AFML-TR-74-56

MERCUROUS CHLORIDE POLARIZER MATERIAL

TECHNICAL REPORT AFML-TR-74-56

1 JUNE 1974

Approved for public release; distribution unlimited.

AIR FORCE MATERIALS LABORATORY

AIR FORCE SYSTEMS COMMAND

WRIGHT-PATTERSON AIR FORCE BASE, OHIO 45433
NOTICE

When Government drawings, specifications, or other data are used for

any purpose other than in connection with a definitely related Government

procurement operation, the United States Government thereby incurs no responsibility nor any obligation whatsoever; and the fact that the government may have formulated, furnished, or in any way supplied the said

drawings, specifications, or other data, is not to be regarded by

implication or otherwise as in any manner licensing the holder or any

other person or corporation, or conveying any rights or permission to

manufacture, use, or sell any patented invention that may in any way be

related thereto.

This technical report has been reviewed and is approved for publication.

Vincent L. Donlan

Project Engineer

FOR THE COMMANDER

Chief, Laser and Optical Materials Branch

Air Force Materials Laboratory

Copies of this report should not be returned unless return is required

by security considerations, contractual obligations, or notice on a specific

document

.

AIR FORCE/56780/25 October 1974 — 200
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BEFORE COMPLETING FORM

1. REPORT NUMBER

AFML-TR-74-56

2. GOVT ACCESSION NO. 3. RECIPIENT'S CATALOG NUMBER

4. TITLE (and Subtttle)

Mercurous Chloride Polarizer Material

5. TYPE OF REPORT a PERIOD COVERED

Final Technical Report

15 June 73 - 15 Dec 73

6. PERFORMING ORG. REPORT NUMBER NBSIR 74-507

7. AUTHORfs;

Richard Forman, William S. Brower,

Harry S. Parker

8. CONTRACT OR GRANT NUMBERfsJ

F33615-73-M-6752

9. PERFORMING ORGANIZATION NAME AND ADDRESS

National Bureau of Standards

Washington DC 20234

10. PROGRAM ELEMENT. PROJECT, TASK AREA & WORK UNIT NUMBERS

PE61101F

J0# ILIR0040

11. CONTROLLING OFFICE NAME AND ADDRESS Air Force Materials Laboratory

Air Force Systems Command

Wright-Patterson AFB, OH 45433

12. REPORT DATE

1 April 1974

13. NUMBER OF PAGES 25

14. MONITORING AGENCY NAME & ADDRESSfi/ di/ferent Irom Controlling Oflice) 15. SECURITY CLASS, (ol Ihls report) unclassified

15o. DECL ASSI Fl C ATI ON/ DOWN G R ADI N G SCHEDULE

16. DISTRIBUTION STATEMENT fof fhls ReporO

Approved for public release; distribution unlimited.

17. DISTRIBUTION STATEMENT (ol the abstract entered In Block 20, If different from Report)

18. SUPPLEMENTARY NOTES

19. KEY WORDS (Continue on reverse side if necessary and Identify by block number)

mercurous chloride; polarizer material; visible; infrared

20. ABSTRACT (Continue on reverse side it necessary and identify by block number) This contract report covers a feasibility study on the potential use of mercurous chloride crystals for construction of prism polarizers, primarily for

the infrared region of the spectrum. In particular, the research has

consisted of crystal growth from the vapor phase and optical characterization

of the absorption, dichroism, and birefringence.

Crystal growth has been performed in quartz or Vycor ampoules sealed off

under a pressure of 10"^ torr or better. Crystals are grown in a vertical

modified Bridgman furnace by vapor transport. Repeated regrowth has been

DD , jAr73 1473 EDITION OF 1 NOV 65 IS OBSOLETE UNCLASSIFIED

SECURITY CLASSIFICATION OF THIS PAGE (When Data Entered)
SECU RIT Y CLASSIFICATION OF THIS PAGEflWien Data Entered) foiind to produce a continuous improvement in clarity, the best samples being

virtually colorless. The growth rates used of 2 mm/day in tubes of 17 mm

inside diameter have been found to be satisfactory for producing the crystals

used to date, but strain patterns and a tendency to polycrystallinity

indicate that slower growth rates should be attempted. Conditions for

growth have not yet been optimized, but reasonably good growing conditions

have been delineated.

