National Bureau of Standards

OCT 7

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NBS TECHNICAL NOTE

Reference book not to be

taken from the library.

293

Research on Crystal Growth

and Characterization at the

National Bureau of Standards

June 1966

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U.S DEPARTMENT OF COMMERCE

National Bureau of Standards
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NBS TECHNICAL NOTE 293

ISSUED AUGUST 26, 1966

Research on Crystal Growth

and Characterization at the

National Bureau of Standards

June 1966

Edited by Harry C. Allen, Jr. Institute for Materials Research

National Bureau of Standards

Washington, D.C., 20234

Supported by the

Advanced Research Projects Agency

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide a means for making available scientific data that are of transient or limited interest. Technical Notes may be

listed or referred to in the open literature.

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

Washington, D.C., 20402 - Price 25 cents

RESEARCH ON CRYSTAL GROWTH AND CHARACTERIZATION

AT THE NATIONAL BUREAU OF STANDARDS

Supported by the

Advanced Research Projects AgencyEdited by

Harry C. Allen, Jr.

The National Bureau of Standards with partial

support from the Advanced Research Projects Agency

of the Department of Defense is continuing a wide

program of studies involving crystalline materials.

These include investigation of methods and theory

of growth, study of detection and effects of defects,

determination of physical properties, refinement of

chemical analysis, and determination of stability

relations and atomic structure. The types of materials range from organic compounds, through metals, and

inorganic salts to refractory oxides. This report

summarizes progress in those projects wholly or partially supported by ARPA.

Key words: Crystal growth, crystal characterization,

and crystalline materials.

1. INTRODUCTION

The National Bureau of Standards continues a wide and active

program in many aspects of material research. The NBS has been

encouraged by the Advanced Research Projects Agency of the Department

of Defense to extend the study of crystals, particularly in the

field of crystal growth and has aided with financial support. This

report describes briefly the current state of the many projects here

in the general field of crystal studies supported by ARPA. These

items should be considered as progress reports, not in the nature

of finished studies. The formal papers resulting from the work

are listed in these pages.
To aid in integrating the crystal program here and to encourage

cross linking between participants in the program, a number of

meetings, discussions and lectures have been arranged. In many

cases these report and discuss our current work, in others scientists

from outside material science centers appear.

2. CRYSTAL GROWTH

2.1 Theory of Morphological Stability of Solids

Growing by Diffusion or Heat Flow

S. R. Coriell, R. L. Parker

Crystallization of Metals Section

Research has continued on the stability of the shape of

particles growing in a diffusion field. Currently, the effect of

interface kinetics on the stability is being studied. It has been

found that stability is greatly enhanced, by several orders of

magnitude in certain cases, if the interface kinetics are slow.

The effect of anisotropic surface tension on the stability of a planar interface has also been considered. The effect of the

anisotropy has been found to involve the second derivative of

the surface tension.

Publications and Talks

S. R. Coriell and R. L. Parker, Role of Surface Diffusion in

Stabilizing the Surface of a Solid Growing from Solution or Vapor;

J. Appl. Phys., 37, 1548 (1966).

S. R. Coriell and R. L. Parker, Interface Kinetics and the Stability

of the Shape of a Solid Sphere Growing from the Melt; International

Conference on Crystal Growth, Boston, Mass., June 20-24, 1966.

2.2 Interface Kinetics of Growth of

Metal Crystals from the Melt

J. G. Early, H. Utech, W. S. Brower

Crystallization of Metals Section

Studies have been initiated to study thermal convection effects

on crystal growth. Apparatus has been constructed and experiments

have been carried out on convection effects in several tin-bismuth

alloys under controlled freezing conditions. Experiments have been
planned and will shortly be initiated to study possible convection

effects on the interface kinetics of growth. These experiments

will determine possible effects of experimental geometry on the

technique for studying interface kinetics.

Publications and Talks

H. P. Utech, W. S. Brower and J. G. Early, The Effects of Thermal

Convection on Crystal Growth in Horizontal Boats; International

Conference on Crystal Growth, Boston, Mass., June 20-24, 1966.

2.3 Electrodeposition of Single Crystals

F. Ogburn, J. Smit

Electrolysis and Metal Deposition Section

Efforts at crystal fabrication were confined to the deposition

of silver from solutions of reagent grade silver nitrate in distilled water. The deposition equipment was designed so that the

cathode potential or current could be held constant at any predetermined value. Measurements were made of the electrical current,

cathode potential and rate of growth of individual dentrites

.

