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Crystal Structure

Transformations in

Binary Halides
NATIONAL BUREAU OF STANDARDS

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r rV>J t±'LC - ^ >- » £ C 10 3UNITED STATES DEPARTMENT OF COMMERCE • Peter G. Peterson, Secretary

U 5 1 3 NATIONAL BUREAU OF STANDARDS • Lawrence M. Rushner, Acting Director ko.-Hl ~ JMwoA. I^€,

1 m£-

Crystal Structure Transformations

in Binary Halides

C. N. R. Rao and M. Natarajan

Department of Chemistry

Indian Institute of Technology

Kanpur-16, India

NSRDS

NSRDS-NBS 41

Nat. Stand. Ref. Data Ser., Nat. Bur. Stand. (U.S.), 41, 53 pages (July 1972)

CODEN: NSRDAP

© 1972 by the Secretary of Commerce on Behalf of the United States Government

Issued July 1972

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C., 20402 (Order by SD Catalog No. C 13.48:41)- Price 55 cents Stock Number 0303-0964
Library of Congress Catalog Card Number: 78-186213
Foreword

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data and critical reviews of long-term interest to the scientific and technical community.

Lawrence M. Kushner, Acting Director

hi
Preface

Many solids undergo transformations from one crystal structure to another. Studies of such

phase transformations are of great value in understanding the nature and properties of solid

materials. Recent literature abounds in information on the phase transformations of solids and

it is often difficult to obtain relevant data or references on the transformations of any specific

solid. We, therefore, considered it worthwhile to collect the literature on the phase transformations

of various types of inorganic solids and present them in an organized manner.

In this and subsequent reviews are summarized the important data and conclusions regarding

the crystal structure transformations of a few series of inorganic solids. In doing so, we have

surveyed most of the material abstracted up to 1970 in Chemical Abstracts and Solid State

Abstracts. We have, however, not listed references to the entire published literature on the trans- formations of a given solid, but have presented only the crucial ones. We believe that all the

literature can be traced back through the references listed at the end of each section. In reporting

the data on the transformations of a solid we have indicated the methods employed as well as the

important data and conclusions from the study. For each solid we have given the crystal structure

data for the stable phase around room temperature and at atmospheric pressure making use of the

standard patterns of the NBS and ACA Crystal Data; we have indicated the temperature wherever

data are available.

The crystal structure transformations of binary halides form the subject matter of this review.

This will be followed by reviews on the transformations of binary oxides and other systems.

IV
Contents

Page Foreword Ill Preface IV

Symbols Employed 1 Introduction 1

1. Hydrogen halides 4

2. Alkali halides 8

3. Ammonium halides 15

4. Alkaline earth halides 24

5. Group IIIA halides 25

6. Group IVA halides 27

7. Group VA halides 30

8. 3d Transition metal halides 31

9. 4d Transition metal halides 34

10. Sd Transition metal halides 40

11. Rare-earth fluorides. 41

12. Inert gas halides 42

13. Halogens ; 43

V

Crystal Structure Transformations in Binary Halides*

C. N. R. Rao and M. Natarajan

A critical survey of the data describing crystal structure transformations in binary halides is compiled. Data on thermodynamic, crystallographic, spectroscopic and electronic properties are given for each transformation. Experimental techniques used to obtain the data are named and com- ments on the data are included in the tables. The literature is surveyed up to 1970. References have

been selected on the basis of their pertinence to the data which are cited and do not represent all the available literature. Key words: Binary halides; crystal structure transformation; electronic data; phase transformation: spectroscopic data; thermodynamic data; x-ray diffraction data. Symbols Employed

P = Pressure

T = Temperature in K

Tt — Transformation temperature

AT = Thermal hysteresis in a reversible trans- formation

PT = Transformation pressure

AHtr — Enthalpy change accompanying the

transformation

Cp = Heat capacity at constant pressure

AS — Entropy change

AV = Change in volume accompanying a transition

p = Density

a: = Volume thermal expansion coefficient

7) — Linear thermal expansion coefficient

e = Dielectric constant

Ps = Spontaneous polarization

Ec = Coercive field tan 8 = Dissipation factor

cr = Electrical conductivity

En = Energy of activation

ECation migration energy

Es = Formation energy of Schottky defects

atm = Atmosphere

cal = Calorie

eV = Electron volt DTA = Differential thermal analysis

IR = Infrared

Z — Number of molecules per unit cell NMR = Nuclear magnetic resonance NQR = Nuclear quadrupole resonance EPR = Electron paramagnetic resonance

Introduction

There has been increasing activity in recent years

in the area of crystal structure transformations in inorganic solids. A variety of experimental techniques has been employed to study these phase

Supported by Project G—77 of the National Bureau of Standards under the Special

International Programs.

transformations. Valuable information has been

obtained on the crystallography, thermodynamics,

and kinetics as well as on the changes in optical,

dielectric, magnetic and electrical properties

accompanying the transformations of a large number

of inorganic solids. The subject of phase transformations in inorganic solids has been reviewed by

Staveley [l], 1 Ubbelohde [2] and Rao [3, 4]. We shall

now briefly describe the general features of crystal

structure transformations.

Thermodynamics of Transformations. Classical

thermodynamics gives a simple treatment of the

equilibrium between two phases. The equilibrium

properties of each phase are defined by the Gibbs

free energy function, G, the pressure, P, and the

temperature, T. The G-P—T surfaces of two phases

are considered to be independent of each other and

in a first order phase transformation the surfaces

intersect at the transition point. If the transformation takes place from a low-temperature to a high

temperature form, there will be an entropy increase

and an associated change in volume during the

transformation. The classical thermodynamic ap- proach in describing phase transformations is inadequate since a variety of systems are known

to exhibit transformations occurring over a wide

range of temperatures. These systems show

anomalous changes in specific heats and specific

volumes and show evidence of “premonitory”

phenomena. The transition temperature, Tt , in

these transformations is taken to be that at which

the heat capacity or the coefficient of expansion

shows a maximum variation. This class of trans- formations which is not predicted by classical

thermodynamics has often been referred to as “gradual, smeared or diffuse” transformations.

Essentially, the difference between these transformations and the first order transformations described by classical thermodynamics lies in the

nature or/and the magnitude of the discontinuity

1 Figures in bracket indicate the literature references that appear at the end of each section.

1
of the thermodynamic energy functions and their

derivatives. In first order transformations, the difference in

the free energies of the two phases is given by

0 = AC = G'z — Gi = A

— TAS 4- PAV, where AE is the difference in internal energies of the two phases;

if PAV in a transformation is small, one can compare the stabilities of the two phases in terms of

their Helmholtz free energies: 0 = AA = AE — TAS.

According to this equation, the transformation

results from the compensation of the lattice energy

difference by the entropy difference at Tt

. At Tt , the free-energy curves intersect each other at AG = 0 . These phase transformations involve

step-wise increase in energy with increase in temperature and show discontinuity in the energy as

well as in many other properties of the system

at T(. Many instances are known where phase trans- formations are induced by application of pressure;

in these transformations the PAV term is very

important. Wherever there is a decrease in volume

during a transformation, pressure would favor such

a transformation and the thermodynamic variables

are simply correlated by the Clausius-Clapeyron

equation. Since the phase produced by the application of pressure will have a lower volume, the

decrease in volume is compensated by an increase

in the AE or in the coordination number.

In the second- (or higher) order transformations,

energy increases gradually with temperature until

the rate of increase of energy sharply falls. As a

result of this, discontinuity is found in specific

heats or specific volumes. Since lambda-shaped

specific heat curves are generally obtained in these

transformations, these are also termed lambdapoint transformations. Second- or higher-order

transformations occur over a temperature interval

and are generally associated with increase in dis- order with rising temperature. If a solid has perfect

order at absolute zero of temperature, a rise in

temperature disturbs the order of the structure

and with progressive increase in temperature, the

structure gets more and more distorted until the

transformation temperature Tt is reached. One

can introduce an order parameter £ which is equal

to unity at absolute zero of temperature and becomes zero at Tt

. Although for the sake of definition one can classify

transformations as first-order and second-order

types, in reality 'it is difficult to find out exactly

whether a transformation belongs to either of these

two types. By definition first-order transformations

should occur at one temperature and two phases

of the same substance should not coexist at temperatures other than Tt

. If a single crystal of a substance is taken through a transition point, the

crystal should break up into a random assembly of one or more crystals of the other phase at T, (depending on the magnitude of AV ). If the temperature is then decreased below Tt , the first phase

should reform randomly from the second phase

(if the two phases are not crystallographically re- lated). While transformations classified as first order are structurally discontinuous and involve

drastic variations in energy, there are indications

of higher order effects such as premonitory phenomena and thermal hysteresis in many of these

transformations.

In continuous or second- (or higher) order trans- formations, premonitory changes are often found.

Even though in these transitions there should be no discontinuity of structure, x-ray studies have

shown that the two structures coexist over a narrow

region of temperature. In some cases, transformations are found to proceed through the formation

of “hybrid crystals" where the two structures co- exist within a general pattern of orientation.

There are many phase transformations which

strictly belong to neither first order nor to second

order. For example, the ferroelectric transformation

of KH2PO4 should theoretically be first order but

conforms more closely to second order. The trans- formation of BaTiC>3 and related materials show

lambda-like changes in properties as well as small

changes in latent heat. There is superposition of

second-order behavior on many first order trans- formations as in the transformation of alkali

sulphates. It often happens that transformations

have observable heat effects and yet the approach

to the transformation is evidenced by a gradual

change of properties. Even the order-disorder

change in second order transformations may be seen as an abrupt change towards the termination.

Irreversible crystal structure transformations are found in many systems where one of the polymorphs

is metastable in a particular temperature (or pres- sure) range. In such transformations, a polymorph

transforms to the other form on heating above a particular temperature and remains in the new

transformed structure even after cooling. It is not possible to assign any critical transformation temperature in an irreversible transformation and the

transformation will be a function of time as well as temperature. One can only define a temperature

below which a transformation does not take place.

Typical examples of irreversible transformations

are anatase- rutile (of Ti02), aragonite-calcite (of

CaCO.3) and cubic-hexagonal or cubic-monoclinic

transitions of rare-earth sesquioxides. Many of

these systems showing irreversible transformations

are associated with small changes in internal energy,

but large energies of activation may involve the

breaking of the primary coordination or change in bond type.

Simple thermodynamic considerations indicate

that one polymorph of a solid can be more stable

under a specified set of conditions than the other.

It should be realized, however, that it is difficult to strictly define the thermodynamic stability of a polymorph since a variety of physical factors as well as compositional differences affect the stability

of polymorphs markedly. Thus, if there is any

difference in the stoichiometry between two poly2
morphs, one cannot really classify them as belonging to the same polymorphic set. Similarly, if different samples of a solid contain different

quantities of impurities, the regions of thermodynamic stability will vary. A better insight into the nature of phase trans- formations can be obtained by correlating structural

changes and the thermodynamics of transformations. The thermodynamics of transformations gives

some indication as to the nature of structural

changes accompanying transformations, but a better understanding can be obtained by considering

the geometrical or crystallographic mechanisms.

Since phase transformations in solids are associated with increase in entropy, one can visualize

the structural origin of the entropy increases. For

example, in first-order transformations, the phases

with high internal energy and low density will have

high entropy as well because of the greater magnitude of voids in the structure. In second-order or continuous transformations, the entropy changes

have been related in many systems with the potential barrier to rotation. While going from the lowto the high-temperature forms in these systems,

there will be some randomization of the structure. Ubbelohde [2] has discussed the structural origins

of entropy increase in phase transformations in

solids.

Structural Changes Accompanying Transformations. Buerger [5] has classified various types of

thermal transformations based on structural

changes involving the primary or higher coordination. Transformations where there are changes in

the primary coordination will involve a more

drastic change in energy and structure rather than

those where there are changes only in the higher

coordination. This can be visualized in the case of

ionic crystals, where the energies of the ionic bonds

vary inversely as the distance and the energies

of the van der Waals bonds vary inversely as the

sixth power of distance. Accordingly, one can

classify transformations of the first order into two

categories: first coordination transformations and

higher coordination transformations.

Changes in primary coordination can take place

by a reconstructive transformation, where the first coordination bonds are broken and reformed. Such

transformations will involve high energies of

activation and will be sluggish. Further, there may

be no symmetry relation between the two phases.

Reconstructive transformations give rise to large

discontinuities such as cell dimensions, symmetry,

internal energy, specific heat, etc. Changes in

primary coordination may also take place through a

dilatational mechanism as in the thermal trans- formation of cesium chloride (479 °C) or ammonium

chloride (184 °C). Dilatational transformations are

likely to be rapid compared to reconstructive

transformations.

There are some transformations where only a

part of the structure may undergo a change in the

primary coordination. The ferroelectric transformations of barium titanate and lead titanate are

likely to belong to this category since the coordination of titanium goes from 6 to 5 due to the shifting

of the position of titanium from the center of the

octahedron towards the vertex. Many of the transformations involving changes in

higher coordination may also have to proceed

through the breaking of the primary bonds and for

this reason, the energy changes and other features

of the reconstructive transformations involving

higher coordination may resemble those of the

reconstructive first order transformations. In some

transformations changes in higher coordination can

be effected by a distortion of the primary bond. Such

transformations may be called distortional or dis- placive transformations. These transformations

will involve considerably smaller changes in energy

and are usually fast. In the displacive transformations, the high temperature form is usually more

open and has higher specific volume, specific heat

and symmetry. In addition to the transformations

discussed above, Buerger also discusses disorder

transformations of two types: rotational and

substitutional transformations.

Experimental Techniques Employed to Study

Phase Transformations. A variety of techniques

have been employed for the study of phase trans- formations in addition to x-ray diffraction which

is an essential part of the study of any crystal

structure transformation. Once a phase transition

is identified in a solid by any technique, one has

to investigate the same by other methods depending on the particular properties of interest. For

example, the ferroelectric transformation of barium

titanate has been investigated by the anomalies

in heat capacity, breaks in thermal expansion

curves and variations in optical properties (including

vibrational spectra) in addition to changes in crystal

structure and dielectric properties. The semiconductor-to-metal transitions in VO2 and V2O3 have

been studied by measurements of changes in magnetic susceptibility, heat capacity, electrical

conductivity in addition to methods like nuclear

magnetic resonance spectroscopy, Mbssbauer

spectroscopy, neutron diffraction, and differential

thermal analysis. The transformation of KNO2 provides another interesting example where a variety of methods of relevance have been employed

to study the transformation. The reversible transition of KNO2 was established by differential thermal

analysis; x-ray diffraction was utilized to prove

that the change was from monoclinic to cubic

structure. Based on the crystal structures and also

analogy with NaNC> 2 , it was suspected that the

transformation was associated with a change from

a ferroelectric to a paraelectric state. In fact, the

dielectric hysteresis loop disappeared around

40 °C (Curie temperature, Tc); the dielectric

anomaly was also observed at this temperature.

A plot of electrical conductivity against temperature

indicated a break at 40 °C. A study of the temperature-dependent infrared spectrum of KNO2 showed

456-089 0 - 72 -2 3
that the absorbance of the NO2 deformation fre- quency (830 cm-1

) showed a marked increase in

intensity at the Curie temperature. This is similar

to the behavior observed in the ferroelectric-paraelectric transformation of KNO3 . The literature

has many such examples where several techniques

have been employed to examine phase transformations in solids.

X-ray diffraction methods are by far the most

important tools for the study of crystal structure

transformations. Identification of structures of

various phases by x-ray diffraction is very essential

irrespective of what other techniques one employs

to identify or examine the changes in the system

accompanying the transformation. Single crystal

work with Laue and Weissenberg photographs are

particularly useful for a detailed knowledge of the

mechanism of a phase transformation. Neutron

diffraction has been employed effectively as a tool

to examine the positions of light atoms as well as to study magnetic structures. Both electronic and infrared spectroscopy are useful tools, provided a characteristic absorption

band of the substance shows variations during a transformation. Wide-line NMR spectroscopy has

been useful in the study of transformations containing suitable nuclei.

Measurements of (a.c. or d.c.) conductivity and

dielectric constant are important techniques for the

study of phase transformations. The transformations

are generally indicated by a break in the conductivity versus temperature curves. Magnetic

transitions (paramagnetic ^ ferromagnetic or paramagnetic-antiferromagnetic) are generally studied

1. Hydroge

Phase transitions of solid hydrogen halides have

been investigated by many workers with a variety

of experimental methods. The transitions generally

take place at low temperatures. Dielectric relaxation, dielectric anomalies, specific heat, NMR and

calorimetric measurements are some of the methods

employed for these studies in addition to x-ray and

neutron diffraction. While in HC1 there is only one transition at ~ 98.36 K, in HBr and HI there are two transitions.

The transformation in HC1 involves a change from

the face centered cubic structure to a low symmetry

structure (face centered orthorhombic); the transition is of first order. HBr undergoes two transitions, one at ~ 89 K (III —^ I) and the other at — 1 14 K

(II —^ I). Phases III, II, and I are encountered suc- cessively when liquid HBr is cooled. The transition

at 89 K is first order while the one at 114 K is a second order transition. Two transitions in HI are known at —70.1 K (III —> I) and at —125.7 K

(II —* I); these are similar to the transitions in

HBr. A third transition at 25 K has been reported in

HI from infrared studies. The theory of phase transitions in hydrogen halides has been studied by

Pauling and others. Some of these transitions are

by employing magnetic susceptibility measurements. Changes of magnetic anisotropy as a function of temperature has also been used to study

polymorphic changes. Mbssbauer spectroscopy

containing (or doped with) the appropriate muclei

provides valuable information on such transitions

if the spectra are recorded at different temperatures.

Measurements of heat capacities as a function of

temperature are valuable in the study of phase

transformations (particularly in the case of second

or higher order transitions). Differential thermal

analysis is another powerful tool in the study of

phase transformations. This technique has been

employed to study the enthalpy change, activation

energy and thermal hysteresis in transformations.

In principle, any technique that can measure a property of the substance which undergoes marked

change during the transformation can be employed

for the study of phase transformations. Dilatometric

measurements, change in Young’s modulus, thermal

expansion of solids are some of the classical

methods for the study of phase transformations.

References

[1] Staveley, L. A. K., Quart. Rev. (London) 3,64 (1949).

[2] Ubbelohde, A. R., Quart. Rev. (London) 1 1, 246 (1957).

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

1967).

[4] Rao, C. N. R., in Modern Aspects of Solid State Chemistry,

Ed. C. N. R. Rao (Plenum Press, New York, 1970).

