystal structure. Cubic; space group, Pm3m; Z = 2; a = 5.585 A. — [1].

a-PtCh

Crystal structure. Hexagonal; space group, C6/mmm;

Z = 1; a =3.10 A; c = 4.29-4.41 A. Samples are poorly crystallized;

Pt-O-O-Pt-O-O chain structure

in the c direction.

[1].

0-PtO2

Crystal structure and

electrical resistivity.

Orthorhombic; space group, Pnnm;

Z = 2; a =4.487 ±0.0005 A; 6 =

4.536±0.0005 A; c = 3.137±

0.0005 A. p (300 K) *10

6

ficm; £0=0.2 eV. Decomposes in air at ~860-920 K.

Distortion from the rutile struc- ture is small; semiconductor be- havior (in the range 4.2-300 K).

[1. 7, 9].

References

[1] Muller, O., and Roy, R., J. Less-Common Metals 16,

129 (1968).

[2] Moore, W. J., Jr. and Pauling, L., J. Am. Chem. Soc.

63, 1392 (1941).

[3] Suzuki, T., Z. Naturforsch. 12a, 497 (1957).

[4] Every, R. L„ J. Electrochem. Soc. 112, 524 (1965).

[5] Fryburg, G. C., J. Chem. Phys. 42, 4051 (1965).

[6] Shishakov, N. A., Andreeva, V. V., and Andruschenko,

N. K., Stroenie i Mekhanism Obrazovaniya Okisnykh

Plenok na Metallakh, Akad. Nauk. SSSR, Moscow,

1959, Chap. 6.

[7] Shannon, R. D., Solid State Commun. 6, 139 (1968);

7, 257 (1969).

[8] Schneider, S. J., and McDaniel, C. L., J. Am. Ceram.

Soc. 52, 519 (1969).

[9] Rogers, D. B., Shannon, R. D., Sleight, A. W., and

Gillson, J. L., Inorg. Chem. 8, 841 (1969).

[10]

Muller, O., and Roy, R., J. Less-Common Metals 19,

209 (1969).

III. 9. Gold Oxides

Although Shishakov [1] earlier reported the formation of hexagonal AU3O2 on the surface of heated

gold metal at 770 K, recent studies by Muller and

Roy [2] indicate AU2 O3 to be the only crystalline

oxide phase in the Au-0 system. Attempted preparation of Au2 0 has not been successful [3]. Au203 is cubic (a = 4.832 A; space group, P43m)

[2, 4], The physical properties are not known in

detail. The infrared band's of AU2O3 have been re- ported in the range 515-660 cm-1

. References

[1] Shishakov, N. A., Kristallografiya 2, 686 (1957).

[2] Muller, O., and Roy, R., Am. Ceram. Soc. Bull. 46, 881

(1967).

[3] Suzuki, T., J. Phys. Soc. Japan 15, 2018 (1960).

[4] Schwarzmann, E., and Gramann, G., Z. Naturforsch.

25b, 1308 (1970).

128
III. 10. Mercury Oxides

HgO is the well known oxide in the Hg-0 system

[1-3]. Vannerberg [4] reported the formation of two

different forms (a- and /3-) of the mercury peroxide,

Hg02; a-Hg02 has a rhombohedral structure whereas

/3-form is orthorhombic.

HgO: HgO exists in the red and yellow forms but

these are not polymorphs; they differ in the particle

size and give identical x-ray patterns [2], The stable

form is orthorhombic [1-3, 5, 6] and has a structure

with zig-zag -Hg-O-Hg- chains (Hg-O = 2.03 A);

there is only weak bonding between the chains.

Laruelle [5] has described a rhombohedral form of

HgO obtained by precipitation from aqueous solutions; it transforms irreversibly to the orthorhombic

form at ~470 K and at this temperature, the trans- formation is a slow process. HgO is diamagnetic

[7] and the diamagnetic susceptibility is almost the same for both the yellow and red forms. HgO decomposes at high temperatures to the metal; other

properties are not known.

Mercury oxides

Oxide and description of the study

Data Remarks and inferences References

HgO

Crystal structure and

properties.

Orthorhombic; space group, Pnma;

Z = 4; a =6.6129 ±0.0009 A; 6 =

5.5208 ±0.0007 A; c = 3.5219 ±

0.0005 A. A rhombohedral form

can be precipitated from solution and it transforms irreversibly to the orthorhombic modification at ~470 K; detailed structure not known. The red and yellow colors of HgO seem to be due to the

particle size effect. HgO is dia- magnetic; x (red form) = —0.221 X10-6 emu/g; x (yellow

form) = —0.2 16 X10-6 emu/g.

X increases in the range 298-573

K. x-T plot shows peaks at 323

(red form) and 387 K (yellow

form); the reason is not clear.

HgO has a zig-zag structure associ- ated with considerable covalency

in the Hg-0 bond. Detailed

properties are not known.

[1-3,5].

Infrared and Raman

spectral studies. Low frequency band is at 67 cm-1

; Raman bands at 331 and 550 cm-1

. The data are interpreted

in terms of the chain structure of HgO.

[8, 9].

HgO2

Crystal structure. a-form: rhombohedral; a =4.74 A; a ~90°. /3-from: orthorhombic;

space group, Pbca; Z = 4; a =

6.080 A; 6 = 6.010 A; c =4.800 A. Detailed data on the properties

are lacking.

[4].

References

[1] Zachariasen, W., Z. Physik. Chem. 128, 421 (1927).

[2] Roth, W. L., Acta Cryst. 9, 277 (1956).