The character of the crystalline silver deposits is highly

dependent on deposition conditions. In the course of the present

investigation it was observed that non-metallic impurities in the

silver nitrate appeared to prevent or discourage the growth of

dendrites and crystal facets. However, prolonged electrolysis

reduces the impurity level sufficiently to permit reproducible

dendritic growth and faceting. At high cathode potentials, dendrites

grow rapidly with extensive branching. Decreasing potentials reduce

branching and also reduce the growth rate of individual dendrites.

A further reduction in potential results in monoacicular growth.

At still lower potentials acicular growth ceases and equiaxed growth

commences

.

The progress of a single acicular growth has been observed

while varying the cathode potential. In so doing it was possible to

determine the growth rate as a function of the cathode potential.

Growth rates of up to 25 mm/h (70,000 A/s) were observed.

A combination of optical and optical goniometric, metallographic

and x-ray diffraction techniques are used to determine dendritic

morphology and structure. To this time, however, only acicular

deposits have been examined. These examinations have revealed, in

every case, a twin structure or structures In the subject specimen.
To this point no less than five distinct twin and multiple twin

structures have been uncovered. As expected for the F.C.C. system

the x-ray diffraction patterns indicate the (111) plane to be the

composition plan of the twin structure and <110> direction to be

the growth direction of the dendrites. An exception to the above

occurred in a dendrite of planar form grown from a purposefully

doped solution containing an additional metal ion. In this case

the direction of growth proved to be the <112> direction. The twin

composition plane remained the (111) plane.

Twin boundaries, parallel to the growth direction, are always

present in the silver dendrites and, hence, seem to participate in

the growth mechanism, possibly from a re-entrant angle mechanism

rather than a dislocation mechanism. Unfortunately, no clear cut

picture of the role of twinning with respect to acicular growth

exists at this time. Several twin boundary configurations have

been observed and may possibly be related to the dendritic growth

rate. This is yet another aspect of the general problem that needs

elucidation.

2.4 Copper Crystal Growth

A. F. Clark, J. J. Gniewek, J. C. Moulder and R. L. Powell

Cryogenic Properties of Solids Section

Copper is used extensively by the solid state physicist to

study transport properties, band structure, and Fermi surface effects.

The effective electronic purity of large copper crystals available

one year ago (resistance ratios of 2000 to 4000) was not good enough

for many measurements. During the last year project members developed

methods and produced copper crystals that are an order of magnitude

better than those previously available: electrical resistance ratios

of over 40,000 (the last single crystal was 44,000), chemical impurities

of 0.1 ppm or less for any impurity, and physical dislocation densities

of about 10s /cnv3

The crystals are grown by a modified Bridgman technique in a graphite-lined quartz crucible with a vacuum of about 10 torr. The

heating is supplied by an induction furnace; about 3kW is necessary

to melt the boules. After solidification and re-cleaning the boules

are transferred to an annealing furnace for a long time anneal (up

to one week) at 1000 °C in a controlled oxidizing atmosphere. With

short anneal times there is a very marked impurity gradient toward

the center of the boule.

The production boules are about 2.9cm in diameter and over

3.0cm long. Small oriented samples have been effectively sparktrepanned from the larger boules. Studies on the damage of spark
machining show that even on fine cutting ranges there is significant

damage within 0.2mm of the surface. Methods have also been developed

for controlled etching and removal of the damaged surfaces

.

Two serious problems remain to be solved. First, most of the

boules are not single crystals, but contain many striation regions

that make the boule unusable. Their exact cause is still unknown.

Until that problem is solved the method remains inefficient. Second,

controlled dilute impurity crystals should be prepared in order to

test various theories of impurity effects in the ideal limit of an

almost pure crystal. Dilue impurity crystals would be very helpful

in several metal physics experiments.

Publications

J. J. Gniewek, A. F. Clark and J. C. Moulder, Spark Planning

Damage in Copper; NBS Tech. Note 321 (1965).

J. J. Gniewek and A. F. Clark, Preparation of Copper Crystals

with Low Electrical Resistivity; J. Appl. Physics 36., 3358 (1965).

2.5 High Temperature Crystal Growth from Solvents

H. S. Parker

Crystal Chemistry Section

The development and evaluation of methods for the growth of

highly perfect crystals at temperatures below the melting point

of the crystal and to obtain an understanding of the parameters

affecting the growth process continued. The technique of temperature

gradient zone melting, proposed by Pfann (Trans. AIME 203 , 961 [1955],

is currently being investigated to determine its applicability to

crystal growth of oxides having the scheelite (CaWO ) structure. As

a potential laser host, crystals having this structure are of

technological as well as scientific interest.