[5] Buerger, M. J., in Phase Transformations in Solids, Ed.

R. Smoluchowski (John Wiley, New York, 1957) also see Fortschr. Mineral. 39,9 (1961). Halides

recently found to be associated with a change from

a ferroelectric to a paraelectric state. The gradual transitions in hydrogen halides were examined by Pauling [24] who attributed the transitions in HBr and HI to the incipient rotation about one crystal axis and the expansion of the crystal

only along this axis. The difference between the

low (a:) and high (/3) forms of HC1 has been likened

by Hettner [13], to that between liquids and solids. Molecules in the a-state vibrate about fixed and

regularly oriented equilibrium positions while in the /3- state only the equilibrium positions of several

temporary groups are oriented with respect to one

another. On the basis of classical mechanics,

Kirkwood [17] accounts for the transitions in hydrogen halides, in terms of hindered rotation. The

effects of dielectric relaxation during phase changes

in hydrogen and deuterium halides have been dis- cussed by Bauer [ 2]. Powels [28] has discussed the

mechanism of transitions from a knowledge of the

experimental results including those from calori- metric, dielectric, structural and thermal expansion

measurements.

Krieger and James [19] proposed a model for a crystal exhibiting successive orientational or rota- 4
tional transitions as in hydrogen halides. The model

consists of an array of classical rotors with next

neighbor coupling. The nature of transitions in

HC1, HBr, HI, DC1, DBr and DI have been successfully explained with this model. Recently,

Kobayashi and co-workers [18] have suggested that

the phase transitions in hydrogen halides can be

understood qualitatively on the basis of a two

dimensional rotational transition model with two

sublattices.

The deuterium analogues of HC1, HBr and HI also

exhibit transitions corresponding to those in hydrogen halides. Thus, DC1 exhibits the II —* I transition

at ~ 105 K with an enthalpy of 320 cal • mol-1

; both

DBr and DI exhibit the III —

>

I transition at ~ 93.5 K

(A//tr ~ 169 cal -mol-1

) and ~ 77.3 K (AHtr -175

cal mol-1

) respectively and the II —> I transition at ~ 120 K (A Htr ~ 303 cal mol-1

) and ~ 128 K (A Htr ~ 386.0 cal • mol -1

) respectively. The Tt and AHtr are higher compared to those in the respective hydrogen

analogues. Just as in hydrogen halides, phase transitions in the deuterium halides have been investigated by a variety of experimental methods.

Hydrogen Halides

Substances and measurement techniques Data Remarks References

Hydrogen Chloride, HC1

Calorimetric measurements like molar heat, specific heat etc., as a function of temperatureTt - 98.36 K

AHtr ~ 284.3 cal • mol -1 AS ~ 2.89 e.u.

Transition is indicated by anomalies in molar heat or in the Cp versus T

curves. The entropy of HC1 was

calculated to verify the third law.

17,91.

X-ray and neutron diffraction Cubic HC1 has a = 5.44 ± 0.01 A at 98.5 K. At 85 K, solid HC1 has a tetragonal form, with c/a = 1.10 where a — 5.27 A, c — 5.797 A. At

92.4 K, the structure is face centered orthorhombic with the space group, Bb2im. a = 5.082 A,

b = 5.410 A, c = 5.826 A. At

118.5 K, the structure is FCC, with

a = 5.482 A. {Tt ~ 98.4 K)

The Bb2im-space group indicates that HC1 structure is non-centro

symetric in this phase. The structure

at 118.5 K is an average of disordered

structures.

[22, 30],

NMR spectroscopy Linewidth is measured as function of temperature.

The transition is found to be an order- disorder type. No evidence was found

for a rotational transition.

[1]-

IR spectra of crystalline HC1 at 68 K, in KBr solvent Two fundamental vibrations at 2704

and 2746 cm -1, are observed. The

angle H — Cl . . . H is ~ 107°.

The basic structural unit in HC1 is a zigzag hydrogen bonded chain; the crystal appears to be built from

anti-parallel pairs of such chains, but

the relative position of the chains is uncertain.

[15].

Dielectric constant, e, and tan 8 measurements, as a function of temperature in various frequency ranges

Tt ~~ 98.4 K The dielectric behavior of solid HC1 is predicted by Pauling’s theory of free rotation of molecules in crystals. The

dielectric behavior of liquid and dis- ordered phases of HC1 can be ex- plained in terms of Onsager’s

equation.

[5. 35],

Dielectric hysteresis and ferro- electric behavior of solid HC1 at low

temperature

Spontaneous polarization. Ps, in HC1

is 1.2 fxc cm -2 at 90 K. The coercive

field. Ec , is ~ 3.3 kV cm-1

. The

ferroelectric loop disappears at ~ 98.4 K.

This observation seems to support

Kobayashi’s two sublattice theory

of rotational transition in HC1.

[16].

5
Hydrogen Halides — Continued

Substances and measurement techniques Data Remarks References

Hydrogen bromide, HBr

Calorimetric measurements like molar heat, specific heat etc., as a function of temperature

T, (III -» I) - 90 K; (II —>1) ~ 118 K; The A// tr values are in the range 150-250 cal • mol -1

. Transitions

are indicated by anomalies in

C,, versus T curves.

The A// tr values are not consistent with other experimental results. The transitions are probably akin

to those in NH^Cl.

[7.10]

X-ray diffraction Existence of solid HBr in the (space group V7 or V33 i.e., D7 or D33 structure in the 85-90 K range is established beyond doubt. Below - 100 K a=5.76 A. Below 100 K,

HBr exhibits rhombic symmetry

with axial ratios, a:b:c=0. 985:1:

1.075, where a=5.555 A, 6=5.64 A

and c = 6.063 A; Z = 4; p = 2.81 g cm-3

.

The x-ray studies of Ruhemann and

Simon [29] between 82-120 K, however, do not indicate any fundamental crystallographic

changes during the transition. They propose a FCC structure throughout this temperature range.

[22,23,29], •

NMR spectroscopy Line width is measured as a function of temperature.A basic factor affecting the line width

is the characteristic flipping time

i.e., the average time to change its orientation. The results suggest that the phase transition in HBr is an

order-disorder type.

[!]

Infrared of HBr crystals at 68 K. Two fundamental vibrations at 2404

and 2438 cm -1 are observed. The basic structural unit in HBr

crystals is a zigzag hydrogen bonded

chain.

[15].

Measurement of e as a function of frequency, oj and temperature

e varies with oj only below 95 K. The

dispersion domains are displaced

towards the high frequency with T. One of the dispersions is of constant amplitude while the other is temperature dependent,

and exists even after Tt

. Definite hysteresis effects were

observed in the transitions.

In addition to the two dispersions

mentioned, a third one was indicated

at higher frequencies. A rotation tunnel effect is introduced to explain

the variation of the critical frequency

with temperature. Such an effect is shown to be present in HBr.

[6, 25, 26, 27],

e and tan 8 measurements The sharp increase in e suggests a

first order transition at —89.8 K

while a sharp increase in tan 8 suggests a second order

transition at — 113.6 K.

The complex e is ascribed to a skewed

arc function found for the low- temperature phase. Deviations of the

relaxation rates can be explained in terms of cooperative interaction

effects in the disordered phase.

[12, 20]

Static dielectric constant and

dispersion loss measurements.

e stat above 117 K agrees well with Onsager’s equation. In phase

II, e rises to peak value of 200 at 89 K and falls to 32 at — 72 K in phase III.

Phase transitions are indicated by

anomalies in e.

[3],

Dielectric hysteresis in HBr single

crystals.

The polarization versus field curves show a ferroelectric hysteresis

loop. The measured spontaneous

polarization, P.„ is about

0.4 g.c cm'2 and the coercive field, Er is —4.3 KV cm -1

.

The phase transition in HBr i.e., the second order change is thus

possibly associated with a change

from ferroelectric to paraelectric

state thus conforming to the two

sublattice model of Kobayashi.

[16].

Effect of ultrasonic waves on thermal

hysteresis of transition in HBr. The hysteresis is not affected by

ultrasonic waves or by

inoculation.

The position of the equilibrium curve

in the transition interval could not be determined.

[«]•

6
Hydrogen Halides — Continued

Substances and measurement techniques Data Remarks References

Hydrogen iodide, HI

Calorimetric measurements like molar heat, specific heat etc., as a function of temperature

For III -> I, Tt , 72 K; AHtr ~ 150

cal - 1

, -124 K;

A//, r ~ 395 cal • mol - h

The transitions are indicated by anomalies in the molar heat/Cp v -rsus temperature curves.

1 7.11].

X-ray diffraction HI has a face centered tetragonal structure with c/a= 1.08 (a = 6.10 A) at ~ 100 K. This structure changes to a FCC structure above

127 K: a = 6.19 A at 125 K; p=3.17 g cm-3

.

The transitions in HI may be similar to those in NHjCl. 121,22,23.29],

NMR Spectroscopy The line width is measured as a function of temperature.

The line width is affected by the average time the molecules take to change from one orientation to another. This indicates the nature of the transition to be an order-disorder

type.

IH

Infrared study of a new phase transi- tion in HI, in the range 6 to 79 K. At 25 K, HI undergoes a phase

change to a phase with more

molecules in the unit cell and also the phase below 25 K is one of less symmetry than the high phase

above 25 K.

The transition at 25 K is readily re- versible and rapid. No hysteresis is evidenced in these studies.

[14].

Deuterium chloride, DC1

Calorimetric measurement of molar

heat of DC1. Tt ~ 105.03 K, A Htr ~ 320.1cal • mol '

1 Both T, and AHlr are higher than in HC1. The transition is probably a first order one.

[4|.

X-ray and neutron diffraction (as well

as heat capacity measurement).

T, ~ 105 K. At 92.4 K, the structure is face-centered orthorhombic, with

a = 5.0685 A, b = 5.3990 A and

c — 5.828 A. At 1 18.5 K. The struc- ture is face-centered cubic with a — 5.475 A. The space group at 77 K is Bb2im (same as HC1).

The FCC structure at 118.5 K is only an

average of disordered structures,

similar to that in HC1. On the other hand the unit cell of DC1 shows an

anisotropic reduction in lattice constants compared to HC1.

130],

Deuterium bromide, DBr

Calorimetric measurement of molar

heat as a function of temperature.

T, ~ 93.5 K (III -> I) AHiT ~ 196.7 cal • mol -1

. T, ~

120.26 K. AHlr ~ 303 cal mol -1

. T, ~ 128.28 K. A//tr ~

386.4 cal • mol -1

.

The T, and AHlr are higher than in HBr. The second transition appears to go through a metastable

state.

[4J.

X-ray and neutron diffraction (as well

as heat capacity).

Two second order transitions at ~ 93.5 K (III -» I) and at ~ 120.3 K

(II I) are noticed. At 84 K, the space group is Bb2im while at 107 K the space group is Bbcm.

At 84 K, the structure is ortho- rhombic with a = 5.44 ± 0.02 A,

b = 5.614 ±0.005 A, and c = 6.12 ±0.005 A; z= 4. At 107 K, the structure is orthorhombic with a = 5.559 ± 0.005 A, b = 5.649 ± 0.005 A, and c = 6.095 ± 0.005 A.

Below 93.5 K, DBr is isomorphous with DC1 (i.e.. it has an ordered crystal structure, composed of parallel zig- zag chains). Between 93.5 K and

120.3 K, DBr has a disordered

structure where the disorder is orientational with two equilibrium

orientations for each molecule.

[31]

Kinetics of DBr transition at 93.5 K

by x-ray and neutron diffraction methods.

Tt~ 93.5 K AV~ 3.66 A3 per unit

cell. AV/Vo ~ 1.94 percent. A

hysteresis of about 0.60 was

observed.

Kinetics and the nature of the transi- tion are discussed in terms of these experimental results.

[32]

7
Hydrogen Halides — Continued

Substances and measurement techniques Data Remarks References

Deuterium iodide, DI

Calorimetric measurement of molar

heat as a function of temperature.

T, ~ 77.3 K (III -> I); AHlr ~ 175.6

cal • mol ~'. T, — 128.28 K (II —» I); AHlr ~ 386.4 cal • mol ~

'.

T, and AH{r are higher than for HI

transitions.

[4,34].

References

r 1 1 Alpert. N. L., Phys. Rev. 75, 398 (1949).

[2] Bauer. E., compt. rend. 272 (1952).

[3] Brown, N. L., and Cole, R. H., J. Chem. Phys. 21, 1920

(1953).

[4] Clusius, K., and Wolf. G., Z. Naturforsh 2a, 495 (1947).

[5] Cone. R. M., Denison. G. H., and Kemp. J. D., J. Am. Chem.

Soc. 53,1278(1931).

[6] Damkiihler, G., Ann. Physik. 31, 76 (1938).

[7] Eucken. A., and Karwat. E., Z. physik. Chem. 112, 467

(1924).

[8] Eucken. A., and Giittner, W., Math, physik., Klasse, Fach- gruppe. II [N.F.] 2, 167 (1937).

[9] Giauque, W. F., and Wiebe. R., J. Am. Chem. Soc. 50,

101 (1928).

[10] Giauque. W. F., and Wiebe, R., J. Am. Chem. Soc. 50,

2193 (1928).

[11] Giauque, W. F., and Wiebe, R., J. Am. Chem. Soc. 51,

1441 (1929).

[12] Groenewegen. P. P. M., and Cole, R. H., J. Chem. Phys.

46 (3), 1069 (1967).

[13] Hettner. G., Ann. physik 32, 141 (1938).

[14] Hiebert, G. L., and Hornig, D. F., J. Chem. Phys. 26,

1762 (1957).

[15] Hornig, D. F., and Osberg, W. E., J. Chem. Phys. 23 , 662

(1955).

[16] Hoshino, S., Shimaoka, K., and Niimura, N., Phys. Rev.

Letters 27, 1286 (1967).

2. Alkali

All the alkali halides with the exception of CsCl,

CsBr, and Csl crystallize in the NaCl structure

(Fm3m), the three cesium halides crystallizing in

the CsCl structure (Pm3m). CsCl undergoes a

first order phase transformation at ~ 480 °C (at

1 atm pressure) from the Pm3m structure to the

Fm3m structure. The transformation in CsCl is reversible and is associated with considerable

thermal hysteresis, AT, as well as an appreciable

change in molar volume, AF. The transformation of

CsCl is likely to proceed by a mechanism involving

dilatation along the body diagonal; the role of lattice

vacancies in the mechanism of transformation is not clearly established. The phase transition in

CsCl has been studied with the aid of a variety of

experimental tools. Fm3m structures for CsBr,

and Csl have been reported in evaporated thin

films at low temperatures. No other transformation

data are available for CsBr, Csl, or CsF.

No phase transformations are known in lithium

[17] Kirkwood, J. G., J. Chem. Phys. 8, 205 (1940).

[18] Kobayashi, K. K., Hanamura, E., and Shishido, F., Phys.

Letters 28A, 718 (1968).

[19] Krieger, T. J., and James, H. M., J. Chem. Phys. 22 , 796

(1954).

[20] Lacerda, L., and Brot, C., compt. rend. 236, 916 (1953).

[21] Mauer, F. A., Keffer, C. J., Reeves, R. B., and Robinson,

D. W.,J. Chem. Phys. 42 (2), 1465(1965).

[22] Natta,G„ Nature 127,235(1931).

[23] Natta, G., Gazz. Chim. ital, 63, 425 (1933).

[24] Pauling, L., Phys. Rev. 36, 430 (1930).

[25] Powles, J. G., compt. rend. 230, 836 (1950).

[26] Powles, J. G., Nature 165,686(1950).

[27] Powles, J. G., J. Phys. Radium 13,121(1952).

[28] Powles, J. G., Trans. Faraday Soc. 48, 430 (1952).

[29] Ruhemann, B., and Simon, F., Z. physik. B15, 389 (1932).

[30] Sandor, E., and Farrow, R. F. C., Nature 213, 171 (1967).

[31] Sandor, E., and Johnson, M. W., Nature 217, 541 (1968).

[32] Sandor, E., and Johnson, M. W., Nature 2 2 3 , 730 (1969).

[33] Smyth, C. P., and Hitchcock, C. S., J. Am. Chem. Soc. 55,

1830 (1933).

[34] Staveley, L. A. K., Quartr. Rev. 3,64 (1949).

[35] Swenson, R. W., and Cole, R. H., J. Chem. Phys. 22,

284 (1954).

Halides

halides. Pressure transformation (from Fm3m to Pm3m) data have been reported for sodium, potassium and rubidium halides. The thermal transformation data on CsCl as well as the pressure transition

data on other alkali halides have been employed

to evaluate the lattice energies of the halides in the

Pm3m and Fm3m phases. The Born treatment of

ionic solids have been employed with fair success

to explain the relative stabilities of the Pm3m and

Fm3m phases.

The phase transformation in CsCl at 753 K

involves a change from the Pm3m structure to the

NaCl structure. Menary, Ubbelohde and Woodward

[40] suggested that a large increase in vacancies

occured in the neighborhood of the transition temperature. However, Hoodless and Morrison [21]

showed from electrical conductivity measurements

that the transitions may not involve a disruption

of the lattice and that the mechanism is likely to be dilatational as suggested by Buerger [10].

8
The Born-Mayer model [23] has been widely employed to calculate the lattice energies of ionic

solids especially of alkali halides. May [38] employed

a three-parameter repulsive term and a higher van

der Waals contribution to account for the greater

stability of the Pm3m phase of CsCl. The use of

a higher van der Waals term was criticized by

Tosi and Fumi [68, 69] who employed structure- dependent repulsive parameters to account for the

phase transition in CsCl. Recently, Rao and

co-workers [45, 60-62] have employed a four parameter repulsive term in the Born treatment to explain

the phase transition in CsCl. The model also ac- counts for the stabilization of the Fm3m phase by

added KC1 or RbCl as well as for the opposite behavior of the CsCl + CsBr solid solutions [45, 60].

It is definitely established that all the potassium

and rubidium halides with the likely exception of

the fluorides undergo pressure transformations

from the NaCl structure (Fm3m) to the CsCl

structure (Pm3m) [7, 25, 47]. The product of the

volume change, AV, accompanying the transition

and the transition pressure, P, may be taken to be

equal to the difference between the lattice energies

of the two structures at the transition pressure [25]. Several workers have attempted to calculate the

lattice energy differences between the two phases

with the aid of the Born model of ionic solids.