Single crystal samples have been oriented by Laue back-reflection techniques and measurements made of optical absorption, dichroism, and birefringence. Proper sample handling procedures have been determined to

minimize thermal shock and thermal etching. The index of refraction has

been measured at selected wavelengths, and calorimetric absorption studies

have been performed at AFML on some of the samples produced.

UNCLASSIFIED

SECURITY CLASSIFICATION OF THIS PAGErW^en Data Entered)
FOREWORD

This report covers work performed under Delivery Order F33615-73-M-6752

and covers the period 15 June 1973 to 15 December 1973. Vincent L.

Donlan, AFML/LPL, Wright-Patterson AFB, Ohio 45433, was the project

engineer.

This research was sponsored by AFML Laboratory Director's Funds.

iii
TABLE OF CONTENTS

SECTION PAGE

I. Introduction 1

II. Research Conducted 3

A. Crystal Growth 3

B. Sample Preparation 10

C. Optical Properties 19

III. Suggestions for Further Developmental Work . . 22

A. Crystal Growth 22

B. Chemical and Physical Properties 23

IV. References 25

iv
MERCUROUS CHLORIDE POLARIZER MATERIAL

I . Introduction

The purpose of this research was to determine whether mercurous

chloride was a suitable material for construction of prism polarizers

for the infrared region of the spectrum. At the present time,

polarizers suitable for use with high power lasers in the infrared

region of the spectrum either do not exist or are of limited utility.

This statement should properly be qualified, as Rochon or Wollaston

prisms could be fabricated from a number of materials. These types

of prism polarizers suffer from the problem that the ordinary and

extraordinary rays both emerge from the same surface with only a small

angular displacement between them. Extant infrared polarizers are not

easily applied to all experiments.

The first type utilizes Brewster angle reflection to remove

one of the beams. This is the principle used in the pile-of-plates

polarizer, where the transmitted beam through the stack is used.

By utilizing materials with high index of refraction, such as silicon

or germanium, polarizers can be made to operate from the reflected

beam. In general, with polarizers which utilize Brewster angle

reflection, a n\amber of optical surfaces are used at reasonably large

angles of incidence. To obtain a high quality output beam, all of

these optical surfaces must be of very high optical quality, which

is usually difficult to achieve.

The second method of producing polarized infrared light, which can

be used if power densities are low enough, is the so-called wire-grid

polarizer. In this case, the polarized light is produced by an essentially transmission-type diffraction grating. In order to obtain

a high degree of polarization, the spacing of the grating must be

small compared to the wavelength of the light. This means that the

grating spacing must be exceedingly small {'^ 3000 lines/mm) . Gratings

with these fine spacings have relatively poor optical quality at the

present time. In addition, the thin metallic "wires" which comprise

the grating can only handle relatively small amounts of incident

power without destruction of the wires.

The most favorable method for producing polarized laser beams

would be to use a Glan-type prism polarizer in which one polarization

appears as a beam at 90° to the incident beam while the other is

transmitted directly through. Generally, an absorbing material is

placed to absorb the 90° reflected beam. To produce polarizers

of this type, one uses uniaxial crystals with substantial birefringences. At the present time, the only material which has proved

practical in prism polarizers of this type has been calcite. Due to

lattice absorption, calcite polarizing prisms are not usable beyond

2.3 ym in the infrared.

1
A further problem exists in producing laser quality prisms

for the near infrared from calcite, and that is the availability of

single-crystal calcite. All calcite prisms produced to date have

been made from the naturally occurring mineral. The known deposits

of this mineral are now almost exhausted of materials of laser quality,

and large efforts have been devoted to producing the material

artificially. To date, to our knowledge, this effort has had extremely

limited success with only very small crystals produced.

With the above considerations in view, we undertook to examine

the literature for other promising uniaxial crystals with sufficient

birefringence to make possible the manufacture of Glan-type polarizers.

After a literature search, we decided that mercurous chloride appeared to

be a good candidate, though there was a paucity of data on its

physical properties. The birefringence was enormous, the powdered

material was reported to be yellowish-white, and it was composed of

heavy-enough atoms to expect transmission in the infrared to wavelengths

at least the vicinity of the important CO2 laser at 10.6 ym. A hazardous

method of crystal growth has also been reported in 1970 which was related to the method we had used earlier for the growth of ammonium

chloride from the vapor phase

.