During this reporting period, the major effort has been

directed towards the growth of crystals of CaMoO using Lip SO

as the solvent. The temperature gradient zone melting technique

involves causing a thin layer of solute plus solvent to traverse

a sandwich composed of single crystal seed/( solute + solvent)/ source

material by placing the assemblage in a temperature gradient.
Stable zones have been formed successfully and regrowths of

the order of 1 - 2 mm have been obtained at temperatures In the

800 - 960° C temperature range, which is 500 - 650° C below the

melting point of CaMoO . Orientation and preliminary etch pit

studies of the regrown crystals indicate that the orientation of

the seed is preserved in the regrown crystal and that the number of

dislocations is reduced by approximately an order of magnitude as

compared with the Czochralski-grown material.

The temperature gradient and temperature in the zone region

have been measured during a growth run, and it was found that the

AT/Ax was 250°C/cm at an average temperature of 815 °C for the zone.

The measurements of temperature were made by inserting small

(0.025 mm x 0.5 mm) thermocouple junctions into the sandwich

assembly and "growing" them in place by passing a solvent zone

through the assembly before the actual growth experiment and

temperature measurements were made.

In order to establish the liquidus slope in the CaMoO -Li2 S04 system, a series of liquidus determinations has been carried out

at concentrations of CaMo04 ranging from the eutectic composition, ~ 15 weight percent CaMo04, to 50 weight percent CaMoO . Liquidus

temperatures ranged from 20 °C to 1022 °C. The average liquidus

slope over the temperature range where the crystal growth experiments were conducted was found to be 0.14 weight percent per 1°C.

Tiller (J. Appl. Phys . 34, 2757 [1963] has suggested that

the zone travel rate should depend on the zone size and growth

mechanism as well as other factors such as the temperature gradient

A series of growth experiments is in progress to see if this

phenomenon is evident in this system for zone thicknesses ranging

from 1.3 x 10 _s to 8.4 x 10 -8 cm. Although the measurements are

not completed, a dependence is apparent.

Measurements of the lithium concentration profile across

the specimens in the direction of zone travel are currently being

made.

Talks

H. S. Parker, The Growth of CaMo04 by a Temperature Gradient Zone

Melting Technique, International Conference on Crystal Growth,

June 20-24, 1966.
2.6 Kinetics of Crystal Growth from Solution

J. L. Torgesen, R. W. Jackson

The growth on the {100} plane of ammonium dehydrogen

phosphate (ADP) in solution is by a screw dislocation mechanism.

The growth spirals are elliptical in shape with the short axis

parallel to the {001} plane of the crystal. Neighboring dislocations of opposite hand (Trank-Read) are common. Growth on

the {101} plane of ADP proceeds by the movement of layers from

the edges toward the center of the face. Complete layer annihitation

is frequent. If Cr+3 ion is added to the growth bath as an

impurity the growth spirals on {100} assume a rectangular shape

but no observable changes in layer morphology on {101} are produced.

Growth layer morphology and growth mechanism effects have

been studied microscopically on the {100} and {101} faces of

ammonium dehydrogen phosphate (ADP) . Studies were made on

solutions of pure ADP and solutions contaminated with Cr+3 by

means of a microscope attachment developed here. Complete reversibility of the growth spiral shapes is evident. This reversibility correlates with the habit effects and Cr+3 impurity

retention in macro-growth systems. Single crystals grown for a time in Cr+3 contaminated solution, producing tapered habit and

retention of Cr+3 in the tapered vicinal faces {100}, revert to

the normal habit on the addition of ethylene-dramine tetraacetic

acid which complexes with the Cr+3 thus inhibiting the Cr+3 activity.

The growth morphology of single crystals of sparingly

soluble salts grown in a silica gel medium has proved difficult

to study microscopically because of excessive light scattering in

the gel. Work is continuing on this 'problem with particular

attention begin paid to the transition from the initially formed

yellow phase of HgTz to the red phase.

Publications and Talks

J. L. Torgesen and R. W. Jackson, Growth Layers in Ammonium

Dehydrogen Phosphate; Science 148, 952 (1962).
2.7 The Crystallization Habits of Organic Polymers

F. Khoury and J. D. Barnes

Polymer Physics Section

The general objective of this project is to characterize the

crystallization habits exhibited by organic polymers, when they are

crystallized both from solution and from the melt, with a view of

providing a broad basis of understanding the mechanisms of crystallization of these long chain substances at the molecular level.