Structure-dependent [68, 69] as well as the fourparameter [62] repulsive terms in the Born-treatment

have been employed to explain these transformations. The need for a higher van der Waals term in

the Born model to account for the transition energies

of KX and RbX (A = C1, Br or I) has been pointed

out [62]. An atomistic formulation of compressibility has also been used to discuss the pressure

transitions of alkali halides [53]. Madan and Sharma

[65] have calculated the lattice energies of all the

alkali halides with the Lennard-Jones (12:6)

potential.

Alkali Halides

Substances and measurement techniques Data Remarks References

Sodium fluoride, NaF

(Fm3m, a = 4.6344 A at 299 K)

Volumetric investigation of high

pressure transformation in NaF.

Transition begins at ~ 18.3±1 kbar

and 430.5 K. AF~5%.

Transition is reversible and incomplete even at 45 kbar.

[48].

Quantum mechanical calculation,

(with nearest neighbor repulsion

only) of transition pressures for NaF and NaCl for the Fm3mPm3m change.

PT — 280 kbar. The value is slightly different from the known experimental data.

[37],

Sodium chloride, NaCl

(Fm3m, a=5.6402 A at 299 K)

X-ray investigation of the high pressure transformation employing a diamond anvil (up to ~ 18 kbar)

Pt, 17.5-18 kbar

a (Pm3m)=3.36±0.04 A at 298 K

and 1 atm. A F 1.0±0.05 cm3 mol-1 AS— 1.5±0.3 cal • deg -1 • mol -1

.

An extreme value of 300 kbar is also reported for Pt- Experimental and

calculated transition pressures

are compared for some more halides.

[4. 17, 18, 28],

Latent heat of the Fm3m-Pm3m

transformation from the temperature

dependence of Pt up to 473 K.

AHtT ~ 660±370 cal • mol -1 at 298 K. The AHtr is comparable to the AHtr for the thermal transition of CsCl.

[50],

Dilatometric study on NaCl single

crystals with organosilicon liquids

as the dilatometric medium.

Pt ~ 21 kbar at 613 K

— 25 kbar at 803 K

( dP/dT) is positive.

[12].

Shear induced transition in shock

loaded NaCl employing a piezoelectric quartz (up to 30 kbar).

Fm3m-Pm3m transition occurred and conformed to a first order. NaCl is unreliable as a high-pressure

gauge. [b 33].

Equation of state for NaCl between

0-500 kbar and 273-1773 K. Pressure as a function of lattice constant and temperature are obtained for the equation of state.

[15, 16].

Dielectric constant and elastic constant measured as a function of pressure up to ~ 25 kbar at room

temperature.

No anomaly (characteristic of a structural change) was found in elastic or dielectric constants.

[11]-

9
Alkali Halides — Continued

Substances and measurement techniques Data Remarks References

Sodium bromide, NaBr

(Fm3m. a =5.9772 A at 299 K)

High pressure transition by volume

discontinuity method.

PT ~ 10-14 kbar at 473 K. The transition is very sluggish. [49, 52],

Sodium iodide, Nal

(Fm3m. «=6.4728 A at 299 K)

Temperature dependence of

transition pressure. Pt~~ 10-14 kbar, at 473 K. There is a transition to the Pm3m structure.

[52],

Volume discontinuity method upto ~ 50 kbar and 437 K. PT ~ 10.00±2.5 kbar at 437 K; AF~8%.

[49],

Potassiu mfluoride, KF (Fm3m,

a — 5.347 A at 299 K)

High pressure transition investigated

by x-rays. Pt ~ 88 kbar from theory No experimental PT is known for KF. [17].

Lattice energies are evaluated for both Pm3m and Fm3m structures. Pt is estimated from available pres- sure transition data (calcd).Pt ~ 20 kbar [73],

Potassium chloride , KC1 (Fm3m,

a — 6.2931 A at 298 K)

High pressure studies on KC1 and

x-ray investigation.

Pt~ 19-20 kbar. A V~ 0.055 cm3 g

-1 AHtr 10.0 ± 8 cal • mol-1

. a (Pm3m) = 3.61 A at 32 kbar.

Lattice energies are estimated for both Pm3m and Fm3m structures to explain the stability etc.[7, 27, 30, 50, 44, 73],

Optical method — by noting the

shift in the vibrational frequency of an added ion like CN - in KC1 crystal

subjected to pressure.

PT - 19-30 kbar Transition is indicated by a decrease

in the transparency of the salt at Pt-

[31,66].

Calculation of P^from experimental

data on coefficient of thermal expansion and compressibility.

PT ~ 22-36 kbar. AF/F- 0.08 [17, 64],

Potassium bromide, KBr (Fm3m,

a = 6.60 A at 298 K) Pressure transition in KBr by

volume discontinuity method.

PT ~ 17-19.3 kbar. AV ~ 0.0325 cm3 g

-1

. Transition pressure is independent of temperature. Transition is sluggish at room temperature. Around 323 K,

hysteresis is about 30 bar. The ex- perimental data are in agreement

with Bridgman’s reports [7|.

[7, 30, 52].

Optical study of polymorphic change

under pressure. Pt ~ 18 kbar. Transition was ob- served by noticing an abrupt

change in the transmission of the material as it passes through the phase transition. Also, the transi- tion was indicated by a discon

tinuity in the shift of the vibrational frequency of the added CN“

ion.

[31,66].

Calculation of Pt- Pt ~ 18 kbar [73], 10
Alkali Halides — Continued

Substances and measurement techniques Data Remarks References

Potassium iodide, KI (F m3m,

a = 7.0655 A at 298 K)

High pressure studies on KI. Diamond squeezers are usually

employed in addition to piston

displacement and volume discontinuity method etc., and examined

by x-ray.

Pt 18-21 kbar. Transition is almost complete around 24 kbar. Internal pressure in KI after the transition

is ~ 20.8 kbar as estimated from

AF/Fu value. a Pm.?m = 4.13 ±0.02

A at 298 K.

The value of Pt agrees well with

calculated value of 21 kbar. Pt

appears to be independent of temperature. Transition is sluggish at 298 K but faster at 423 K.

[7, 26, 17, 30,

52, 73],

Rubidium fluoride, RbF

(Fm3m, a =5.64 A at 298 K;

a lower value of a has been re- ported by Lew [33a]).

Pressure temperature curves for RbF.Pt~ 22 kbar at -973 K. Transition takes place to the Pm3m

structure.

[51].

Estimation of Pt Pt~ 11.76 kbar. [73],

Rubidium chloride, RbCl

(Fm3m, a =6. 58 10 A at 300 K).

Thin layers of RbCl evaporated

onto amorphous bases at low

temperatures and examined by

electron diffraction.

a Pm3 m—3.97 A at 193 K. The Pm3m phase slowly disappears as the temperature is increased.

[5].

High pressure transition in RbCl

and x-ray investigation.

Pt —31 kbar (calculated) — 5 kbar at 133 K (observed)

A// tr 130±20 cal • mol -1

. PT is - 7.5 kbar at 973 K.

The volume contraction in RbCl is comparable to that in CsCl for the Pm3m-Fm3m phase. Thus the high pressure phase of RbCl is CsCl type

only.

[6, 17, 24, 47, 50,51,68,

69],

Investigation of pressure effects on

the compressive strength and

stress-strain behavior of RbCl.

Pt ~ 7.5 kbar. A simultaneous

deformation and polymorphic

transformation were observed.

The simultaneous deformation and

transformation are attributed to the crack-healing effect. Cracks appear

at about 1% deformation and vanish

in large deformations, especially at the buckled surface of the sample. At

pressures above Pt, the cracks do

not disappear but transform to a system of slanted fissures.

[34].

Rubidium bromide, RbBr

(Fm3m, a=6.889A at 298 K)

Thin layers of RbBr evaporated

onto amorphous bases and ex- amined by electron diffraction at low temperatures.

«pm3m =4.24 A at 230 K. The intensity of the Pm3m lines disappear with rise in temperature.

[5],

Pressure transition in RbBr by

x-ray investigation, in the range 0-40 kbar and 273-873 K.

Pt ~ 25 kbar (theory) — 4.6 kbar (observed). — 5.5 kbar at 873 K.

The experimental observations can be

fitted to Simon equation.

[6. 17, 47,51.

52],

RbBr grown from aqueous solution on oriented silver films and x-ray

investigation.

The films have RbBr with Pm3m

structure where a=4.06 A. If the

film is grown from vapor, the structure is Fm3m.

Vapor grown RbBr films change to Pm3m if dissolved in water and grown from solution on silver

films.

[64],

11

456-089 0 - 72 -3
Alkali Halides — Continued

Substances and

measurement techniques Data Remarks References

Rubidium iodide, Rbl

(Fm3m, a = 7.342 A at 300 K)

Thin layers of Rbl evaporated onto amorphous bases and examined by

electron diffraction.

«i>m3m

== 4.59 A at 193 K. The intensity of the Pm3m lines slowly disappears as the temperature is increased.

[5].

Pressure-temperature curves for Rbl from 0-40 kbar and 273 to 873 K. Pt ~ 4.8 kbar at 873 K, above

which the Pm3m phase is possible

The experimental points fit to Simon

equation.

[51].

Optical investigation of the Fm3mPm3m pressure transition in Rbl

single crystals. A high pressure

vessel with a modified Vickers

metallographic microscope and

time lapse photographic technique

are employed.

Pt ~ 3.5 kbar. The transition was affected by the nature of the crystal surfaces. The

lattice dynamics of the transition has been discussed in terms of the shifts

in the optical and acoustical mode

frequencies. Results of this discus- sion are not compatible with the experimental thermal expansion data on Rbl.

[13, 14].

Dilatometric studies of the Fm3mPm3m transition in Rbl; construction of the P—T diagram.

Pt ~ 4.3 kbar at 298 K. (dT/dP) = 1.4° bar -1 for the equilibrium

curve.

Hysteresis effects are observed in the

transition.

[54],

Cesium chloride, CsCl (Pm3m,

a = 4.123 A at 298 K)

Calorimetric methods Tt, 743 K AHlT, 580-707 cal mol-' [2, 29, 32].

Application of the Clausius- Clapeyron relation to the transition. The relation is applicable with

limited utility.

[36],

DTA of pure.CsCl and solid solutions with other alkali halides and x-ray

diffraction studies of CsCl transi- tion at various temperatures.

Transition ( T„ 745-753 K) is of first order AHlr , 600-880 cal • mol-1

; A T ~ 33°; A V ~ 7.34 cm3 mol-1

. With incorporation of KC1 or RbCl,

the Tt and AHlr are decreased

with increase in KC1 or RbCl. With

CsBr, Tt and AHtr are increased.

The Fm3m phase of CsCl gets stabi- lized at ordinary temperatures in solid solutions with 30% KC1 or 35%

RbCl. With KClor RbCl solid solutions, first order characteristics of the CsCl transition are still preserved. In CsBr solid solutions higher order components are present in the

transition and the Pm3m structure is retained at ordinary temperatures.

[45, 56, 59, 60, 73],

Temperature variable x-ray diffrac- tion studies on CsCl powder as well as single crystals.

T, , 742-753 K. The lattice const, extrapolated to 298 K is, a = 6.91 A

for the Fm3m phase (at 753 K, a = 7.02 A). In CsCl + CsBr solid solutions coexistence of Pm3m and

Fm3m phases is observed in the

x-ray patterns in the neighborhood

of the transition temperature.

The lattice constants of the Pm3m as well as the Fm3m phases decrease

with increasing percent of KC1 or RbCl. On the other hand the lattice constant increases with CsBr content. The molar volume change, AV,

slightly increases in KC1 solid solu- tions indicating that the transitions are first order. In CsBr solid solu- tions, AV decreases indicating the presence of higher order components

in these transitions.

[22, 23, 40, 45, 57,58,71,

74, 75],

Electron diffraction studies of CsCl

transition.

T,, 753 K; a Fm3 m = 6.923 A at 227 K

and 6.940 A at 298 K (extrapolated

values).

The transformation is believed to proceed through a defect mechanism.

Hybridization and mixed crystal (of both Pm3m and Fm3m structures)

formation were noticed just below Tt

.

[5,41,63],

12
Alkali Halides — Continued

Substances and measurement techniques Data Remarks References

Ionic conductivity measurements of CsCl and its solid solutions with KC1, RbCl, CsBr and SrCl2.

The Tt is indicated by a break in the

log cr versus 1/T plots. The activation energies from the plot give values of cation migration energy, £7" ~ 0.6 eV and Schottky defect formation energies, Es . Es — 1.4-

1.8 eV in pure CsCl in Pm3m phase.

The value of Ex in the Fm3m phase,

is — 2.0 eV. The cation migration energy decreases slightly with K+ or Cs+ content and increases with Br- content. Addition of Sr+2 slightly decreases the Tt of CsCl.

The conductivity experiments on single

crystals as well as polycrystalline

pellets give results which do not vary much. The role of vancancies in the mechanism of the transition is not definitive. Though vancancies

may aid the transition by decreasing

the Tt , the mechanism would still involve dilatation along the body

diagonal.

[3,42,43,46,

46a, 72].

Self-diffusion and electrical conductivity measurements. En for conduction — 1.26 eV; E„ for

diffusion: 1.53 eV for Cs 127 1.27 eV for Cl36

.

Conduction is only ionic. Schottky

defects are predominant. The

transformation does not involve an

extensive disruption of lattice. The

transformation is essentially dilata- tional with an increase in lattice vacancies around 7Y

20,21].

Electronographic investigation of evaporated thin films. Both Pm3m and Fm3m structures were noticed.

[19].

Study of the phase transition by

S

:!5 emanation from CsCl35 and

its solid solution with CsBr.

T, is indicated by the increased

radioactivity. 77 is increased

by CsBr addition.

[35].

Kinetics of the CsCl transition above

733 K, by electron diffraction studies. The kinetics become difficult to follow because CsCl starts to sublime at — 730 K and is extremely

sensitive to moisture.

Both Pm3m and Fm3m phases coexist near T,.

[70],

Cesium bromide, CsBr,

(Pm3m, a=4.2953 A at 298 K)

X-ray diffraction up to melting

point.

Some extra weak lines are observed

in the x-ray spectra, which do not belong to the Pm3m structure.

The weak lines are with 2 to 4 times

the basic lattice period of the stable Pm3m structure.

[55,71],

Thin films of CsBr evaporated on to surface of mica, CaCo.j or amorphous

bases at low temperatures and

examined by electron diffraction.

Fm3m structure of CsBr was noticed <2=7.2300 A — 7.253 A at - 155 K. The Fm3m lines are of weak intensity

and disappear with time.

[5,63],

Evaluation of Tt and AHtr from ex- perimental data from CsCl+CsBr

solid solutions and thermodynamic

considerations.

For CsBr, Tt — 1153 K for Pm3m_

Fm3m transition; A// tr (calcd)=300

cal mol -1

.

The transition temperature is higher than the melting point of CsBr.

[72],

Cesium iodide, Csl

(Pm3m, a=4.5679 A at 299 K)

X-ray diffraction of Csl up to the melting point.

No transition was observed. [71].

Thin films of Csl evaporated on to amorphous bases or mica or CaCO:i at very low temperatures and ex- amined by electron diffraction.

Both Pm3m and Fm3m structures were observed with the intensity of Fm3m structures becoming weak

with time. a Fm:s m =7.631 A at 148 K.

The intensity of the lines due to the Fm3m structure weakens slowly with

increase of temperature and

completely disappears at room

temperature.

[5,63],

13
Alkali Halides — Continued

Substances and measurement techniques Data Remarks References

Csl evaporated on to K and Rb

halides (as thin films) and examined

by electronograph.

Microcrystals of NaCl and CsCl type

structures were observed. With

increasing thickness of the Csl

films the lines due to Pm3m

structure increase in intensity.

Microcrystals of Fm3m structure were

unstable in air and recrystallized

into Pm3m structure. Exchange of Cs and Rb cations was observed to take place in the presence of moisture.

[19].

References

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[7] Bridgman, P. W., Phys. Rev. 48, 893 (1935).

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[9]

Buckle. E. R., Diss. Faraday Soc. 30, 46 (1960).

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R. Smoluchowski (John Wiley & Sons. Inc., New York,

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(1969).

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S. S.R., page 123 (1961).

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541 (1965).

[35] Makarov, L. L., Aloi, A. S., and Stupin, D. Yu., Radiokhimiva 11, 116 (1969).

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1961 (1965).

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[38] May, A., phys. Rev. 52, 339 (1937); ibid. 54, 629 (1938).

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[40] Menary, J. W., Ubbelohde, A. R., and Woodward, I., Proc. Roy. Soc. (London) A208, 158 (1951).

[41] Morlin, Z., and Tremmel, J., Acta. Phys. 21, 129 (1966).

[42] Morlin, Z., Acta. Phys. 21, 137 (1966).

[43J Morlin, Z., Acta. Phys. 24, 277 (1968).

[44] Nagasaki, H., and Minomura, S., J. Phys. Soc. Japan 19,

1496 (1964).

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Soc. 66, 2497 (1970).

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Soc. A, 2293 (1971).

[47] Pauling, L., Z. Krist 69, 35 (1928).

[48] Pistorius, C. W. F. T., and Admiraal, L. J., Nature 201,

1321 (1964).

[49] Pistorius, C. W. F. T., Nature 2 04, 467 (1964).

[50] Pistorius, C. W. F. T„ J. Phys. Chem. Solids 25, 1477 (1964).

[51] Pistorius, C. W. F. T., J. Chem. Phys. 43, 1557 (1965).

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Soc. 81, 6148 (1959). 14
3. Ammonium Halides

Phase transitions in ammonium halides as well as

their deuterium analogues are well documented in

the literature. A wide variety of experimental methods have been employed to study these transitions.

While a higher order X-transition at low temperatures

is common to all the halides, a Pm3m(CsCl)-Fm3m

(NaCl) transition is exhibited by ammonium chlorides, bromides and iodides and their deuterated

analogues at higher temperatures. The latter is a

first order transition and is accompanied by considerable thermal hysteresis. In addition to the X- and Pm3m-Fm3m transitions, a third transition is reported in NtUBr at ~ 100 K. The X-transitions of

ammonium halides are associated with a change in

the rotational motion of the NHf ion about the ter- nary axis. Pressure transitions in NH4X have been

reported. Lattice energies of all the four ammonium

halides have been estimated theoretically [38] and

the calculated values are in good agreement with the

observed values.