[3] Aurivillius, K., Acta Chem. Scand. 10, 852 (1956); 18,

1305 (1964).

[4] Vennerberg, N. G., Arkiv for Kemi 13, 515 (1959).

[5] Laruelle, P., Compt. Rend. (Paris) 241, 802 (1955).

[6] Rooymans, C. J. M., Philips Res. Rep. Suppl. 1, 1 (1968).

[7] Mikhail, H., Hanafy, Z., and Salem, T. M., J. Chem.

Phys. 35, 1185 (1961).

[8] Edmond, D., and Armand, H., J. Chim. Phys. Physicochim. Biol. 65, 1030 (1968).

[9] Cooney, R. P. J., and Hall, J. R., Austr. J. Chem. 22,

331 (1969).

129
IV. Some Recent Studies

Titanium Oxides

Rao and co-workers [1] have recently found that pressure has no effect on the TuOj transition; this is consistent with

the band-crossing mechanism wherein the a

T and eT bands

cross as the c/a ratio increases. The very recent study of Marezio et al. [2] published in 1973 on the three phases of Ti 4C>7 has clearly established the

structural aspects of the transitions. The room temperature

()hase is metallic with an average valence of 3.5 for Ti. In the ow-temperature insulating nonmagnetic state, there is a separation into strings of Ti3+ and Ti4+ with the 3+ sites forming metal-metal bonds. In the intermediate phase, there

is no evidence for charge separation or long range order of bonds.

The atomic vacancies in TiO have been discussed by Goodenough [3]. ESR studies on Ti306 have been reported by Schlenker et

al. [5]. References

[1]

Viswanathan, B., Devi, S. U., and Rao, C. N. R., Pramana

1, 48 (1973).

[2]

Marezio, M., et al., J. Solid State Chem. 6, 213 (1973);

Phys. Rev. Letters 28, 1390 (1973).

[3]

Goodenough, J. B., Phys. Rev. 135, 2764 (1972).

[4]

Honig, J. M., Wahnsiedler, W. E., and Dimmock, J. O.,

J. Solid State Chem. 5, 452 (1972).

[5]

Schlenker, C., Buder, R., Schlenker, M., Houlihan, J. F., and Mulay, L. N., Phys. Stat. Solidii (b) 54, 247

(1972).

Vanadium Oxides

Recent studies of Honig and co-workers [1] on Vo. 99 Cro.01 O1.6 in the 200 to 900 K range indicate that the electrical anomaly is extrinsic and is caused by the coexistence of phases over a wide temperature range. If this be the case, much of the earlier arguments based on the I-M transition will be

invalid. Caruthers et al. [2] have recently calculated the band struc- ture of VO2 . The influence of atomic vacancies on the band structure of VO has been discussed by Goodenough [3]. For a recent ESCA study on V3Os, see [4], For recent data on V2O6 and Vn 02n-i, see [5]. Heat capacity studies on V„ Chn-i are given in [6].

Recent single crystal studies on V3Oi 3 [7] confirm a negative

volume change ( — 0.5%), a positive enthalpy change (-f-400

cal/mol) and a semiconductor-metal transition at 150 K.

There is no change in the crystal system.

Marezio et al. [8] have studied some structural aspects of Vi_* Cr* O2 in detail. References

[1] Honig, J. M., Chandrasekhar, G. V., and Sinha, A. P. B., Phys. Rev. Letts. 32, 13 (1974).

[2] Caruthers, E., Kleinman, L., and Zhang, H. I., Phys. Rev. B7, 3753 (1973).

[3] Goodenough, J. B., Phys. Rev. B5, 2764 (1972).

[4] Honig, J. M., Van Zandt, L. L., Board, R. D., and Weaver,

H. E., Phys. Rev. B6, 1323 (1972).

[5] Kachi, S., Kosuge, K., and Okinaka, H., J. Solid State Chem. 6, 258 (1973).

[6] McWhan, D. B., Remeika, J. P., Maita, J. P., Kinaka,

H. O., Kosuge, K., and Kachi, S., Phys. Rev. B7, 326

(1973).

[7] Saeki, J., Kimizuka, N., Ishii, M., Kawada, I., Nakamo,

M., Ichinose, A., and Nakahira, M., J. Cryst. Growth

18, 101 (1973).

[8] Marezio, M., McWhan, D. B., Remeika, J. P., and

Dernier, D. P., Phys. Rev. B5, 2541 (1972).

[9] Tewari, S., Solid State Commun. 11, 1139 (1972).

Nickel Oxides

Recent measurements [1] on highly pure NiO crystals gave

the hole mobility, /u<j/ =20-50 cm2/ Vs at 300 K; m * = 1.5m0; and <*f* = 1.6. Reference

[1] Spear, W. E., and Tannhanser, D. S., Phys. Rev. B7,

831 (1973).

Niobium Oxides

Recently, measurements of electrical properties of NbO in high magnetic fields have been reported by Honig et al. [1] who have interpreted the results in terms of a nearly free electron model in which holes and electrons contribute jointly to conduction process; magnetoresistance anomalies have also been examined by these workers.

Reference

[1] Honig, J. M., Wahnsiedler, W. E., and Ekland, P. C., J. Solid State Chem. 6, 230 (1973).

130

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Crystal Chemistry, Phase Transitions and Related Aspects

5. Publication Date May 1974

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C. N. R. Rao and G. V. Subba Rao

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A survey is made of the data describing the thermodynamics

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electrical, magnetic, and other properties which accompany phase

transitions are discussed-Nearly complete coverage of the literature

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17.

KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Crystal structure transformations ; critical data, transition metal oxides; electronic

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