A paper entitled "The Spherulitic Crystallization of Isotactic

Polypropylene from Solution: On the Evolution of Monoclinic

Spherulites from Dendritic Chain-Folded Crystal Precursors" has

been published. The study reported therein provides a detailed

insight into the nature and origin of the atypical mode of growth

and morphology of monoclinic spherulites of polypropylene which

are not only formed on crystallizing the polymer from concentrated

solutions, but which also represent the most common form of crystallization of the polymer from the melt. It is shown that such

spherulites evolve from dendritic crystal precursors which propagate

through a mode of twinning involving an 80° change in orientation

of the chain molecules in secondary chain-folded lamellar branches

with respect to that in primary branches. This mode of twinning

which is exceptional to polypropylene is the source of the fundamental differences between the mode of development of monoclinic

polypropylene spherulites and that generally associated with the

formation of spherulites in other polymers.

A study has been carried out during the past year of the

growth habits exhibited by polychlorotrif luoroethylene crystals

grown from dilute solutions of the polymer in low molecular weight

polychlorotrif luorethylene oil over a range of temperatures. Lamellar

crystals have been grown which exhibit various degrees of curvature,

while planar or nearly planar chain folded lamellae are formed at

low supercoolings , high supercoolings yield hollow bowl shaped

crystals whose walls posses a laminated structure consisting of

several superimposed chain folded lamellae, each of which consists

of a mosaic of fold domains. The progressive transition in the

habit of the lamellar crystals from planar to highly curved with

increasing supercooling parallels in many respects an analogous trend

which has been previously observed on crystallizing poly(4-methylpentene1) from solution. A preliminary study of the crystallization of

polyoxymethylene from solution has also indicated that curved

lamellar crystals of this polymer are also formed at high undercoolings. These various observations suggest that the formation
of curved lamellar crystals of polymers is a phenomenon of somewhat general occurrence under conditions which favor rapid

crystallization. While this phenomenon and the elucidation of

its origin on a structural basis is of interest per se, it is all

the more interesting in view of evidence which suggests that it is

intrinsically associated with the as yet not fully understood

growth processes leading to the formation of the more common types

of spherulitic structures in polymers.

Two papers are in the process of completion in which the

formation of curved crystals of poly(4-methylpentene-l) and

poly(chlorotrif luoroethylene) are described and in which the

possible nature of the structural factors leading to the formation

of such crystals are discussed. In particular, the influence of

lattice distortions induced by the molecular folds in the

constituent lamellae of such crystals is considered.

Additional studies are planned with a view of establishing

whether the phenomenon of curved crystal formation is of more

general occurrence. The bearing of this phenomenon and its origin

in relation to the mechanism of growth of the common types of

spherulitic structures in polymers will be investigated further.

Publications

F. Khoury, The Spherulitic Crystallizations of Isotactic Polypropylene from Solution, J. Research, NBS , 70A, 29 (1966).

2.8 The Crystallization Kinetics of Mu It i -Component Systems

J. I. Lauritzen, Jr., E. A. DiMarzio, and E. Passaglia

Polymers Division, Polymer Physics Section

In systems consisting of several components there are a number of kinetic problems which can be considered as the kinetics

of growth of a "mixed chain", i.e., a chain whose v/th unit may be

any of the components of the system. Examples of such systems are

(1) a mixture of monomers which polymerize to form a copolymer,

(2) a mixture of several components which crystallize to form a solid solution, the chain In this case being the growth of a crystal

strip at, say, a screw dislocation edge, (c) a polymer melt or

solution crystallizing to form a chain folded crystal, the chain

being a strip growing on the edge of crystal and the 'components"

being the possible lengths of the segments of molecules between

folds.
In these problems it is desirable to be able to calculate

the rate of growth of the chain as a function of the relevant

parameters of the system, the average composition of the chain

at various rates of growth, and the various pair and higher

distributions. A theory which permits the calculation of these

quantities has been formulated, and a MS has been prepared for

publication.