Much of the theoretical work has been devoted

to the X-transitions in ammonium halides. These X- transitions are believed to be due to the onset of NH^

ion rotation by Pauling [28a]. While the X-changes'

in NH4CI and NtLBr have been attributed to an

order-disorder change in the NHJ ion orientation

[8a, 25], a temperature-dependent one-dimensional

free rotation has been suggested in the case of NHJ

[30]. The low temperature phase of NH4CI consists

of parallel oriented NH| groups while the high temperature phase is characterized by randomly oriented NH| groups [27]. In NHUBr, the low temperature

phase consists of an antiparallel orientation of NH+

4 ions and the high temperature phase is similar to that of NH4CI [26]. Neutron diffraction studies

[22, 25] have confirmed the NHj ion orientations in

these halides. The polarization effect of the halide

ions is considered to be responsible for the differ- ence in the NHJ ion orientation in the low temperature phases of NH4CI and NH4Br [26]. NMR [10a,

15] and IR [36, 54] studies have been employed to investigate the problem of NH| ion rotation. Sonin

[40] has discussed the possible antiferroelectricity in

NH4I and NH4Br from a correlation of physical properties with structure. The Pippard equations and the phase diagram for

NH4CI have been analyzed in terms of a compressible Ising model; the Pippard relations are completely

consistent with the occurrence of a first-order transition in the immediate vicinity of a X-point [33]. Anomalous heat capacity variations in ammonium halides

at low temperatures have been ascribed to second

order phase transitions [53]. However, calculations

based on available data show that the behavior is described by the Clausius-Clapeyron equation for

the first order transitions rather than Ehrenfest’s

equation for the second order transitions. An

abrupt change in dielectric constant at the transition points also indicate a first order transition. A

compressible Ising model has also been employed

to show that the anomalous AF due to ordering is essentially identical with the anomalous changes in

the elastic constant, C44 [9, 9a].

Thermal Transitions. Ammonium fluoride is hexagonal at room temperature and belongs to B4 group (similar to hexagonal Agl or wurtzite

type); this is the only ammonium halide which

exists in the hexagonal structure. No transition is known in NH4F above 298 K. However, a X-transition

at 242 K and a pressure transition around 4000 atm

have been reported.

Below 450 K, ammonium chloride possesses the

B2 structure with the Pm3m (CsCl type) space

group. Phase transitions in NH4 C1 have been the

subject of detailed investigation by many workers.

The following is the sequence of phase transitions

in ammonium chloride. ~ 240 K ^ ~ 456 K ^ ^

fusion

(Pm3m) (Pm3m) (Fm3m)

-liquid

NH4CI

Forms III and II are both CsCl type structures, the

transition between them being of X-type. Form I is of NaCl type (Fm3m) with <2 = 6.520 A at 523 K.

Thus the phase transition at 456 Kis a Pm3m-Fm3m

transformation similar to the transition in CsCl.

The Pm3m-Fm3m transition is of first order and is associated with considerable thermal hysteresis.

IR, Raman and NMR spectroscopy, x-ray diffraction, DTA as well as measurements of piezoelectricity, dielectric constant, heat capacity and heat of

solution have been employed for the study of the

transitions.

Ammonium bromide is body centered cubic

(Pm3m, CsCl type) below 411 K. Above this temperature, the structure is face-centered cubic (Fm3m,

NaCl type). Just as in NH4 CI, there are two transitions in NH4 Br. The Pm3m-Fm3m transition takes

place at —411 K with appreciable volume change

and thermal hysteresis. The transition has been

found to be first order. The X-change takes place

around 235 K. In addition to these two transitions a

third transition at — 102 K has also been reported.

The transitions at 235 K and 102 K are also accompanied by hysteresis effects; these transitions have

been attributed to changes in the NHJ ion orientation in III and IV phases.

T„ 102 K

IV< > III (Pm3m) Tetragonal

235 K TT 411 K T fusion

> IF » I

< > X-change (Pm3m) (Fm3m)

(nearly

CsCl type)

liquid

NH4 Br

15
393 K fusion The room temperature structure of NH4 I is Fm3m. This transforms to the Pm3m structure

(CsCl type) at about 257 K. A ^-transition in NHJ

takes place at -231.3 K. It is interesting to note

that the stability range of phase II (CsCl-type) is narrowed down to 25°.

231.3 K 257 K fusion

IIP >II< > I

< » Tetragonal (X-change) (Pm3m) (Fm3m)

(nearly liquid

CsCl type) NH4I

Deuteroammonium halides, ND4 X, exhibit the

Pm3m-Fm3m phase transitions with the exception

of ND4 I. But a X-transition is common to all of them

just as in their hydrogen counterparts. The Pm3mFm3m transitions retain their first-order characteristics in the deuterated solids. The transition

temperatures of ND4X are generally lower than in

the corresponding NH4 X, except the III— IV transition of ND4 Br. The Pm3 m-Fm3 m transition in ND4 C1 takes

place at 448 K while the X-change occurs at ~ 250

K. These transition temperatures are lower than

the corresponding values in NH4 C1. While the

Pm3m-Fm3m transition is accompanied by con- siderable hysteresis and volume change, the

X-change has no hysteresis effects. There are three transitions in ND4 Br corre- sponding to those in NH4 Br: 164 K 215 K

IV < > HI < > II (Pm3m) (nearly CsCl type; X- change (Pm3m)

tetragonal)

II < > I « > liquid

(Pm3m) (Fm3m) ND4 Br

The III— IV transition is faster than in NH4 Br;

T, is also higher in the former case. The AV in ND4 Br transitions are comparable to those in NH4 Br. No literature data seem to be available on ND4 I after Staveley’s report [47]. The III— II transition takes place at ~ 229 K, with no thermal

hysteresis and negligible volume change (~ 0.1

cm3 mol-1

). This is likely to be a higher order trans- formation.

Pressure Transitions. Pressure transitions in ammonium halides are not reported extensively in the literature, compared to thermal transitions.

The earliest work on pressure-temperature relationships in ammonium halides seems to be that of

Bridgman [7 a] who reported data for NH4CI,

NFCBr and NH4I between 273 K and 473 K and

12,000 kg cm -2

. The only reports on the transition of NH4F (at ~ 3800 atm and liquid nitrogen

temperatures) are that of Stevenson [48], and

Swenson and Tedeschi [50a]. Stevenson [49]

also reports data on NH4CI, NFCBr, NH4I and

ND4 Br. Adams and Davis [1] report four polymorphic forms for NH4 I. Pressure effects on the

X-transition of NH4 C1 have been investigated by

Klynev [17] as well as Trappeniers and co-workers

[52]. A systematic investigation of pressure transitions in ammonium halides has been carried out by Pistorius [29] recently. A brief account of phase transitions in ammonium halides has been

reported recently by Hovi [ 11 a]. This review

summarizes the experimental results from di- alatometric, x-ray, calorimetric and NMR studies;

the effect of deuteration of ammonium halides is also included.

Ammonium Halides

Substances and measurement techniques Data Remarks References

Ammonium fluoride, NH4 F (P6mc,

a = 4.44 A and c — 7.17 A at 298 K)

Dielectric constant as a function

of temperature.

Anomaly in e at ~ 242 K. Tt ~ 241.5 JK. The anomaly in e is due to the A-transition in NH4 F.

[16].

Ammonium chloride, NH4CI

(Pm3m, a — 3.8756 A at 299 K)

Pm3m-Fm3m transition

X-ray diffraction = 3.868 A at 291 K

a Fm'Sm — 6.52 A at 523 K

Tt ~ 457.3 K.

Changes of volume, densities ac- companying the transition are

listed.

[46a].

Specific heat as a function of temperature

T, -456.1 K, A//tr ~ 1030

cal • mol -1 Transition is indicated by an anomalyin the Clt (T) curves.

[6 , 37,

55a]. 16
Ammonium Halides — Continued

Substances and measurement techniques Data Remarks References

Thermal analysis Tt ~ 458 K; AHn ~ 1073 cal • Hysteresis is shown as a difference in [32a, 37, mol-1

. T 8 13.25,; AC 8 7.14 cm3 mol-1

.

the Tt in heating and cooling

curves.

42, 57],

Differential Calorimetry Tt ~ 456.1 K; A7/tr ~ 1073 cal • mol -1

. A detailed discussion of the possible sources of error and their corrections in evaluating the AHtT is fur- nished. The data are likely to be the best for NH4 C1 transition and very

reliable.

[2]-

Dielectric constant as a function of temperature

T, ~ 458 K. For single crystals e is 7.08 at 293 K

and 2 MHz.

[16].

Dilatometric studies Tt ~ 456.1 K; AC ~ 19.31% at Tt ( dT,ldP) = 0.0724° atm" 1

. These results compare well with the

results of other investigations. The

transition conforms to first order as indicated by Clausius-Clapeyron

equation.

[31].

Review articles Tt ~ 456 K

AStr ~ 2.2 cal deg-1 • mol-1 AC 8 5.26 cm3 • mol-1

.

\-transition

[11a, 24, 47,

52a],

X-ray diffraction down to 80 K Tk ~ 243 K

a = 3.82 A at 88 K.

[46, 46a],

Thermal analysis Tk ~ 242.5 K. The transition is due to a change in the rotational motion of NH| ion. Hysteresis effects are observed in the transition.

[18. 19].

Differential calorimetry 7\ ~ 242.1 K

A T ~ 0.3° AH ~ 109 cal • mol -1

.

[4],

Dielectric constant as a function

of temperature

Tk ~ 242.5 K. Transition is indicated by an anomaly

in e (T) curve.

[16].

Dilatometric studies; with CBr4 dV-7= is maximum at 7\ ~ 242.4 K

d 1 AT ~ 0.30°.

Hysteresis was about 1° if CBr4 was [43, 51], as the dilatometric liquid. used.

Thermal expansion of NH4 CI and a is maximum between (7\), 242 Transition is believed to be due to an [7]- x-ray investigation. and 242.5 K. order-disorder change.

Temperature dependence studies T\ ~ 242.8 K; the temperature There appears to be an indirect re- [58a], of proton and deuteron spin- trend of the nuclear correlation lation between the temperature

lattice relaxation in NH4 C1. time tc is in qualitative agree- ment with volume-temperature

dependence.

dependence of tc and the degree of order in the crystal lattice in the region of the A-transition. ESR and DTA studies on doped Tk is indicated by a peak in DTA; With increasing Cu +2 ion, Tk de- [20. 28. 35]. NH4 CI samples containing a in ESR, Tk is indicated where creases. The intensity of the signal

paramagnetic ion, like Cu+2

, the first derivative of the hyper- in VO.f doped NH4 C1 vanishes at Ni +2 or VO £ fine constant with respect to temperature is a maximum.

Tk ~ 241 K. In the electronic ab- sorption spectrum of Ni +2 doped

NH4 CI, a blue shift of 100-150 cm -1 observed at ~ 235 K in the

positions of the band maxima is attributed to the anomalous lattice contraction of the NH4 C1 lattice.

17
Ammonium Halides—Continued

Substances and measurement techniques Data Remarks References

Vibrational spectra of thin non- No indication of fine structure due Low temperature modification belongs [56], scattering films of NH4 C1 at to free rotation of the NH| ion to T'(l sym. is which the NH| ion 301, 195 and 83 K. was found. Evidence for the presence of a torsional lattice mode involving the NH^ ions both above and below 7\ was

found.

symmetry is 7V Transition is probably of the order-disorder type.

Total scattering cross section per An apparent change in cross sec- The transition involves a change in [23], H atom measured as a function

of temperature by using sub- thermal neutrons.

tion is observed at T the rotational motion of the NH|

ion from torsional oscillation to relatively free rotation.

Review articles

Ammonium bromide, NH4 Br (Pm3m, a — 4.059 A at 299 K)

T„ 247.2 K; AS ~ 0.82 eu. AV~~ 0.16 cm3 mol-1

.

Pm3m-Fm3m transition

[11a, 24, 47,

52a],

X-ray diffraction of NH4 Br from The lattice constants are: AV calculated from x-ray data shows [32, 46a], 293 to 467 K. a, = 6.85187 + 1.38 x 10-5

r +

5.385 x 10 -7

t

2 a„ = 4.5409 + 2.1 16 X 10-5

* + : 3.29 X 10"7

t

2 where t is in °C. AFj n = 0.0767 cmV1

.

good agreement with the value from dilatometric studies.

Solubility as a function of temperature by a close tube method.

T, ~ 410 K The Tt is indicated by the point of cross section of the solubility curves

of I and II phases.

[41].

Dielectric constant as a function

of temperature.

T, ~ 410 K For single crystal NH4 Br, e — 7.24

at 293 K and 2 MHz.

[16].

Thermal analysis T,~ 410.4 K; AT -24°

AHtT ~ 882 cal • mol -1 AV ~ 9.5 cm3 mol-1

.

[33a, 37, 57].

Dilatometric method (Kinetics of Tt ~ 410.2 K Total volume changes, variation of [8, 31], the transition, of NH4 Br at AV ~ 18.85% density and thermal expansion

410.2 K).

( 0.0878° atm -1

. E„ for the transition is ~ 15000

cal • mol -1

.

coefficients are discussed. A dis- cussion of thermal expansion be- havior is given.

Rate of transition of super- The transition rate is very high and It is inferred that the transition has [21]. cooled high-temperature modifi- cation (NaCl type) of NH4 Br

crystallizing from highly super- saturated aqueous solution into the low temperature CsCl phase

(by oscillographic recording of the photocurrent change ac- companying the I —» II transition).

comparable to the speed of sound.

a martensite character.

18
Ammonium Halides — Continued

Substances and measurement techniques Data Remarks References

Measurement of specific volumes

by hydrostatic weighing in 293-413 K range.

AC ~ 7.4866 cm3 mol-1 at Tt for NH4 Br AV ~ 7.4704 cm3 mol-1 at Tt (392.5 K) for ND4 Br.

Effect of deuteration of NH4 Br is discussed; AV values from the lit- erature are compared and

discussed.

[15a],

IR spectra of NH4 Br thin and

nonscattering films at 301, 105 and 83 K.

Review articles

The spectra in phase II is in agreement with the symmetry,

D4h. No evidence for free rota- tion at any temperature is found.

T, ~ 414 K

AV ~ 6.34 cm3 mol-1

. Review of phase changes in NH4 Brupto 1964.

[56],

[11a, 24, 47,

52a].

\-transition

X-ray diffraction between 295 and

148 K. For phase II, aj = 4.0534+ 1.6 X 10-4

t (where

£= 1, 2, 3)

cii= 4.0539 + 1.34 X 10-4

t (where

£=1, 2) where t is in °C. For Phase III, a3 = 4.0537 — 2.72 x 10-4

t V, the molar volume (in cm3 mol-1! is given by

F=40.113 +4.82 x 10-3t for phase II V= 40.042 — 2.34 x 10-3 t3.64 x 10-5

t 2 for phase III. Thermal expansion coefficient,

' a = 1.24 X 10-4 for phase II a = — 6.0 X 10-5 — 1.8 X 10 -6

t

2 for phase III.

a.i = a 2 = a 3 for phase II a i = a 2 9^ a 3 for phase III

[12, 46a],

DTA 7\ ~ 234.9 K. Transition is believed to be due to the change in the rotational motion

of NH| ion.

[19].

Dielectric constant as a function

of temperature.

Tk ~ 236 K. [16].

Dilatometric methods with

toluene as the dilatometric

liquid.

7\ ~ 234 K. Transformation is heterogeneous and

is accompanied by a small

hysteresis.

[44],

Total scattering cross section per H-atom measured as a function employing subthermal

neutrons.

T\ is indicated by a change in the cross section. Transition is due to a change in motion of the NH^ ion from tor- sional oscillation to relatively free

rotation.

[23],

ESR Investigation of NH4 Br

single crystals doped with Cu +2 ion through \-point.

The hyperfine constant shows a maximum at ~238 K. The maximum change at ~ 238 K is due to the \-change which occurs

at ~ 235 K.

[34].

Review articles Tt ~ 234.4 K

AS ~ 0.34 e.u AV ~ — 0.16 cm mol -1

Reviews on phase changes in NH4 Br

upto 1964.

[11a, 24, 47, 52 a].

Ammonium iodide, NH4 I (Fm3m,

a = 7.2613 A at 299 K)

456-089 0 - 72 -4 19
Ammonium Halides — Continued

Substances and

measurement techniques Data Remarks References

Pm3m-Fm3m transition X-ray diffraction. Tt - 257 K

AV — 16.96%

a = 7.259 A at 295 K (Fm3m)

a = 4.334 A at 252 K (Pm3m)

Effect of particle size on the time

for half- conversion in the transi- tion is investigated. Below Tt , the needed super-cooling increases with decreasing particle size.

[12a, 13, 46a],

Differential calorimetry Tt - 257 K

AHtr — 809 cal • mol-1 The method is applied for the first time for the study of a transition below 273 K.

[3].

Thermal analysis Tt - 253 to 259 K. Moisture affects the Tt. [37],

IR investigation of NH^I in phase I.

Tt ~ 255.4 K. In phase I, one hydrogen bond is formed with I - and the NH| ion

rotates freely about this bond.

[30],

Review articles Tt - 258 K

AV — 8.14 cm3 mol-1

. k-transition

Reviews upto 1964. [11a, 24, 47,

52a],

X-ray diffraction a, =4. 3387+2.41 X 10 -4

f (£=1,2,3)

(ai=a 2 —a3 for phase II) a;=4.3439+2.28X 10 -4

t (i = l,2); a3 = 4.2999-1.071 X 10-3 t-6.41

XlO~6

t 2 (ai =«2 + a 3 for phase

III).

Molar volume changes are given as a function of temperature.

[13].

Differential thermal analysis. 7\~ 231.35 K. Transition is likely to be associated with a change in the NH| ion

rotation.

[19].

Molar volume as a function of temperature.T\ — 233 to 213 K. Above the T\ the structure is regular

and below T\ the structure is tetragonal.

[44],

Dielectric constant (e' and e") measurements. Two maxima between 253 and 233 K

are observed. The rotation of NHj ion along the N—H axis cannot explain these maxima.

[16, 24].

Review articles. 7\ — 233 K

AS — 0.3 eu AT~0.

AV 0.1 cm3 mol-1

.

[11a, 24, 47,

52a],

Deuteroammonium chloride, ND4 CI (Pm3m. a — 3.8682 A at 291 K)

X-ray diffraction. aPm3m=3.8190 A at 88 K; apm3 m = 3.8682 A at 291 K. Tt — 448 K (for II-I transition). 7\~ 249.4 K.

[7, 46a].

Neutron diffraction. The nature of the NH+ ion rotation

is examined.

The neutron diffraction results are in agreement with spectroscopic

results of Wagner and Hornig.