2.8 Thermal Generation of Vacancies and Substitutional

Sites in Crystalline Polymers

G. T. Davis and R. K. Eby

Polymer Physics Section

The thermal expansion of the macroscopic specific volume

of polyethylene exceeds that of the unit cell. It has been

proposed that this may result from thermal generation of vacancies

and substitutional sites (Eby, 1962). There are a number of

limitations on the data needed to test experimentally either this

or any other proposal: reversible with temperature, lattice and

volume measurements on the same sample, very accurate measurements,

samples isothermally crystallized, samples of well characterized

fractions as well as whole polymer, and measurements of the lamella

thickness of the samples used. Data satisfying these requirements

have not been available. Therefore, apparatus and samples have

been prepared to obtain these data. A stirred bath has been

developed for dilatometric measurements between -135 and +60° C

and a commercial x-ray cell has been modified for use in the same

range. (This range can be extended for both measurements.) Initial

results for fractions and whole polyethylene are consistent with

the thermal generation of vacancies and new substitutional sites.

This work has been reported at the March 1966 Meeting of the

American Physical Society.

Publication

R. K. Eby, J. Appl. Phys . 33, 2253 (1962).

2.9 Point Defects in Crystalline Copolymers

J. P. Colson, L. H. Bolz, and R. K. Eby

Polymer Physics Section

Research concerning the inclusion of perf luoromethyl groups

as point defects in the crystals of copolymers of tetraf luoroethylene

10
and hexaf luoropropylene has been briefly reported in NBS Technical

Note 260. Two papers discussing this work have been published.

Publications

L. H. Bolz and R. K. Eby, J. Research NBS 69A, 481 (1965) and

J. P. Colson and R. K. Eby, J. Appl. Phys . 37, July (1966).

This work which has not been reported before has been published

in J. Acoust. Soc. Am 39_, 506 (1966). (R. K. Eby and J. P. Colson)

2.10 Homogeneous Nucleation Studies

J. D. Hoffman, G. S. Ross and L. J. Frolen

and

F. Gornick

Polymers Division

During the current report period, studies of the isothermal

rate of homogeneous crystal nucleation in highly supercooled melts

of unfractionated linear polyethylene have been completed. The

temperature dependence of the nucleation rate was determined and

analysed in accord with current theories of homogeneous nucleation

of chain-folded polymer crystals. Statistical analysis was

accomplished by means of OMNITAB computer program. An estimate of

the quantity (o2 oe) was determined when a and a e are respectively

the lateral and end-surface free energies of the crystal. If the

melting point of polyethylene is assumed to be 417.86°K then (cs ce

) is 8068. Combining this value with an independent estimate of o=9 .

6

-2 -2

ergs cm , a value of 87.5 ergs cm is obtained for the chain

folded surface.

Isothermal, radial growth rates for spherulites of unfractionated

polyethylene have also been measured. These experiments give an

estimate of (ao ). Again using the values of 417.86°K for the

i e -2 melting point and 9.6 ergs cm for a, the calculated value for a -2 i is 80.4 ergs cm which is in good agreement with the value obtained

from the nucleation experiments.

The program developed for "PILOT" by Dr. George Moore of the

Metallurgy Division for counting procedures has been further

11
investigated and was found to be unsatisfactory in its present state

for these studies. However, experimental techniques for preparing

samples which are stable for several months have been developed.

Consequently the same field can be used for many experiments and are

counted by a comparative technique which allows individual droplets

to be identified and their freezing behavior from experiment to experiment to be studied.

Droplet experiments on one fraction (50,000 Mw) of linear

polyethylene have been completed and are presently being analysed.

The preliminary value for ae is 95.7 ergs cm" . Slow cooling and

spherulite growth experiments are currently in progress and are

expected to be completed within the next two months.

During the next reporting period the dependence of the nucleation

and growth rates upon molecular weight will be investigated using

several fractions of polyethylene prepared in this laboratory.

Publications and Talks

J. D. Hoffman, Nucleation Controlled Crystal Growth and ChainFolding in Polymers, International Conference on Crystal Growth,

Boston, Massachusetts, June 21-24, 1966.

F. Gornick, Crystal Nucleation in Polyethylene: The Droplet

Experiment, International Conference on Crystal Growth, Boston,

Massachusetts, June 21-24, 1966.

F. Gornick and J. D. Hoffman, Nucleation in Polymers, Ind. Eng

.

Chem. 58, 41 (1966).

J. D. Hoffman, Theoretical Aspects of Polymer Crystallization with

Chain Folds: Bulk Polymers. SPE Transactions, J, 1, (1964).

2.11 Nucleation Rate of Polymer Crystals

J. D. Hoffman and James J. Weeks, Jr.

Polymers Division

The analysis of the data for the crystallization of Polybutene-1

from the subcooled liquid to form II crystal was completed. A paper

giving the results of the investigation has been published.