[ 22],

IR spectra of their non-scattering

films at 301.295 and 83 K. The evidence for the order-disorder nature of the second order

transition is confirmed.

No evidence of fine structure due to free rotation of NH^ ion was found. Evidence for the presence of a tor- sional lattice mode involving the NH^

ion both above and below A-point was available. Low temperature

modification belongs to the T'd symmetry in which the NH| ion symmetry is Tfj . A-point transitions are probably order-disorder type.

[56],

20
Ammonium Halides — Continued

Substances and measurement techniques Data Remarks References

Review article.

Deuteroammonium bromide, ND+Br

(Pm3m, a=4.06 A at 299 K)

C, — 448 K for Pm3m-Fm3m transition. T\ — 249.3 K: AF— 0.13 cm mol-1

.

[11a, 47].

X-ray diffraction. T„ I —»• II — 393 K; AV - 7.61 cm

mol-1 at 398 K:

7\, II -> III - 215 K; AF- -0.293 cm mol-1 at 215 K.

Ill -> IV - 164 K; AF- 1.08 cm mol-1 at 164 K.

Lattice constants

a=6.889 A at 469 K (phase I) a=4.084 A at 385 K (phase II) a=4.047 A, c=4.062 A at 195 K

(phase III); a=4.016 A at 152 K

(phase IV).

Replacement of H by D lowers the Tt while the volume changes, AV, are

not very much affected. In ND4Br some metastable states (with BCC

structure) were observed in the range 408-399 K.

[14. 46a].

Neutron diffraction. The nature of rotation of NH-f ion

is examined.

The freely rotating model is not tenable with the experimental

results.

[22],

Heat capacity in the range 17 to 300 K. Two maxima in the Cp versus T curve

are noted. The first maxima is associated with the polymorphic

transition in the crystal lattice and

the second one with the ordering of

the NH^ ion tetrahedra with

respect to the two equilibrium

positions in the unit cell. 7\ — 215

K; AH\ — 307 cal • mol -1

. Tt — 166 K; AHtr — 340.5 cal mol -1

. AH

values are obtained from the inte- grated areas under the peaks.

[50],

IR spectra of thin nonscattering films The symmetry of ND4Br in Phase III No evidence for free rotation at any [56]. of ND4Br at 301. 95 and 83 K. is DIh

. in accordance with x-ray

data. temperature studied, was found. Below 173 K, ND4Br transforms to phase IV where the symmetry is Ta

just as in NH4C1.

Spin-lattice relaxation and phase In 99 percent of the sample, a hyster- Assuming the activation energy for the [58b]. transition in ND4Br, at 163.9 K. esis effect was observed. With a 94 percent ND4 Br no such hysteresis effect was observed.

transition to be a function only of the

lattice type, the behavior of 99%

ND4Br can be explained in terms of coexistence of two phases at 163.9 K.

Neutron diffraction study of the Four different models are considered The freely rotating ND^ ion model is [22]. NaCl phase. to explain the experimentally

observed phenomena of transition. not tenable, with the observed exper- imental situation. IR absorption data on thin films by Wagner and Hornig

support a model involving a single- axis rotation of NH4+ ion. Review articles. Tt — 405 K (Pm3m-Fm3m)

7\ — 215 K; AF — —0.17 cm mol -1

, AT — 0.06°.

[11a, 24,47].

21
Ammonium Halides — Continued

Substances and measurement techniques Data Remarks References

Pressure Transitions ofAmmonium

Halides Pressure transition in NH+F. Pt ~ 3800 atmospheres at room

temperature; volume change is 28%. The high pressure phase is FCC at this pressure. With still high pressures of ~ 11 kbar a BCC phase is obtained where the volume change is ~ 11 % of the zero pressure volume.

The phase change occurs at a very high pressure at liquid nitrogen temperature. The recovery begins only

when the pressure is completely

removed. The phase change involves

a collapse of the wurtzite structure

into a cubic modification.

[48, 50a],

X-ray investigation of high pressure phases of NH+F and ND4F, under

purely hydrostatic conditions.

The high pressure phase in both the cases is likely to be tetragonal. The

transitions are observed at 3.64 ± 0.02 kbar in both fluorides.

[24a].

DTA study of pressure effects on A- transition in NH4CI and ND4CI up

to 2700 atmospheres.

Transition entropy and enthalpy

decrease up to 1500 atmospheres

and remain unchanged thereafter.

The fact that the entropy of transition is reduced by increased pressure, cannot be explained on the basis of an

order-disorder theory which pictures the transition as due to a reorientation of NHi" ion over two sites (AS = 7? In 2 ).

[52],

Spectral investigation of the A- transition in NH4 CI up to 23 kbar

pressure in the range 3000-7000 cm -1

.

The A-transition takes place around

13-15 kbar at room temperature.

The transition was followed by

noticing the change of the absorption band with pressure.

The change is associated with the suppression of the random orientation of the NH<if ions, in the crystal

lattice. Lattice vibration frequencies

are affected to a greater extent than

the intramolecular vibration fre- quencies by pressure.

[17].

Ultrasonic investigation of the high pressure transition in NH 4Br single crystals.

The adiabatic elastic constants of NH4Br crystals have been

measured at 200 MHz as a function of temperature and

pressure in the ranges 180-240 K

and 0—6 kbar. A new high pressure ordered phase desig- nated as 0 n was noticed in this range.

The behavior of NH4Br is compared

with that of NH4 C1 near its order- disorder phase transition.

[ 10].

DTA of ammonium halides up to 40 kbar; the pressure-temperature curves of the halides are given.

NH4F has a triple point at 16.8 kbar and 582 K. The melting curve of NH 4F shows an increase up to 706 K at 40 kbar.The A-changes in NH4CI and ND4 CI

possibly change to first order

transitions above 30 kbar.

[29],

Volumetric study of the pressure

transitions in NH4CI and NH4 Br up to 12000 Kg cm-2 between 273 K

and 473 K.

A// tr ~100 cal • mol _, (NH 4Cl). A7/ tr ~ 160 cal • mol - 1 (NH4Br). The AHlr in NH4 CI is independent

of pressure. These anomalies

may in part be explained with

Pauling’s suggestion that there is a change from torsional oscillation to rotational motion of NH^ ions; further considerations may be necessary to account for the different behaviors of NH4 CI and NH4 Br.

[7a],

22
Ammonium Halides — Continued

Substances and measurement techniques Data Remarks References

Piston displacement technique

down to liquid nitrogen temperatures, on NH-iCl, NH4Br, NH4I and ND_iCl, ND4 Br and ND4 I.

Transition temperature, K The Ammonium tetrahedra are placed on the face of the cube as in the a-phase or at the center of the cube for all the other phases. In

/3-, y-, and 8-phases the NH^"

tetrahedra are located at the center of the cube formed by the

halide ions. The tetrahedra are oriented in a parallel sense in 8-phase from cell to cell; in y-phase there is antiparallel orientation while in the /3-phase the

orientation is randomly.

[49].

a—/3 /3-y £-8 y—8

NH4 C1 ND4C1 NH4Br ND4Br NH4I ND4I

458

411 398

257 254

*

* 235 215

231 227

243 249

*

**

*

*

* 100

169

*

*

*No transition takes place at atmospheric pressures.

Modified Bridgman’s device with diamond and beryllium anvils employed to study pressure changes up to 20 kbars, in NH4I.

NH4I which exists in Fm3m

structure at room temperature

changes to Pm3m at —500 bars.

The transition conforms to first order at the start but second order

effects are prominent during the major interval of the change.

[1].

References

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Belarus. S.S.R. Ser. Fiz. Tech. Navuk 57 (1957).

[8a] Frenkel, J., Acta. Physicochim, U.R.S.S. 3,23 (1935).

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(1968).

[9a] Garland, C. W., and Jones. J. S., Bull. Am. Phys. Soc.

Ser. 2,8,354(1963).

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5282 (1968).

[10a] Gutoswsky, H. S., Pake, G. E., and Bersohn, R.. J. Chem.

Phys. 22,643(1954).

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(1964).

[11a] Hovi, V.. Proc. Intern Conf. Sci & Tech, of Non-metallic

Crystals (New Delhi India) (1969).

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[12a] Hovi. V., Paavola. K., and Nurmi, E., Ann. Acad. Sci. Fennicae A, 328 (1969).

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2 1 5,1 (1966).

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938 (1968): Ann. Acad. Sci. Fennicae, Ser. A. 291 (1968).

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463(1962).

[15a] Jaakkola, S., Piiyhonen. J.. and Simola, K: Ann. Acad.

Sci. Fennicae. A, 295 (i%8).

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Sci. Fennicae, Ser. A., 168, 8 (1964). [32a] Rao. K. J., and Rao, C. N. R.. J. Materials. Sci. 1 , 268 (1966).

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[34] Sastry, M. D., and Venkateswarlu, P., Proc. Ind. Acad.

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97 (1933). 23
[44] Smits, A., Ketelaar. J. A. A., and Muller, G. J., Z. physik.

Chem. A175 , 359 (1936).

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1141(1964).

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1161(1966).

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1103(1966).

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71(4), 952 (1967); J. Chem. Phys. 47(7), 2361 (1967). 4. Alkaline Earth (Group IIA) Halides

Substances and measurement techniques Data Remarks Reterences

Beryllium fluoride

,

BeF2 (Tetragonal, «=6.60A and

c = 6.74 A)

X-ray diffraction and DTA. Three DTA peaks are observed

at 308, 673 and 818 K. The transi- tion at 308 K is a change from

tetragonal to cubic structure, with a=6.78A; this is an edge

centered cubic structure. (8 molecules in the unit cell]. .

The peak at 673 K does not produce

any new form. The products after heating in the range 673 to 803 K

showed x-ray patterns of cubic

form only.

[5, 8a],

High pressure transitions up to 873 K and 60 kbar. At 773°, BeF-i transforms from the quartz form to the coesite form,

under pressure. At room temperature transition occurs at ~ 15.5 kbar.

Some high pressure phases are

also produced by grinding.

[1 to 4].

Beryllium chloride, BeCl2 (orthorhombic, Ibam, a=5.36A,

6=9.86 A and c=5.26A)

X-ray diffraction and DTA. On cooling the melt rapidly, the a

phase is obtained. The structure

of ct'-BeCl2 is similar to SiS. The a'-phase on heating to 523 K

gives a cubic modification Q3'). The f3' form on further heating to 613 K produces the

stable (3 form. If, however, the melt

is allowed to slow equilibrium

cooling the a form is encountered

at ~ 698 K and further to f3 form

at -678 K.

[6,9].

Heat capacity in the range 13 to 715 K. Tt is — 676 K for the oc — (3 transition; AHlr — 1495±50 cal mol-1

. Thermodynamic properties for BeCl2 are tabulated for the range 0-750 K.

[8].

Beryllium iodide, Bel>

(Tetragonal, P4/nmm.

a=6.12±0.01 A and

c=10.63±0.04 A ) Two forms of Bel2 are known with a

transition at 623 K. Form I is tetragonal; this transformsto Form II (orthorhombic) above

623 K. The lattice constants of Form II are: a= 16.48±0.02 A, 6=16.702±0.01 A and c=11.629±

±0.01 A. AF= 0.

The number of molecules per unit

cell is 4 in the low temperature phase,

while there are 32 molecules in the high temperature phase.

[7]-

24
Alkaline Earth (Group IIA) Halides — Continued

Substances and measurement techniques Data Remarks References

Fluorides ofcalcium. Magnesium

Strontium and Barium

Changes in lattice spaeings of CaF2 , SrF2 . and BaF2 investigated by

x-ray diffraction at pressures up to 65 kbar in a tetrahedral anvil press.

Transitions to structures of lower symmetry take place at —30 kbar

in BaF2 and at — 60 kbar in SrF2 .

The dependence of the repulsive

potential on interionie distances is estimated. V arious alternative assignments of repulsive parameters

6 and p for the individual ion-ion interactions are discussed in the light of the Born model. The repulsive

parameters appropriate to alkali halides are insufficient to provide the observed repulsion energy.

[10].

X-ray studies up to 160 kbar. CaF2 transforms reversibly to a-PbCl2 structure (space group V16 Pbnm) around 80 kbars; a=3.50A. 6=5.64.4. and

c=6.96 A (AV/V~10%). In SrF2 and BaF2 . the high pressure a-PbCl2 phases could be re- covered: in BaF2 . the transition

is around 20 kbar. The lattice parameters of the a-PbCl2 phases are: SrF2 : g=3.804-A: 6=6.287 A: c=7.469 A; AF/F~ 8%

BaF2 : a=4. 035.4: 6=6.676 A: c=7.879 A; AV/V ~ 11%.

MgF2 reveals no transformation up to 160 kbar.

[2a].

References

[1] Dachille, F., and Roy, R., Z. Krist. Ill, 451 (1959).

[2] Dachille. F.. and Roy, R.. Nature 186 , 34 (1960).

[2a] Dandekar, D. P., and Jamieson. J. C., personal communication (1971).

[3] Douglas. T. B., STAR. 2 , 1348A (1964).

[4] Kirkina. D. F.. Novoselova. A. V., and Simanov. Yu. P.. Proc. Akad. Sci., U.S.S.R.. Sect. Chem. 107 , 201 (1956).

[5] Kirkina. D. F.. Novoselova. A. V'., and Simanov, Yu. P., Doklady Akad. Nauk. S.S S.R. 107 , 837 (1956).

[6] Kuvyrkin. O. N., Breusov, O. N., Novoselova. A. V., and

Semenenko. K. N.. Zhur. Fiz. Khim. 34 , 343 (1960).

[7] Johnson. R. E., Staritzky. E., and Douglass, R. M., J. Am.

Chem. Soc. 79 , 2037 H957).

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(11), 3839 (1965).

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Moskov Gosudarst. Univ. im. M. V. Lomonosova No. 174,7(1955).

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(1952).

[10]

Smith. H. I., and Chen, J. H.. Bull. Am. Phys. Soc. 11,

414 (1966).

5. Group IIIA Halides

There are no data available regarding the phase

transitions of the halides of boron and gallium.

A1F3 is known to exist in two polymorphic forms, a and /3 [7a]. NQR investigations have shown that

Inl3 undergoes a sluggish transformation under

pressure at low temperatures [4a]. There are, however, a few reports on the phase transitions of

thallium halides. T1F is orthorhombic [8] and undergoes a phase change at ~354 K to a tetragonal

structure [10]. T1C1 [5, 9, 15], TIBr [15, 16] and

Til [11] which crystallize in the CsCl structure

transform to the NaCl structure when evaporated

into thin films on suitable substrates at low temperatures [3, 7, 12, 13]. The abnormal NaCl structures

have been examined by electron diffraction as well

as x-ray diffraction techniques. Lattice energies

of the thallium halides have been calculated by

Mayer [8a], Altshuller [1] as well as Sharma and

Madan [14]. No pressure transformation is known

in T1C1 or TIBr up to 500 kbar [11a]; three high

pressure polymorphs of T1F are known [10].

25
Group III A Halides

Substances and

measurement techniques Data Remarks References

Aluminum fluoride

,

A1F3 (Rhombohedral, R32-D^;

u=5.039 A, a=58°31' at 298 K)

Heat capacity as a function of temperature.

T, -718 K

AHlr — 160 cal • mol -1

.

[7a].

Indium triiodide, Inl 3 NQR investigations and pressure- dependence studies. The sample changes from normal

yellow to brick red color at low

temperatures, indicating a sluggish transition, under

pressure.

Extrapolation of data seems to suggest

that the transition may be feasible around 200 K at atmospheric

pressure. Earlier observations indi- cated a transition around 77 K, at atmospheric pressures.

[4a, 13a],

Thallium fluoride, T1F

(orthorhombic Dfj]-Fmmm.

a=5.495 A, 6=6.08 A and

c=5.18A at 298 K)

X-ray and DTA investigation of high temperature and high pressure

transitions.

Tt —354 K. The space group of the

tetragonal structure (I) is D \

7h~ 14

l mmm; o=3.771 A and c=6.115 A

at 408 K, z=2. A high pressure

transition is observed at — 12.6 kbars and 128.5 K. TIF(II) is orthorhombic with D^-Fmmm;

a=5.18 A, 6=5.495 A, c=6.08 A; 2=4.

The orthorhombic to tetragonal

transition is accompanied by a decrease in density. Pressure- temperature curves of T1C1, TlBr

and Til are also presented.

[10].

Thallium chloride, T1C1

(Pm3m, a=3.8427 A at 298 K)

Thin films of T1C1 evaporated on to amorphous bases at low

temperatures and investigated by

electron and x-ray diffraction.

Abnormal NaCl structure was

observed with a=6.30-6.37A

around 208 K.

The stability of the NaCl phase is influenced by the base and the temperature.

[3, 7, 12, 13].

Thallium bromide, TlBr

(Pm3m, a=3.9588 A at 298 K)

Thin films of TlBr evaporated on to amorphous bases and investigated

by electron and x-ray diffraction.

a Fm3m=6.58—6.63 A around 213 K. The NaCl structure disappears with

increase in temperature.

[3, 7, 12, 13].

Thallium iodide. Til (orthorhombic, Djj-Cmcm,

a=4.582 A, 6= 12.92 A and

c=2.251 A at 298 K)

Thin layers of Til evaporated on to amorphous bases (or ionic crystals)

and examined by electron and x-ray

diffraction.

The CsCl type structure of Til has ®pm3 m =4. 205 A at 288 K. a Fm3m=6.94 A (on LiF substrate).

The CsCl structure passes onto the orthorhombic structure near room

temperature. It is interesting that Til deposited on LiF has the NaCl

structure.

[3, 5a, 7, 12

13].

26
Group IIIA Halides — Continued

Substances and measurement techniques Data Remarks References

X-ray and DTA investigation; pressure transition in TIL

Til transforms from a layer ortho- rhombic to cubic structure at 430 K. A// tr ~ 293 ±38 cal • mol -1 (from

DTA) ~ 321 cal • mol -1 (from

Clausius-Clapeyron equation),

ao ~ 4.2099 A at 293 K for the high

temperature form (extrapolated)

Pt 4.7 ±0.1 kbar at 298 K. AF ~ 3.3% at Pt. a shows in the range 296-443 K. aa = 15.90± 0.5 X 10~6 (°C) -1 a6 = 48.7 ±0.3 x 10-6

(oC)-* ar = 60.4±0.6X 10"6 (°C)-'. For the cubic phase,

a„ = 51.0 ±0.3 X 10“6 (°C)-\

between 443-573 K.

The transition at 430 K is also accompanied by about 35% change in dielectric constant e. Structural and thermodynamic aspects of the

transition are discussed.