A 64,600 molecular weight fraction of polyethylene was used to

obtain low-angle X-ray diffraction data during crystallization and

melting of the sample. The smaller of the two main low-angle spacings

(L^) most closely agrees with the lamellar thickness previously

determined by Frank Anderson of Chemstrand on this same sample by

means of electron microscopy. The larger spacing (L,)

12
corresponds exactly to the width of Anderson's type II lamellae.

By making diffraction scans during melting, it was found that the

L« peak was not shifted to larger spacings as had been expected;

whereas, the L.. peak did shift toward smaller angles. The uniformity

and temperature dependence of lamellar width are still unexplained.

Publication

J. Powers, J. D. Hoffman, J. J. Weeks, and F. A. Quinn, Jr.;

Crystallization Kinetics and Polymorphic Transformations in

Polybutene-L; J. Research NBS 69A, 335 (1965).

3. CRYSTAL CHARACTERIZATION

3.1 Soft X-ray Spectra of the Solid State

R. D. Deslattes and B. Paretzkin

Crystal Chemistry Section

The x-ray topographic work has continued to produce results.

The mismatched minus position topography has been perfected by

the production of a set of six perfect reference crystals which

permit strain microscopy of arbitrary specimen with a noise level

more than an order of magnitude below the part per million level.

The sensitivity of the method has enabled the undertaking of

several interesting problems involving a better understanding of

the physics of the technique or more detailed materials characterization. Some of these areas are, (1) mechanisms of image formation,

(2) surface relaxation of strain fields about single dislocations,

(3) point defect measurements such as the measurement of lattice

parameter changes associated with the diffusion of foreign atoms

into host lattices, device type application of the measurements to

integrated circuits and a potentially new system of geochronology

based on the isotopic variability (as revealed by strain) in

perfect natural crystals of silicates and borates.

Further, the ability to produce perfect single crystals finds

applications in metrology. Some examples are angle standards, an

atomic mass standard and density standards.

Publications and Talks

R. D. Deslattes, H. S. Peiser, J. A. Beardon and J. S. Thomsen,

Metrologia, July 1966.

13
3.2 Crystal Defect Studies by Optical Means

R. F. Blunt and M. I. Cohen

Solid State Physics Section

Color centers, produced mainly by 50 kV x-rays, have been

studied in the rutile structured MgF2, largely at room temperature.

An initial slightly anisotropic absorption band near 260 nm is

tentatively identified as an F-center. Subsequent optical

bleaching results in the formation of a strong bsnd at 370 nm

and several weak bands including one at 320 nm. Both of these

appear only for E I c and are believed to be due to M-centers

.

The choice of models is dictated in part by the observation of

typical trapped electron ESR spectra in irradiated samples

.

Bleaching studies show that the 320 nm band arises from "centers"

lying along the [110] directions, while the 370 nm band seems to

be cylindrically symmetric about the c-axis, and also that the

320 nm is reversibly converted into the 370 nm band. Selection

rules have been derived for (Is) 2 - (ls2p) transitions in each of

the four possible M-center configurations permitted in the rutile

structure. It is found that of the two "types" having F-F bands

in the (001) plane, one identifies rather well with the 370 nm

band and the other with the 320 nm band.

An investigation of the extrinsic and intrinsic properties of

the absorption edges of SrTiOg, Ti02, CU2O and other 'oxides" is

under way. A detailed measurement of the absorption edge of SrTi03

has been made and the edge is found to obey the Urbach rule over at

least five orders of magnitude. Similar measurements on Ti02 for

comparison are in progress.

Work has commenced on the application of certain of the

recently devised "modulation" techniques to the study of the

absorption edge and band structure of crystals. An electroref lectance

apparatus of the "Cardona" type has been constructed and measurements

made on a number of crystals including SrTiOo, Ti02, BaTi02, CU2O,

InSb, and CdS

.

Publications

R. F. Blunt, Optical Absorption of Cobalt in Maganese Fluoride,

J. Chem. Phys. 44, 2317 (1966).

14
3.3 Crystal Defect Studies using Magnetic Resonance

Te-Tse Chang

Solid State Physics Section

A superheterdoyne detection system at x-band frequency has

been assembled and is completed. It was estimated that this system

would be able to detect signals as low as 10~13 to 10~TM watt.

This is about an order of magnitude better than the usual commercially

available superheterodyne detection systems for general usage. With

this detection system, the EPR spectrum of Mo-> + in Ti02 at 4°K was

detected and recorded for the first time, since very low microwave

power has to be used to avoid saturation effects. Furthermore,

the saturation and relaxation effects of llo-> + EPR transitions can

now be studied. It was found that the relaxation rate approaches

a linear temperature dependence below 1.5°K, varies as (temp) 5 up

to 3°K, and approaches (temp) 9 dependence at 3°K-4°K. At 77 °K and

up to 150°K, the usual solid state diode detector was used to

observe the Mo^ + EPR signal; the line-width measurement revealed

that the relaxation rate is (temp)" dependent above 77 °K and approaches

a higher order dependence above 130 °K. No Orbach relaxation process

was found.

EPR signals from conduction electrons in SrTiO^ and color

centers in MgF2 are currently under detailed study. The signals

were weak and broad. With superheterodyne detection, the spectrum

may be more readily resolved. Current efforts are spent on identification and interpretation of those signals.

3.4 Thermal Conductivity of Paramagnetic Salts

J. F. Schooley

Cryogenic Physics Section

A new vibrating-coil magnetometer for measuring the magnetic

moment, M = 0.08H, for H less than 1 Oe and at temperatures less

than 0.3 °K was completed and moment data obtained. The magnetic

moment data obtained on SrTiOo specimens were correlated with

semiconductivity and superconductivity theories and with specific

heat data. The superconducting transition temperatures of a series

of ceramic strontium titanate, barium-doped strontium titanate,

and calcium-doped strontium titanate were examined using a mutual

inductance bridge operating at 270 c/s 0.01 Oe

.

15
Publications and Talks

R. S. Kaeser, E. Ambler and J. F. Schooley, Vibrating Coil

Magnetometer for use at very Low Temperatures, Rev. Sci. Inst.

37, 173 (1966).

H. P. Frederikse, J. F. Schooley, R. W. Thurber, E. Pfeiffer and

W. R. Hosier, Superconductivity in Ceramic Mixed Titanites , Phys

.

Rev. Letters, 16, 579 (1966).

E. Ambler, J. H. Colwell, W. R. Hosier and J. F. Schooley,

Magnetism and Critical Fields in SrTiO^ . (Submitted for

publication)

.

3.5 Magnetic Properties of Salt Single Crystals

at Very Low Temperatures

B . W . Mangum

in

and

Relaxation times have been measured for HoClo and ErClo

LaCl3 . The results of these measurements have been analysed

published. The spin-lattice relaxation time for concentrated

NdCl3 was measured by the audio frequency susceptibility method

as a function of temperature at fields up to 100 Oe . The results are

complicated by the presence of spikes in the susceptibility and are

not yet fully understood.

Publications and Talks

B. W. Mangum and R. P. Hudson, Spin-lattice Relaxation in Some Rare

Earth Trihalides, J. Chem. Phys, 44, 704 (19 66).

B. W. Mangum and R. P. Hudson, Spin-lattice Relaxation of Rare

Earth Ions in LaClo , Bull. Am. Phys. Soc, Series II, 10, 1109

(1965).

3.6 Nuclear Magnetic Acoustic Resonance

L. H. Bennett

Alloy Physics Section

Work was continued on the study of the nuclear magnetic

acoustic resonance of various isotopes by means of tests on single

crystal specimens.

16
Improvements were made in both the marginal oscillator and the

matching circuits whereby improved signal-to-noise ratio resonance

figures were secured for the potassium iodide crystal, which is

employed as a criteria for testing the equipment.

Whereby resonance was observed on In in a high purity

single crystal of InSb supplied by Texas Instruments, it was not

possible to observe the same resonance on an Auln2 single crystal.

This can be attributed to the higher purity, and perfection of

structure of the indium antimonide. A new Auln2 crystal has been

ordered having a higher order of purity than the present crystal.

A tantalum single crystal, secured as the highest available

purity produce of several years ago, and with a resistivity ratio

of 75 was tested with no resonance being observed. R. H. Gregory

and E. H. Bommel, Phys . Rev. Letters 1_5, 404 (19 65) reported nuclear

acoustic magnetic resonance results in more recently produced higher

purity tantalum. Of five samples Gregory and Bommel tested only

two, namely, of ultrapure Westinghouse tantalum having a resistivity

ratio greater than 400, gave measurable resonances. They observed

such resonance with a good signal-to-noise ratio at liquid nitrogen

temperatures and with a lower ratio at liquid helium temperatures.

Such resonances were not observable at room temperature, which is

not understood

Ei of 405

test temperature varying from liquid nitrogen to room temperatures.