[4, 11, 17].

Dilatometric study. T, ~ 423-447 K

AF~ 0.003 cm3 g

_1

. The transition is autocatalytic. Presence of gases like CO2 , nitrogen or air does not affect the rate of transition.

[2,6].

References

[1] Altshuller, A. P., J. Chem. Phys. 22, 1136 (1954).

[2] Benton, A. F., and Cool, R. D., J. Phys. Chem. 35, 1762

(193D.

[3] Blackman, M., and Kahn. I. H., Proc. Phys. Soc. 77, 471

(1961). [4] Bradley, R. S., Grace. J. D., and Munro, D. C., Z. Krist. 120,349(1964).

[4a] Brooker, H. R., and Scott, T. A., J. Chem. Phys. 41, 475

(1964). [5] Hambling, P. G., Acta. Cryst. 6, 98 (1953). [5a] Helmholtz, L., Z. Krist. 95, 129 (1936). [6] Ishikawa, F., and Sato, Y., J. Chem. Soc. Japan 55, 930

(1934). [7] Kahn, I. H., Proc. Phys. Soc. (London), 76, 507 (1960). [7a] Kirk-Othmer, Encyclopedia'of Chemical Technology, 2nd

Edition, Vol. 9, p. 529 (John Wiley & Sons., New York,

1966).

[8] Ketelaar, J. A. A., Z. Krist. 92, 30 (1935). [8a] Mayer, J. E., J. Chem. Ph^s. 1, 327 (1933).

[9]

Meyerhoff, V. K., Acta. Cryst. 12 , 330 (1959).'). [10] Pistorius, C. W. F. T., and Clark, J. B., Phys. Rev. 173,

692 (1968). [11] Samara, G. A., Walters, L. C., and Northrop, D. A., J. Phys. Chem. Solids 28, 1875 (1967). [11a] Samara, G. A., and Drickamer, H. G., J. Chem. Phys. 37,

408 (1962). [12] Schulz, L. G., J. Chem. Phys. 18, 996 (1950). [13] Schulz, L. G., Acta Cryst. 4, 487 (1951). [13a] Segel, S. E., and Barnes, R. G., J. Chem. Phys. 25, 578

(1956). [14] Sharma, M. N., and Madan, M. P., Ind. J. Phys. 38, 305

(1964). [15] Smakula, A., and Kalanajs, J., Phys. Rev. 99, 1737 (1955). [16] Westman, S., and Magneli, A., Acta. Chem. Scand. 11, 1587 (1957). [17] Zahner, J. C., and Drickamer, H. G., J. Phys. Chem. Solids 11, 92 (1959).

6. Group IVA Halides

Carbon tetrafluoride exhibits a first order thermal

transformation at ~ 76.2 K and atmospheric pres- sure [6]; the transformation shows no hysteresis.

Stewart [21] observed a volume change of about 1.8

cm3 mol-1 at ~ 89 K. The transition temperature for

CF4 observed by Stewart is, however, higher than

that mentioned in the previous report. CC14 when

cooled below its freezing point undergoes a trans- formation at —225 K [11, 14]; the transition is accompanied by a change from the cubic to the

monoclinic structure. The large enthalpy (— 1080

cal mol-1

) suggests the transition to be of first order.

CBr4 exhibits a similar transformation from a cubic

to a monoclinic structure [10].

Bridgman [3] observed three solid forms of CC14 in the range 253-473 K and up to 12 kbar; Kleiss

and co-workers [12] have reported two pressure

transitions in CClj at — 4 kbar (I—II) and at — 7 kbar

(II—III). Bridgman [4, 5] reports pressure transitions in CBr4. SiF4 undergoes a pressure transition

27
from a BCC structure to a close-packed structure

in the region 5—20 kbar [21]. A pressure transition occurs at —94.3 K in SiF6 [6]. SiCl4 is reported to undergo a pressure transition by Bridgman [3]. Gel2 exists in two hexagonal modifications as evi- denced by electron diffraction studies on thin

films of Gel2 [1]. Electrical resistance measurements

under pressure indicate a polymorphic change in

Snl4 to a metallic state [17]. PbF2 exists in two

modifications [2] while Pbl2 exhibits polytypism

[7, 9]. An x-ray study of over 600 crystals of Pbl2 [8] has revealed some important facts for the formulation of a satisfactory theory of polytypism [9]. Phase changes in Pbl2 have been investigated by

the Hahn emanation method of following the emanation of the 135 Xe from Pb135 I 2 [15]. The theory of

polytypism has been discussed by Schneer [19]; a systematic account of polytypism with literature

references is furnished by Verma and Krishna

[ 22].

Group IVA Halides

Substances and measurement techniques Data Remarks References

Carbon tetrafiuoride, CF4

Calorimetric study T, ~ 76.2 K The transition is reversible with

negligible hysteresis.

[ 6].

Piston displacement method for Isothermal pressure — volume The transition is a first order one. [ 21]. studying high pressure transition curves were examined for solid No crystal structure data are

in CF4

Carbon tetrachloride

,

CC14 (cubic, a = 8.34 ±0.03 A at 238 K)

CF4. A volume change of ~ 1.8 cm3 mol-1

is consistent with cal- culated of ~ 2.0 cm3 mol-1 for the

transition.

available for the low and high pressure phases of CF4 .

X-ray diffraction Below 225.3 K an ordered monoclinic phase of CC14 is reported. a — 20.3 A, 6 = 11.6 A, c = 19.9 A; 0=111°; Z = 32; p= 1.868 gem-3

. Above 225.3 K this phase trans- forms to a hitherto unknown

rhombohedral phase stable up to the melting point.

High pressure studies by Weir et al. [23] furnish the following crystal structure data: CC14 (I) rhombohedral, a = 14.27 A, a = 90°; Z = 21 . CCUdl) monoclinic,

(C2/C or Cc), a = 22.1 A, 6= 11.05 A,c = 25.0 A, 0 = 114°; Z = 32. CCLi(III) orthorhombic, (C222i), a =

11.16 A, 6 = 14.32 A, c = 5.74 A,

Z = 8 .

[17a, 23].

Calorimetric study employing a Tt ~ 224.4 K; A//tr ~ 1080 cal • A suspension of solid CC14 in [11, 14, 16, modified Nernst vacuum calorimeter. mol -1

. methanol was employed. The Tt of CCI4 is very sensitive to impurities

and hence not useful as a thermometric standard.

20],

High pressure changes in CC14 Two transformations at ~ 4 and Observations of the phase changes [12]. investigated by employing a dia- mond high pressure cell. ~ 7 kbar are observed. were made by using a transmitted

light polarizing microscope.

Carbon tetrabromide, CBr4 (Mono- r clinic, a = 21.12 A, b— 12.26 A,

c = 24.14 A; 0 = 125° 3' also a = 21.12 A, b— 12.26 A, c =

21.05 A; 0=110° 10')

Heat capacity of CBr4 in the range Tt ~ 222.13 K. MllT ~ 1594.5 cal Heat capacity data from experiment [13]. 273-333 K. mol-1

. A

S

tr ~ 1.87 cal • mol-1 deg- 1

. and theory are employed to show

the absence of free rotation in CBr4 above Tt

. There is essentially no change in the molecular motion

at the transition point. Dilatometric study The change is from cubic to monoclinic structure.

[18].

Optical investigation of the kinetics Tt — 226.1 K. The activation Optical crystallographic data for [10]. of CBr4 transition in polycrystalline

samples. energy is of the same order of magnitude as the sublimation

energy.

monoclinic CBr4 are obtained.

28
Group IVA Halides — Continued

Substances and measurement techniques Data Remarks References

Silicon tetrafluoride, SiF4 (BCC, I43m

or 123. a = 5.42 ±0.01 A)

Pressure changes in the range 97-

200 K by x-ray diffraction. SiF4 changes to a close-packed

structure at high pressures. Two

transitions are known, one at ~ 8.2 kbar and another at ~ 10.2 kbar.

The overall lattice of the low- pressure phase is molecular rather than ionic.

[21].

Siliconhexafluoride

,

SiFe

Calorimetry. T, ~ 94.3 K. Transition is reversible and discontin- uous; likely to be a first order

transition.

[6],

Germanium diiodide

,

Gels (hexagonal,

P3ml, a = 4.13 A and c = 6.79 A)

Electron diffraction of thin films. Two hexagonal forms have been

identified. Form I w-ith space

group. Q ; a=4.17 A and c=33

0 6v A. Form II with space group, Dgd

; a = 4.17 A and c = 20.16 A. Z is 2 and 3 in I and II respectively.

The structure of the I and II Forms

comprise of four and six-layer

stackings of iodine atoms respectively, the germanium atoms lying

in the octahedral spaces of these

layers.

[1]-

Stannic iodide, Snl4 (cubic, T£-Pa3.

a = 12.273 A at 299 K)

Electrical conductivity measure- ments on Snl4. Snl4 becomes metallic at about 185 kbar as indicated by a sharp decrease in resistance.

The results are in reasonable agree- ment with extrapolated optical-gap

data.

[17].

Lead fluoride, PbF2 (a-PbF2 , ortho- rhombic. D|£-Pnmb. a = 3.899 A.

6 = 6.4421 A, c= 7.6514 A at 298

K). Thermodynamic properties of PbF2 at high temperatures and x-ray in- vestigation of phase transition.

PbF2 exists in two modifications a

and /3. a transforms to /3 at ~ 473

K. /3-PbF2 is cubic with a = 5.940 A at 299 K.

The T, was determined thermographically. The high temperature

cubic form can also be prepared by

the action of HF on PbCOs.

[2. 5a].

Lead iodide, Pbl2 (hexagonal D|d P3ml, a = 4.557 A and c — 6.979 A at 298 K). Polytypism in gelgrown Pbl2 . Growth conditions and various polytypic structures are linked to- gether. Besides the 2H, 4H, 6R. 6H. and 12R polytypes already

known, twenty new polytypes

have been found.

The small extent to which polytypism

has been observed earlier has been interpreted as lending support

to Frank’s screw dislocation theory. However, the experimental

results prove the importance of growth conditions. For various crystal structure data, reference [5a] may be consulted.

[7. 5a, 22],

X-ray study of about 600 crystals

of Pbl2 . A satisfactory epitaxial theory of polytypism has been formulated. A new polytype of the

largest size, 48R is described.

[8].

References

[1] Avilov, A. S., and Imanov, R. M., Sov. Phys-Cryst. 13. 52

(1968):

[2] Banashek. E. I., Patsukova, N. N., and Rassonskaya. I. S., Neorg. Khim Acad. Nauk. S.S.S.R. 27, 223 (1956).

[3] Bridgman. P. W., Phys. Rev. 3. 153 (1914); ibid., 6. 1 (1915).

[4] Bridgman. P. W., Proc. Am. Acad. Arts Sci. 52. 57 (1916). [5] Bridgman. P. W., Proc. Am. Acad. Arts Sci. 72, 227 (1938).

[5a] Crystal Data, ACA Monograph, No. 5, Ed. J. D. H. Donnay,

and Donnay, G., American Crystallographic Association

(1963). [6] Eucken, A., and Schroder, E., Math-Physik. Klasse,

and Donnay. G., Fachgruppe II. 3, 65 (1938). 29
[7] Hanoka, J. I., Vedam, K., and Henisch. H. K.. J. Phys.

Chem. Solids, Suppl. 1, 369 (1967). [8] Hanoka, J. I., and Vand, V., J. Appl. Phys. 39, 5288

(1968). [9] Hanoka. J. I., Vedam. K., and Henisch, H. K., Crystal

Growth. Ed. H. S. Peiser (Pergamon Press. Inc. Oxford.

1967) p. 369. [10] Hartshorne. N. H., and Swift, P. McL., J. Chem. Soc. 3705 (1955). [11] Johnston. H. L., and Long. E. A., J. Am. Chem. Soc. 56,

31 (1934). [12] Kleiss. R. M., Nourbakhsh, M., and Alder, P. N., J. Materials Sci. 3, 657 (1968). [13] Marshall. J. G., Staveley, L. A. K., and Hart, K. R., Trans. Faraday Soc. 52, 19 (1956). [14] McCullough. J. C., and Phipps, H. E., J. Am. Chem. Soc. 50. 2213 (1928).

[15] Narang, V. P., and Yaffe, L., Canadian J. Chem. 46, 491

(1968). [16] Phipps, H. E., and Reedy, J. H., J. Phys. Chem. 40, 89

(1936). [16a] Post, B., Acta. Cryst. 12, 349 (1959). [17] Rigglemann, B. M., and Drickamer, H. G., J. Chem. Phys.

38, 2721 (1963). [17a] Rudman, R., and Post, B., Science 154, 1009 (1966). [18] Sakovich, G. V., Zhur. Fiz. Khim. 34, 2808 (1960). [19] Schneer, C. J., Acta. Cryst. 8, 279 (1955). [20] Skau. E. L., and Meier. H. F., J. Am. Chem. Soc., 51, 3517

(1929). [21] Stewart, J. W„ J. Chem. Phys. 33, 128 (1960). [22] Verma, A. R., and Krishna, P., in Polymorphism and Polytypism in Crystals (John Wiley, New York, 1966). [23] Weir, C. E., Piermarini, G. J., and Block, S., J. Chem.

Phys. 50, 2089 (1969). 7. Group VA Halides

Substances and measurement techniques Data Remarks References

Arsenic triiodide

,

AsL (Trigonal

C2.-R3, a = 7.208 A, c = 21.45 A

at 298 K).

Polymorphism in Asl3 . Two polymorphic forms are known.

An yellow (metastable accicular)

form is obtained by rapid cooling

of a hot benzene or toluene solution. On standing, the yellow

crystals change to a stable red

form. The conversion becomes

faster on heating.

No conversion was observed when the samples were carefully dried and

stored at room temperature in the absence of air, even after a period

of one month.

[3, 4],

Antimony trichloride, SbCl3 and

Antimony tribromide, SbBr3 .

Investigation of phase changes in the range 273 to 450 K and up to 30 kbar.

Two new forms for each of the

halides were noticed. Their phase

diagrams are similar.

Study on Sbl3 and Bil3 showed no

transitions in either of them.

[la].

Bismuth trifluoride, BiF3 ortho- rhombic, Pnma, a = 6.563 A,

6 = 7.021 A, c = 4.845 A at 298

K).

Polymorphism in BiF3 . Two other modifications are re- ported in the literature. An a- form stable up to 473 K is cubic

(Fm3m), with a = 5.865 ±0.06 A. a-changes to /3-form above 473

K.

For various crystal structure data

reference 2 must be consulted.

[1, 4a, 5].

Bismuth triiodide, Bil 3 (Trigonal,

C§j-R3, a = 7.522 A, c = 20.73 A

at 298 K). Two new forms of Bil 3. Form I, hexagonal, R3, with a — 7.5 A and c — 20.65 A. Form II cubic, with a = 20.676 A.

[2].

References

[1] Aurivillius, B., and Lindqvist, T., Acta. Chem. Scand. 9, 1209 (1955). [la] Bowles, B. F., Scott. G. J., and Bable, S. E., Jr., J. Chem.

Phys. 39, 831 (1963). [2] Crystal Data, ACA Monograph, No. 5, Ed. J. D. H. Donnay,

anu Donnay, G., American Crystallographic Association

(1963).

[3] Hass, D., Z. anorg. allgem. chem. 327, 84 (1964). [4] Heyworth, D., Phys. Rev. 38, 351 (1931). [4a] Kirk-Othmer, Encyclopedia of Chemical Technology, 2nd

Edition, Vol. 3, p. 539 (John Wiley & Sons., New York,

1964). [5] Zalkin, A., and Templeton, D. H., J. Am. Chem. Soc. 75, 2453 (1953). 30
8. 3d Transition Metal Halides

Substances and measurement techniques Data Remarks References

Titanium trichloride, TiCl3 (ct- TiCl3 , rhombohedral, R3 , a —

6 = 6.12 A, c= 17.50 A; y= 120°) X-ray diffraction TiCR on reduction with hydrogen

at 1070 to 1173 K gives a-TiCl3 ; this is bidimensional and belongs

to rhombohedral symmetry, R3 . The coordinates of Ti are: 0, 0

V3; chlorine atoms are at, V3, 0, 0.079. TiCR on reduction with aluminum alkyls give /3-form of TiCl3 ; this is a linear polymer.

This has a hexagonal structure with a = b = 6.27 A, c=5.82 A. (3- form on heating to about ~ 573 K

passes on to a new ( 7 ) modification.

[3a, 14].

Chromium chloride, CrCl3 (Monoclinic, C2/m; a = 5.959 A, 6 =

10.321 A, c = 6.114 A; 0 =

108.49°)

X-ray diffraction and NQR

( 35 C1) with CrCl3 single

crystals.

Tt ~ 240 K. The monoclinic struc- ture transforms to a rhombohedral structure with R3 symmetry;

a = 5.942 A, c= 17.333 A.

The transformation seems to be a

first order one.

[13].

Ferrous bromide, FeBr2 (hexagonal,

P3ml, a - 3.74 ± 0.05 A, c =

6.171 A.)

X-ray diffraction FeBr3 undergoes a cooperative second-order transformation at ~ 673 K. The high-temperature

phase is different from hep or random-packed-layer structure of the Cd(OH)2 type.

Tt is markedly affected by the pres- ence of moisture in the sample.

[5].

Ferric fluoride, FeF3 (hexagonal,

R3c, a — 5.20 A, c= 13.40 A

at 291 K)

X-ray diffraction FeF3 grown by sublimation of anhydrous crystals has a cubic symmetry. At 683 K, transition

to a rhombohedral structure takes

place.

For other data on the hexagonal

crystal structure, reference should be made to [4a].

[4].

31
3d Transition Metal Halides — Continued.

Substances and measurement techniques

Manganese fluoride, MnF2 (Rutile, P4/mnm. a = 13.59 ± 0.05 A,

c = 3.3099 ± 0.05 A at 298 K)

Data Remarks References

X-ray investigation of high pressure transitions up to 160 kbar

and at 298 K.

At 33 ± 4 kbars, MnF2 (I) (rutile type) transforms to MnF2 (II) (a distorted CaF2 type) with a = b —

5.03 A, c=5.28 A; density, p =

4.62 g cm-3

at 80 kbar. At

higher pressures (150 kbar) a

third phase, III, is formed (a =

3.25 A, 6 = 5.54 A, c = 6.88 A; p = 4.98 g cm-3

). A metastable

phase, IV, is obtained on re- moving the pressure.

A triclinic form of MnF2 with 1 point group has been mentioned in the literature and referred to in “Crystal Data” [4a]. The lattice constants are: a = 5.13 A, b = 5.31 A, c = 5.07 A, a = 180° 30', /3 = 119° 35' and

y = 88° 33', Dandekar and Jamieson

[4b] suggest that the rutile phase

transforms to a new tetragonal

phase which then transforms re- versibly to the a-PbCl2 form

(a = 3.323 A, 6 = 5.560 A, c =

6.454 A; AF/F = 10%).

[1,4b, 7, 17],

Pressure transitions of CoF2 , NiF2 , and ZnF2

X-ray investigation of high pressure changes in CoF-2 , NiF2 and ZnF2 upto 160 kbar at 298 K.

The phase changes occur from the low pressure rutile phase to the distorted fluorite structure. The lattice constants and densities are given below:

High pressure phase

a, A p, g cm-3

CoF2 4.91 5.44 NiF2 4.84 5.66 ZnF> 4.82 6.13 Low pressure phase

a, A c, A p, g cm-3

CoF2 4.54 3.32 4.70 NiF2 4.51 3.30 4.78 ZnF2 4.41 3.27 5.40

These data have been obtained in 130-150 kbar region at 298 K. Xray diffraction patterns of these

fluorides indicate the presence of a mixture of the high and low pres- sure phases under these conditions. Dandekar and Jamieson [4b] suggest that the initial rutile struc- ture transforms to another tetrag- onal structure (starting around 30

kbar) with a = 4.908 A and c =

4.658 A (at 25 kbar); release of pressure yields a mixture of the two phases. There is no further change in structure up to 120

kbar.

[4b, 7],

Copper (/ ) Chloride CuCI (Tjj—F43m;

a = 5.416 A at 299 K)

X-ray powder pattern of CuCI at high temperatures.

Tt — 683 K. The low temperature

phase of CuCI is FCC (ZnS) with

a = 5.4057 A. The high temperature form has the hexagonal

structure (a = 3.91 A and c = 6.42

A).

The high temperature phase may be

similar to wurtzite

[ 10 ].

32
3 d Transition Metal Halides — Continued

Substances and measurement techniques Data Remarks References

High pressure phases in CuCl by A phase diagram has been con- Phase I is wurtzite type and phase II [ 2 , 16]. studying the P—T curves; conduc- structed indicating the stability is zinc blende type structure. DTA

tivity and TEP measurements in range of various high pressure diagrams are given for various

the 370-670 K and 10-55 kbar. phases. The stability of phase I is in a small range of 673-683 K and

below 5 kbar. Phase II is stable

in the range up to 600 K and 40

kbar. Between 40 and 60 kbar a new phase Ha is reported. IV and

V are stable above 60 kbar up to 140 kbar and below 673 K.

phases and their mutual transitions.

Copper (II) chloride, CuCl 2 (Monoclinic, I2/m, a = 6.70 A, 6 = 3.30

A, c-6.67 A; 0=118° 23')

X-ray investigation of polymorphisn

in CuCl2. There are two polymorphic forms a and 0 with a Tt ~ 761 K.

[ 8].

Copper (I) bromide, CuBr (T^-F43m,

a = 5.6905 A)

Calorimetric and x-ray investigation. y-CuBr is FCC with a = 5.6794 A

at 293 K. This is ZnS type and is like Cul. j3-CuBr is hexagonal, C|v a = 4.04 A, c = 6.58 A at -703 K.' a-CuBr is BCC and is similar to a-Agl. a = 4.53 A at ~ 753 K. A//y-0=14OO cal -mol -1 ASy-0 = 2.1 e.u. mol-1 AH0-a = 700 cal • mol-1 AS/3 _a = 0.9 e.u. mol-1 0 and a are only average

structures.

[6].

Microcalorimetric studies (employ- CuBr (obtained from CuBr2 dis- Immediately after the a-form is [9. 18]. ing DTA) sociation at ~ 453 K) exhibits the y —> 0 —» a transitions by DTA

anomalies.

formed, CuBr starts melting at ~ 770 K.

Copper (/) iodide, Cul (cubic T|-

F43m, y-form, a = 6.0507 A)

DTA and TGA studies on Cul 2 and

Cul. Cul 2 readily loses iodine and trans- forms to the 0 form at ~ 648 K

and finally to the a form at ~ 785

K. The solid melts at ~ 873 K. a

is cubic, 0 is hexagonal (ZnO

type) and y is of ZnS type

structure.

[9, 15].

Investigation of Cul by x-ray Tt , y

-* 0, 642 K The high temperature forms (0 and [ 12 ]. diffraction, heat capacity, etc. 0

-> a, 680 K. a) are average structures.

High pressure polymorphism in In Cul, six phases have been re- For both CuBr and Cul phase dia- [3. 11, 12, CuBr and Cul, by DTA and x-rays. ported: a (FCC phase) 0 (wurtzite) y (zinc blende) as well as hi, 6 2 , and h3 phases are known, y —» hi transition is at 4 kbar,

hi —» 6 2 at 5 kbar and 6 2 —* h 3 at ~ 15 kbar.

In the case of CuBr, a (BCC), 0

(wurtzite) and y (zinc blende) are

all stable below 40 kbar and ~ 800 K.

grams are given. 16]-

33
3 d Transition Metal Halides — Continued

Substances and measurement techniques Data Remarks

Zinc chloride, ZnCL

Dilatometric study The glass transition temperature The rate of crystallization was also was determined by dilatometric measured dilatometrically and

and linear expansion coefficient results are presented in detail. methods. There are three forms of ZnCL

known, a, (3, and y. For the various

structural data reference should be

made to [4a]

Polymorphic Phases of Copper Halides

Designation Structure Range of stability

CuCl I wurtzite Between 650-700 K and below 5 kbar.

II zinc blende Below 650 K and ~ 30 kbar.

III BCC (similar to a-Agl) Above 700 K and 10 kbar. CuBr I BCC (similar to a-Agl) Above 800 K and ~10 kbar.

II wurtzite Between 600-700 K and below — 8 kbar.

III zinc blende Below 600 K and up to — 30 kbar.

Cul I FCC (disordered zinc blende) Between 675-1000 K and up to 30 kbar.

II wurtzite Between 575—675 K and below — 5 kbar.

III zinc blende Below 575 K and — 12 kbar. IV Below —600 K and above — 10 kbar. V Above 20 kbar and — 600 K. VI Above 30 kbar and around 800 K. VII Above 900 K and — 10 kbar.

References

[1] Azzaria, L. M., and Dachille, F., J. Phys. Chem. 65, 88r (1961). [2] Bradley, R. S., Munro, D. C., and Spencer, P. N., Trans. Faraday Soc., 65, 1912 (1969). [3] Bridgman, P. W., Proc. Am. Acad. Arts Sci. 52, 91 (1916); Proc. Nat. Acad. Sci. 1 , 513 (1915). [3a] Cras, J. A., Nature 194, 678 (1962). [4] Croft, W. J., and Kestigian, M., J. Electrochem. Soc. 115, 674 (1968). [4a] Crystal Data, ACA Monograph, No. 5, Ed. I. D. H. Donnay,

and Donnay G., American Crystallographic Association

(1963).

[4b] Dandekar, D. P., and Jamieson, J. C., personal communication (1971).

[4c] Eisenberg, A., and Hilemen, B. J., U.S. Gov. Res. Rep.

39, 108A (1964). [5] Gregory, N. W., and O’Neal, H. E., J. Am. Chem. Soc. 81, 2649 (1959). [6] Hoshino, S., J. Phys. Soc. Japan 7, 560 (1952). [7] Kabalkina, S. S., Vereshchagin, L. F., and Lityagina,

L. M., Sov. Phys. Solid State 11, 847 (1969). [8] Korzhukov, N. G., and Psalidas. V. S., Vestn. Mork. Univ. Khim. 23, 54 (1968).

[9]

Le-Van-My, Perinet, G., and Bianco, P., Bull. Soc. Chim

France 12, 3651 (1965). [10] Lorenz, M. R., and Prener, J. S., Acta. Cryst. 9, 538

(1956). [11] Mao-Chia, Y. W., Schwartz, L. H., and La Mori, P. N.,

J. Phys. Chem. Solids 29, 1633 (1968). [12] Miyake, S., Hoshino, S., and Takenaka, T., J. Phys. Soc. Japan 7, 19 (1952). [13] Morrison, B., and Narath, A., J. Chem. Phys. 40, 1958

(1964). [14] Natta, G., Corrodini, P., Bassi, I. W., and Porri, L., Atti. 'acad. nazl. Lincei, Rend. 24, 121 (1951). [15] Pechkovskii, V. V., and Sofronova, A. V., Zh. Neorgan.

Khim. 10, 1513 (1965). [16] Rapoport, E., and Pistorius, C. W. F. T., Phys. Rev. 172,

838 (1968). [17] Vereshchagin, L. F , Kabalkina, S. S., and Kotilevets, A. A., Sov. Phys. JETP 22, 1181 (1966). [18] Winkler, H. G. F., Z. anorg. u. allgem. chem. 276, 169

(1954). 9. 4d Transition Metal Halides

ZrF4 exists in two polymorphic forms [13a, 42a]. AgF undergoes a pressure transformation from

A transition in MoF6 has been reported from NMR NaCl to the CsCl structure [18b]. There is no thermal

studies [la]. transition in AgCl or AgBr. Agl, which usually

34
exists in the B4 or wurtzite. form (hexagonal P63 me; a=4.592 A, c=7.510 A) and in the B3 or sphalerite form 1 (low cubic, F43m, a=6.495 A)

both of which undergo a thermal transformation

at 418 K to the B23 (high cubic) phase. The trans- formation is reversible and is associated with

large A H as well as considerable thermal hysteresis,

AT and volume change, AV; the transformation is likely to be of first order [39]. The existence of the low-cubic (sphalerite or B3-form) Agl was questioned by Majumdar and

Roy [29]. Burley [12], however, reports the kinetics

of the B3 ~

B4 transition in Agl by x-ray diffractometry at high temperature. The structure of the

B23 (high-cubic) Agl has also been a subject of

interest [22]. Statistical models have been employed to explain the type of one-dimensional

polymorphism (polytypism) in Agl [44]. Accordingly,

B3 and B4 are polytypes of Agl. Cdl2 is another

example of polytypism.

AgBr enters into solid solution with Agl and the

structures of the specific phases depend on the

composition and temperature [3, 4]. The solid

solutions of Agl and AgBr have the B3 and/or B4

structure up to ~ 10 percent AgBr, and with higher

percent of AgBr the NaCl type of structure (B1

phase) is formed [16, 39]. About 25 percent of Agl seems to enter into solid solution with AgBr in the

B1 -phase. However, the literature data available on the solid solution limits and the range of stability

of specific phases are widely different.

Pressure transitions of silver halides with Blstructure have been studied in detail, but there

appears to be some uncertainty regarding the

exact structures. The most commonly postulated

high pressure structure is the CsCl type (B2)

structure [2, 9], essentially because of its higher

anion-cation coordination. X-ray studies have re- vealed this to be the case with ionic solids like

rubidium and potassium halides. However, with

more covalent solids this is not the case. It has

been shown by x-ray studies that AgCl and AgBr

under pressure transform to a cinnabar type of

structure (B9) i.e., a tetragonal one [26. 47]. The

different phases of Agl encountered in the high

pressure transitions have different structures

and ranges of stability [1, 5]. In the following

table the high pressure phases of Agl are listed

along with their structures and stability ranges.

Structure and Stability of Silver Iodide in the High Pressure Area

Designation Structure Lattice constants Range of stability

I B23. high cubic a = 5.074 A at 443 K Stable above ~ 419 K and up to ~ 80 kbar.

II B4. wurtzite 0 = 4.591 A, c= 7.498 A

at 298 K. Stable up to ~ 419 K and below ~ 2 kbar.

II' B3. sphalerite 0 = 6.495 A at 298 K Stable below ~ 413 K and below ~ 2 kbar.

III Bl, NaCl type a = 6.07 at 298 K Stable up to ~ 419 K and above 3-4 kbar. IV Orthorhombic Stable below ~ 320 K and between 2—4 kbar. V B2. CsCl type Stable above 95 kbar.

Lattice energies of silver halides have been

calculated by Mayer [31], Huggins [23], Ladd and

Lee [27], Sharma and Madan [45] and recently

by Natarajan and Rao [39]. Most of these workers

have employed the Born model. Lattice energy

calculations have enabled a study of the relative

stabilities of the various structures of silver halides

and some of their solid solutions [39]. Born model

has been employed to explain the pressure transitions in silver halides as well as the thermal

transition in silver iodide [39]. T ransition pres- sures have been theoretically determined by Jacob

[24] employing the Born model.

Polytypism of Cadmium halides. Both CdBr2 [8] and Cdl2 [18a] are known to exhibit polytypism

[32, 35, 37, 44, 57a]. Single crystal x-ray studies on CdBr2 grown from solution have shown that

6R is by far the most common polytype of CdBr2 .

There are as many as 130 structures for the CdBr2 polytypes [36]; 92 of these belong to the 6R type;

sublimation of CdBr2 also yields the 6R type.

Polytypism in Cdl2 has been found to be due

to screw dislocations [32]. X-ray diffraction studies

show the existence of many polytypes with dif- ferent stacking sequences [31]. The entire

“22 . . . 11" series of structures could be generated by spiral growth from single nonintegral screw dislocations in an “ideal" 4H structure

[33, 34]. Other known structures not belonging

to the “22 . . . 11" series could be generated

by assuming the presence of two or more cooperative dislocations. These considerations suggest

that polytypism is not expected in certain substances because of the nature of their ideal structures. A detailed discussion on polytypism is fur- nished by Verma and Krishna [57a].

35
4d Transition Metal Halides

Substances and measurement techniques Data Remarks References

Zirconium tetrafluoride, ZrF4 (Monoclinic, C!jh

, a = 11.69 A, b =

9.87 A, c= 7.64 A; /3 =

126° 9' at 298 K)

DTA and heat capacity ZrF4 undergoes a phase transition

at ~ 680 K. The high temperature form is cubic (Fm3m) with a — 7.88 A.

Heat capacity measurements in the range 5—307 K do not indicate any

anomaly. DTA indicates a transition at ~ 680 K.

[13a, 42a,

58a].

Molybdenum hexafluoride, MoFe

NMR investigations of phase

change in MoF6 (temperature

dependence of half width of the

19 F line).

Two jumps in half-line-width at ~ 263 K and at ~ 219 K are ob- served. The jump at 263 K is due to the phase transition in MoF6 at ~ 263 K.

The spectra of the two modifications

of MoFe are discussed in relation

to the different types of Mo-F bonds

and other symmetry considerations.

[4

Silver Fluoride, AgF (Fm3m, a =

4.932 ± 0.003 A at 295 K)

X-ray studies at different pressures. Transforms to a new structure (probably B2 or CsCl) at ~ 26 kbar with AF=1.64 cm3 mol; AS ~ 4.2-5. 1 Joule/mole-deg.

The lattice parameter of the B2

phase (extrapolated to 1 bar) is 2.99 A.

[18b].

Silver iodide, Agl (B3-Agl,

F43m, a = 6.495 A at 298 K; B4-Agl, P63 mc, a = 4.592 A,

c= 7.510 A at 298 K)

Structure of various polymorphs

of Agl by x-ray diffraction. Three modifications of Agl are known; a low cubic-sphalerite

(B3) phase stable upto 410 K, a hexagonal-wurtzite (B4 phase)

which is formed when the Agl

melt is cooled and a hot cubic

(B23) form stable above 420 K. The hot cubic phase has a space

centred lattice of I atoms, with a = 5.04 A; Ag ions are dis- tributed over the 30 largest gaps

in the I structure.

The structure of B23-Agl was sug- gested to be tetragonal, but later work confirmed that the structure

is an average between two struc- tures of space groups Im3m and

I4m2. The structure of Agl pre- cipitated from solution depends on

the presence of excess of I - or Ag+ as well as the rate of precipitation.

[6, 7, 10„ 19, 21,22,28,

52, 60].

Agl polymorphism with DTA, X-ray etc. Sphalerite to wurtzite transformation probably takes place over a wide temperature interval. The

transition is associated with a A Htr ~ 100 cal • mol-1

. The

energy of activation is ~ 10.3 ±

0.8 kcal • mol -1

. The low temperature phases of Agl trans- form to the B23 phase at ~ 420

K with a AHtr of about 1600

cal • mol-1

.

The sphalerite-wurtzite transition is more likely to be an athermal

higher order transition. Burley’s

[12] observation, however, suggests that the transition is a first- order one. Presence of iodine vapor seems to facilitate the B3~»B4

change at ~ 393 K. The Tt of B3/

B4 —B23 transition also depends

on the partial pressure of iodine. B3 —

B4 transition is not seen by

x-ray diffraction or DTA when small

heating rates are employed. The

B3/B4 —* B23 transition is a well defined first order transition.

[11, 12,22,

29,30,39,

43,58,59

54],

36
4d Transition Metal Halides — Continued

Substances and measurement techniques Data Remarks References

B3/B4—» B23 transition investigated by emf method.

The emf of the cell, Ag(s) Agl(s) CaCl2

• 6H2 0 Pb(s)

Agl(Ci)

PbI(C2 ) was studied as a function of tem-

| perature. T,~ 419.6 K; AHtT ~

1270 cal • mol-1.

[14, 15].

Electrical Conductivity The high ionic conductivity of the

hot cubic (B23) phase above (420

K) of Agl is in accordance with a random orientation of silver ions

in the lattice. E„ for conduction

in B3 and B4 phases are ~0.2

and —0.6 eV respectively.

The cr versus l/T plots seem to sug- gest the transition of B3—» B4 to occur at —410 K.

[20.38.

39, 53].

X-ray and DTA study of the effect of AgBr on the Agl transition. AgBr lowers the Tt to — 493 K at — 10% AgBr. This seems to be

the solid solution limit in the low and high temperature phases. The

specific phases of the solid solu- tions depend on the temperature

as well as composition.

AgBr and Agl form an eutectic at — 68% AgBr with a eutectic temperature of — 613 K.

[3, 4. 16, 39, 51, 55],

Silver chloride, AgCl (Fm3m, a =

5.5491 A at 299 K) and Silver bromide AgBr (Fm3m. a = 5.7745 A at 299 K).

X-ray diffraction of AgCl supported

in amorphous boron vessel and

subjected to quasi-hydrostatic pres- sures up to 150 kbar.