3.7 Experimental Determination of Atomic Scattering Factors

G. Bur ley

Crystallography Section

The x-ray atomic scattering factors for magneeium oxide

reported previously were further refined, and the results have

been published.

Measurement of the scattering factors for copper metal, using

molybdenum radiation to minimize the anomalous dispersion correction,

has also been completed (Table I). This material was chosen because

it is a transition element with electronic configuration 3dl0 4s l,

where both the closed d shell and the s orbital are spherically

symmetric and a direct comparison of theoretical and experimental

results is possible. As expected, the experimental values are

17
very near those calculated by the Hartree-Fock procedure, but

agreement is improved by comparison with an augmented plane

wave energy band calculation carried out by F. J. Arlinghouse

(Ph.D. thesis, M.I.T. 1965).

New equipment installed during 1965 included a complete

solid state detection unit, comprising a linear amplifier,

pulse height analyzer, printing timer and sealer to be used with

a scintillation counter. Extensive testing and calibration indicates

that long-range stability of better than 1% is now routinely

possible.

The program of surveying compounds for use as standards in

terms of absolute intensity measurements was primarily directed at

sodium chloride. Techniques of sample preparation, and effects

of humidity and of radiation were assessed. No definite conclusions

have been made, but it appears that the reproducibility required

will be difficult to attain. The change of the integrated

intensity with radiation exposure will be further investigated.

The limitations and errors inherent in this technique are

now under study. A least-squares fit of the Fourier transform

of the scattering factor curve to the radial electron density

distribution plus an error curve is being programmed. The effect

of termination of series at both low and high sin0/\ values is

also being considered.

In the parallel program of determining Debye-Waller temperature

factors by measuring intensities at a number of different temperatures both copper and sodium chloride were investigated. Experimental

values agreed well with the corresponding values from low-temperature

specific heat data.

Publications

G. Bur ley, Experimental Atomic Scattering Factors for Magnesium

Oxide, J. Chem. Phys . Solids, 26, 1605 (19 65).

3.8 Deformation and Fracture of Ionic Crystals

S. Wiederhorn

Physical Properties Section

A new experimental technique has been developed to investigate

static fatigue of brittle solids. Using the double-cantilever

cleavage arrangement, it was possible to observe crack motion and to

18
observe crack motion and to accurately measure crack velocities

in glass. Temperature and stress dependence of crack motion in

water indicate the observed fracture to be an activated process

with an apparent activation energy of 19.5 kcal. Crack motion in

nitrogen gas is found to be a complicated function of the partial

pressure of water vapor in the gas. Three regions of crack motion

were delineated. In the first region, crack motion is believed

to be due to a chemical reaction between the water vapour and the

glass with the interfacial chemical reaction being the rate controlling step. In the second region, crack motion is also due

to a chemical reaction between the glass and water vapor, but

the rate is controlled by diffusional transport of water vapor to

crack tip. A combined rate equation has been developed which

satisfactorily explains the experimental data in regions I and II.

A satisfactory explanation for crack propagation in region III is not

yet available, however, the observed crack motion is independent of

moisture content.

The above work will be finished in two months , after which we

hope to determine the effect of pH and impurity ion concentration

on crack motion in glass. We also plan to measure the fracture

energy of several glasses of different compositions.

3.9 Crystals of Organic Molecules

Richard A. Keller and J. John Natterstad

Physical Chemistry Division

During the current period the intramolecular energy transfer

properties of several molecules containing two different, isolated

chromophors were investigated at 77 °K in rigid glass solutions. A

study of the fluorescent and phosphorescent properties of these

molecules showed that, in all cases, both singlet and triplet

excitation energy is transferred to the chromophor with the lowest

singlet or triplet excitation energy. One of the molecules studied

had the two chromophors separated 7-12 A by a steroid bridge. Several

of the molecules had a "spiro" bridge which held the two chromophors

perpendicular to each other. Another series of molecules contained

two chromophors separated by one, two, and three methylene groups.

Work is presently in progress on the study of other chromophors

separated by a steroid bridge.

19
Papers and Talks

"Formation of Crystallites of Benzophenone in Hydrocarbon Glass",

Richard A. Keller and David E. Breen, J. Chem. Phys . 43, 2562

(1965). —

"Intramolecular Energy Transfer Between Triplet States of Weakly

Interacting Chromophors" , David E. Breen and Richard A. Keller,

in preparation.

Book review: "Organic Semiconductors" by Y. Okamoto and Walter

Brenner, Richard A. Keller, J. Frank. Inst. 279., 391 (1965).

ZO

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