At — 25 kbar, AC is 0.9 V0 where V,t is the volume at 298 K

and 1 atm. At higher pressures,

the structure is shown to be that of Hg2 Cl2 ; a = 3.92 A, c=9.03 A

and c/a = 2.30.

This is the first experimental evidence

where a tetragonal structure is assigned to the AgCl high pressure

phase.

[26],

X-ray diffraction Both AgCl and AgBr do not trans- form to a B2 or CsCl-type structure. The high pressure phase in both the cases is of B9 type. For AgCl, a = 4.06 A, c= 7.02 A; c/a= 1.73. AV/K ~ 8% at the

transition. For AgBr, a = 4.0 A,

c=7.15 A; c/a— 1.77.

The tetragonal structure is stabilized due to overlap of electron orbitals resulting in the formation of two

highly covalent collinear bonds. The

Hg2 Cl2 type structure assigned to AgCl at high pressure is questioned.

[47].

V olumetric method Pt ~ 88 kbar for AgCl; — 84 kbar for AgBr.

The high pressure phases were con- sidered to be of B2 structure.

[9]-

Optical observation on single

crystals. Pt ~ 90 kbar for AgCl, Pt ~ 86 kbar for AgBr.

[2].

Pressure-temperature curves for AgCl and AgBr upto 60 kbar and

1073 K.

No change was observed in both the

halides. The pressure was not enough to detect the appearance of the high pressure phases.

[18].

Pressure transitions in Agl:

High pressure transition in Agl

investigated by x-rays. At — 3 kbar the cell symmetry seem to belong to orthorhombic

one. Moore and Kasper [37], however, consider the 3 kbar phase

as tetragonal or pseudotetragonal.

Stability range of six polymorphs is indicated in a P~T diagram. The

phase at 3 kbar is likely to be re- lated to the NaCl structure, according to Moore and Kasper.

[1, 17,37].

High pressure transition of Agl upto 60 kbar and x-ray investigation. At — 60 kbar, Agl has the NaCl(Bl)

structure with a = 6.067 A.

[40],

37
4d Transition Metal Halides — Continued

Substances and measurement techniques Data Remarks References

High pressure transition in Agl up to~ 120 kbar; examination by

microscopic and x-ray diffraction techniques.

Microscopic examination indicates a

transition at ~ 115 kbar. X-ray

studies reveal that at ~ 97 kbar,

the Agl transforms to a tetragonal structure with a = 5.61 1 A, c= 5.02 A and volume change is ~ 13%.

Electrical conductivity measure- ments suggest that the transition involves a significant change to more covalent bonding.

[2.47],

High pressure transformation in Agl up to 100 kbar. and x-ray

investigation.

As many as six polymorphs have

been reported: B3, B4, B23, a 3 kbar phase, B1 structure and a

sixth one.

The 3 kbar phase is yet to be identified exactly. There is a possible

mutual transformation between B3,

B4 and the ‘3 kbar’ phases by a dis- placive mechanism rather than a reconstructive mechanism. This im- mediately suggests that B3, B4 and

the 3 kbar phases are polytypes of Agl and should be very similar to each other energywise and struc- turewise.

[5].

High pressure transition in Agl by

electrical conductivity measure- ments as a function of pressure up

to 100 kbar.

Conductivity increases with pressure

in the low pressure phase (stable upto 2.5 kbar) and then decreases

in the two high pressure phases.

The increase in the low pressure phase is in accordance with — ve thermal expansion. A reversal of the slope above 45 kbar in the (NaCl

type) cubic phase is explained in terms of the formation of I- ions.

Riggleman and Drickamer [42] however find a large increase in conductivity at ~ 97 kbar indicating

a transformation.

[42, 46],

Cadmium fluoride, CdF2

X-ray study up to 160 kbar CdF2 transforms to the o:-PbCl2 structure reversibly around 60

kbar; a=3.368 A; 6=5.690 A,

c = 6.730 A (at 120 kbar?).

[15a],

Cadmium bromide, CdBr2 (hexagonal, R3m, a = 3.957 A, c= 18.668 A)

X-ray study of CdBr2 polytypic

structures.

Cadmium iodide, Cdl2 (hexagonal,

P3ml, a = 4.24 A, c = 6.835 A)

An x-ray study of solution grown

CdBr2 crystals has revealed that 6R is by far the most common

polytype. Other structures en- countered include, 4H, 12H (or 12R), 40H (or ~ 120R) and some

disordered structures.

Sublimation of CdBr2 does not produce any polytype other than 6R. The

relation between screw dislocations and polytypism is discussed.

[7, 36].

X-ray and interferometric studies

of Cdl2 polytypism in relation to crystal growth.

A combined optical and x-ray dif- fraction study was made of polytypism of Cdl2 which grow by a dislocation mechanism. For the 2H and 4H structure types, a cor- relation between spiral step

height and x-ray unit cell is found;

with larger unit-cells no such cor- relation was observed. This is contrary to expectations on the

basis of Frank’s theory and can

not be fully explained.

Thirteen new polytypes of Cdl2 are described. This number probably

may be eighteen. In addition, several crystals resembling 2H and

4H types are also reported.

[56, 57].

38
4d Transition Metal Halides — Continued

Substances and measurement techniques Data Remarks References

Investigation of the rhombohedral

polytypes of Cdl2 and the trans- formation to hexagonal polytypes.

Two rhombohedral polytypes of Cdl2 , 30R and 42R, grown from

vapour phase are reported. Their

complete structures are (221212)3

and (22221212)3 respectively.

Each of these polytypes occurs in syntactic coalescence with a hexagonal polytype having the same cell dimensions as the rhombohedral

one showing that the lattice was

transformed from hexagonal to rhombohedral or vice versa during

crystal growth.

[13].

New polytypes of Cdl2 A 28 layered new polytype of Cdl2 , designated as 28 He with unit

cell dimensions, a = 4.24 A,

c — 95.69 A has been described. The detailed atomic structure has been worked out and its zig zag sequence is found to be 2222222222221111. Another

anomalous polytype of Cdl2 is 26 He.

The screw dislocation theory does not

fully explain the growth of this polytype 28 He which is probably

the largest polytype of Cdl2 known,

with an atomic structure. 26 He

also does not conform to the Frank

screw dislocation theory. So the

structure has been solved by a Fourier transform method as a special case using calculated and

observed x-ray intensities distribution. Other anomalous polytypes of Cdl2 discussed, include 26 Hd, 50 He

etc. Factors affecting the phenomenon of polytypism are enumerated.

[48, 49, 50],

X-ray investigation of polytypism as

a higher order transformation.

Interaction energies for the known

polytypic structures are calculated

for Cdl2 on the basis of Schneer’s

theory of polytypism. Almost

all polytypes are characterised by

a maximum number of unlike

interaction contacts.

This aspect was confirmed, experimentally by x-rays, which suggests a gradual transition from 3C to 2H

structures.

[41].

X-ray study of the phenomenon of polytypism in Cdl2 . X-ray oscillation photographs of Cdl2 crystals reveal the existence

of two different structures in parallel orientation on the same

face of a crystal. This helped in understanding the frequent observation of the syntactic coalescence

of two more polytypes in the same

crystal as a phenomenon of

epitaxial growth.

Prolonged microscopic observations have shown that 3-dimensional

nuclei keep on forming at preferred

sites during the growth of crystals. Subsequent x-ray investigation shows

that it leads to the formation of a new

polytypic structure of lower free- energy.

[25],

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10. 5 d Transition Metal Halides

OsCU is known to exist in two polymorphic

forms [15]. Transitions in mercury halides are

also known. Both mercuric chloride and mercuric

bromide are orthorhombic at room temperature.

No detailed phase transformation data are available

for these two halides. Some information regarding

the isomorphic relations of these halides as well

as their mixed salts are available [11, 12],

Red mercuric iodide is tetragonal and is the

stable phase at room temperature. The change of

red Hgl2 to the orthorhombic yellow variety takes

place reversibly at ~403 K. The effect of state of

aggregation [1], heating rate [3, 5] and the pres- ence of other halides [10] on the Tt have been

studied. The theory of dynamic allotropy has been

found to be applicable to the Hgl2 transition [5].

The kinetics of the tetragonal-orthorhombic

transition in Hgl2 has been studied by Baram [2] and others [3, 6]. The mechanism of the transition

[14] has been discussed by Johansson [7], from

considerations of the structures of the two modifications. The Hgl2 transition has been considered as an example of a topochemical reaction which

takes place in the inner or outer fields of force of the crystalline matter [8, 9]. The high temperature

yellow Hgl2 could be stabilized on A12 C>3 base

provided the sample is free from moisture [17]. Recently, the Hgl2 transition has been studied

with low frequency laser excited Raman spectroscopy [13]. A third variety of Hgl2 has been reported

[16]. A pressure transition in HgL2 was reported by

Bridgman [4].

40
5cl Transition Metal Halides

Substances and measurement

techniques Data Remarks References

Osmium tetrachloride. OsCfi

(cubic, a = 9.5 A at 298 K)

X-ray and magnetic measurements A solution of OsCL when refluxed with SOCL yields the cubic form. This is a dark-brown solid. It belongs to the space-group Ofi -P43 [32] or O

7

— P4i[32]. Os04 and CC14 when reacted in a sealed tube at ~ 743 K. yield the high temperature orthorhombic

form (o= 12.08 A. b= 11.96 A

and c= 11.68 A).

Both the low and high temperature

forms are paramagnetic and display

magnetic susceptibilities as a result of strong spin-orbit coupling; the high temperature form exhibits the temperature dependent magnetic

susceptibility.

[15].

Mercuric iodide. HgL (Tetragonal, o = 4.39 A. c= 12.38 A at 296 K).

Factors affecting the T, of Hgl 2 A freshly prepared sol. of Hgl2 transforms at a lower temperature. The T, increases as precipit- ation continues. Tt is also affected by other halides such as HgCl2 , HgBr etc.

The yellow7 sol. when dried is covered

with a red layer which disappears

slowly.

[1, 10],

Dilatometry and kinetics The transition from red to yellow

variety takes place at ~ 400 K. The dilatometric studies indicate

that the transition is autocatalytic in both the directions. Quenching experiments indicate

that the theory of dynamic allo- tropy is applicable to Hgl2 transition. AF~ 0.0028 cm3 g-1

.

Transition rates in the range 400-

406 K are almost the same in the presence of air, C02 or nitrogen.

However, presence of hydrogen or oxygen has a definite effect on the speed of the conversion of yellow to red form. An inconclusive evidence

for a white modification of Hgl2 is also mentioned.

[2, 3, 5, 6, 8, 9],

Low frequency laser Raman

spectra. Raman bands for red Hgl2 are at 17.5, 29 and 114 cm-1

; in addition,

weaker bands are seen at 46, 142, and 246 cm-1

. For yellow Hgl2 , stronger bands are at 37, 41. and

138 cm-1

; a weaker band is seen

at ~ 278 cm-1

.

Marked changes in the spectra were

attributed to changes in crystal structure.

[13].

References

[1] Balarev, D., Chemie 47, 27 (1952).

[2] Baram. O. M., ukrain. khim. Zhur. 22, 137 (1956).

[3] Benton, A. F., and Cool, R. D., J. Phys. Chem. 35 , 1762

(1931).

[4] Bridgman, P. W., Proc. Am. Acad. Arts Sci. 51, 55 (1915).

[5] Damiens, A., compt. rend. 177,816(1923).

[6] Eade, D. G., and Hartshorne, N. H., J. Chem. Soc. 1636

(1938).

[7] Johansson, G., Arkiv Kemi 8. 153 (1955).

[8] Kohlsciiutter, H. W., kolloid chem. Beihefte 24 , 319(1927).

[9] Kohlsciiutter, V.. kolloid-Z 42 , 254 (1927).

[10]

Losana, L., Gazz. Chim. ital. 56 , 301 (1926).

[11] Luczizkii, W., Mem. Soc. Nat. Kiew. 20, 191 (through

Chemical Abstracts, 1, 28492 ). [12] Luczizkii, V. I., Z. Krist. Min. 46, 297 (through Chemical

Abstracts, 6, 57

9

). [13] Melveger, A. J., Khanna, R. K.. Guscott, B. R., and Lippincott, E. R., Inorg. Chem. 7, 1630 (1968).

[14] Newkirk, J. B., Acta. Met. 4 , 316 (1956).

[15] Paul, M., Z. Naturforsch 24 , 200 (1969).

[16] Seiskind, M., Michel, P., and Grun, J. B., J. Chim. Phys.

56,858(1959).

[17] Zeitlin, H., and Goya. H., Nature 183, 1041 (1959). 1 1 . Rare-Earth Trifluorides

The crystal structures of some of the rare-earth

fluorides like SmF3 , EuF3 , HoF3 , and TmF3 depend

on the method of preparation [2], The fluorides

can thus exist in a hexagonal structure or an orthorhombic one, YF3 is orthorhombic at room temperature. A cubic modification of YF3 reported by

41
Nowachi [1] has not been reproducible. The preparation of the various fluorides and their crystal dimensions have been reported by Zalkin and Templeton

[ 2]-The hexagonal trifluoride seems to be the only

stable form for elements from lanthanum to neodymium. It is likely that it can be produced under

proper conditions for the rest of the rare-earth

elements and also yttrium, although this has not been achieved in several cases. The data on LnF3 structures are best understood if only the hexagonal

form is stable at elevated temperatures even though it may be precipitated from solution under

certain conditions because of greater ease of crystal

nucleation and growth.

References

[1]

Nowacki. W., Z. krist. 100 , 242 (1938).

12. Inert

Xenon fluorides (XeF2 , XeF4 , XeFe, etc.) have

been well characterized in the literature. Two

forms of XeF4 have been reported from x-ray studies.

XeFe has been found to exhibit interesting thermal

[2] Zalkin, A., and Templeton, D. H., J. Am. Chem. Soc. 75,

2453 (1953).

Gas Halides

properties [3]. Heat capacity measurements have

indicated the existence of two phase transitions

[3]

, in the range 200-300 K. The crystal structures

of two XeFe phases have been reported [1].

Inert Gas Halides

Substances and measurement

techniques Data Remarks References

Xenon tetrafluoride, XeF4

X-ray studies Two monoclinic forms I and II are reported for XeF4 . Form I belongs

to P2i/C space group with «=6.64±0.01 A. 6=7.33±0.01 A,

c=6.40±0.01 A; /3=92°40'±5'. p = 4.42 g cm-3

. Form II belongs to P2j/n space group with 0 =5.03 A. 6=5.92 A, c=5.79 A; 0=99° 27'. p = 4.04 g cm-3

.

The greater density of form I suggests

that this is the lower temperature

polymorph of XeF4 .

[2. 4. 5],

Xenon hexafluoride, XeF6

X-ray and neutron diffraction Single crystals of XeFe were grown

by sublimation in a thermal

gradient near room temperature.

The crystal structure belongs to P2i/m or P2i space group (monoclinic) with o=9.33±0.03 A,

6=10.36±0.03 A. c=8.95±0.03 A;

/3=91.9±0.2°. A second modification of XeFe, belongs to F43c or Fm3c, with a=25.34±0.05 A. This (FCC) XeFe transforms to the third form around 295 K with

a hysteresis of ~ 8°.

The crystal structure of the form III is

still to be worked out.

[!]•

Calorimetry Very large specific heat anomalies

are observed in the regions

241-261 K and 288-297 K.

The anomalies are probably due to slow phase transitions.

[3].

References

[1] Agron, P. A., Johnson, C. K., and Levy. H. A.. Inorg. Nucl. [4J Siegel, S., and Gerbert, E., J. Am. Chem. Soc. 85,240(1963).

Chem. Letters 1. 145(1965). [5J Templeton, D. H.. Zalkin, A., Forrester, J. D.. and Willian-

[2] Burns. J. H., J. Phys. Chem. 67, 536 (1963). son. S. M., J. Am. Chem. Soc. 85, 242 (1963).

[3] Malm. J. G.. Schreiner. F., and Osborne, D. W., Inorg. Nucl. Chem. Letters 1 , 97 (1965). 42
13. Halogens

Substances and measurement

techniques Data Remarks References

Fluorine

Heat capacity as a function of temperature

T,~45.55±0.02 K

AHtr . 173.9±0.004cal-mol-'

Entropy of fluorine at 85.02 K = 39.58 ± 0. 16 e.u.

The transition is indicated by an anomaly in C,,(T) curves. The

transition had been suspected by

Murphy and Rubin [3j from an

analysis of the heat capacity data of Kanda |2|.

IH.

Bromine

P—T behavior of single crystals up to 273 K. A new solid phase at high pres- sures (~35 kbar) and at —273 K

is reported with the space group Cmca (orthorhombic) «=4.79 A, 5=6.75 A and l c=8.63A.

|4|.

References

|lj Hu. J. H., White. D., and Johnston, H. L., J. Am. Chem.

Soc. 75,5642(1953).

|2] Kanda. E.. Bull. Chem. Soe. Japan, 12,473 (1937).

[3] Murphy, G. M., and Rubin, E., J. Chem. Phys. 20, 1179

(1952).

|4] Wier, C. E.. Piermarini. G. J., and Block. S., J. Chem. Phys.

50.2089(1969).

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BIBLIOGR^APHJC DATA NBS -NSRDS-41

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Crystal Structure Transformations in Binary Halides

5. Publication Date July 1972

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16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

A critical survey of the data describing crystal structure transformations in binary

halides is compiled. Data on thermodynamic, crystallographic, spectroscopic and

electronic properties are given for each transformation. Experimental techniques

used to obtain the data are named and comments on the data are included in the tables

The literature is surveyed up to 1970. References have been selected on the basis of

their pertinence to the data which are cited and do not represent all the available

literature.

17. KEY WORDS (Alphabetical order, separated by semicolons)

Binary halides; Crystal structure transformation; Electronic data; Phase transformation; Spectroscopic data; Thermodynamic data; X-ra.y diffraction data.

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Catalog No. Cl 3.10: 305. Price $2.00; 50 cents additional for foreign mailing.

NBS Special Publication 305, Supplement 1,

Publications of the NBS, 1968-1969. When ordering, include Catalog No. Cl3.10:305/Suppl. 1. Price

$4.50; $1.25 additional for foreign mailing.

NBS Special Publication 305, Supplement 2,

Publications of the NBS, 1970. When ordering, include Catalog No. C13.10: 305/Suppl. 2. Price

$3.25; 85 cents additional for foreign mailing.

Order NBS publications (except Bibliographic Subscription Services)

from: Superintendent of Documents, Government Printing Office, Washington, D.C. 20402. J
L.L