Small single-crystal samples were produced in our laboratory by

a vapor growth technique and the optical transmission range was

measured. These measurements showed that the material was transparent

from 0.4 ym to 16 ym with no absorption bands visible within these

limits. At this point, the contract was initiated. Its objectives

were, first, to develop techniques of crystal growth, hopefully

suitable for scaling up to larger size crystals, and secondly, to

characterize the optical properties of pure samples, including

measurement of the index of refraction.

In siammary, the objectives of the contract have been achieved.

A technique of crystal growth has been found which should allow

boules of several centimeters diameter by tens of centimeters in

length to be produced. Further, optical absorption at 10.6 ym in

these crystals has been found to be less than 0.02 cm"'-; a thorough

evaluation of the effects of impurities could significantly lower

this value. Measurements of index of refraction have demonstrated

feasibility of construction of Glan-type prism polarizers with

transmission from 0.4 ym to greater than 10.6 ym.

By way of comparison with other potential artificial materials

for polarizers, mercurous chloride appears to have many advantages.

Crystal growth occurs at low temperatures from the vapor phase, a method which is expected to produce material of high purity and

high optical quality. No obvious stoichiometry problems have been

encountered, in sharp contrast to many chalcogenide materials which

2
could be possible candidates. Further, the crystals appear to be

quite inert when pure, and are capable of holding a good optical

finish. The indices of refraction are relatively low for infrared

materials, which is favorable for dielectric coatings to reduce

reflection losses. Optical transmission over the entire visible

region of the spectrum is especially useful as mercurous chloride

optical elements could easily be examined, for homogeneity for example,

at high sensitivity in an interferometer operating in the visible

region; many infrared materials can be examined only in the infrared

region of the spectrum, and apparatus for such examinations are not

generally available.

There appear to be no obstacles to production of laser quality

polarizing prisms from mercurous chloride crystals. Over their

transparency region they should be superior to prisms made from

other known materials, as the higher birefringence means that the

finished prisms will have very large acceptance angles. Further work

is, of course, required to make this promise a reality.

II . Research Conducted

A. Crystal Growth

As mercurous chloride sublimes easily, numerous experimenters

have reported on the growth of millimeter-sized crystals by this

technique. Only one literature reference exists which is devoted

primarily to crystal growth, the paper of 1970 by Barta. He reports

the growth of mm-size crystals from sealed ampoules. He determined

that, for his method, optimal growth occurred at a temperature of 480°C,

which corresponded to a pressure of 15 atm. inside the growth txobe.

Growth by this technique then was obviously not simply scalable up

to larger sizes as the ampoules would burst under the high pressure

developed.

We had encountered a similar situation in some of our earlier

work in the growth of single-crystal ammonium chloride. In this

earlier work, we had resorted to vapor transport in a Bridgman-type

apparatus to somewhat reduce the growth temperatures and pressures

required. Three different geometries have been attempted for mercurous

chloride growth as well as several different furnace profiles.

All of the crystal growth has been performed using commercial

powdered materials for the starting charge. We have examined the

products of a number of manufacturers with the results summarized

in Table I . In each case , the as-received material was sealed in a Vycor* tube evacuated to pressures pf' less than 10~^ torr and then

rapidly s\ablimed to the top of the tube in a vertical gradient furnace

with the hot zone at 350°C and the cooler top at 340 °C. The fibrous

mass at the top was then used for a slow controlled growth run to

produce a single crystal. As the table indicates, best results were

obtained from the materials supplied by Cerac/Pure* . Chemical analysis

of the final grown crystals has not been performed.

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Using the prepurified starting material described above as a charge, final growth was performed in highly evacuated, specially

prepared ampoules. In figure 1, we show the ampoule geometry used

for the early experiments. These ampoules were charged as shown in

the figure and then lowered out of the furnace shown in figure 2 at

a rate of approximately 2 mm/day. The geometry of this furnace was

such that the sample could not be viewed during growth, but from

variation of temperature and pulling rates the optimal growth

conditions were found to be 2 mm/day or less through a temperature

gradient of 10 to 15°C. The larger sample tubes shown in figure 3 were

then prepared and the same furnace used for growth runs with these tubes.

With these larger tubes the optimal conditions for growth were found

to be 1.5 mm/day with a 15 °C gradient.

A persistent difficulty, which we were never able to correct, was

found in using ampoules of this shape, and, along with other factors,

led us to abandon this geometry. In every case, we found that the

crystals became entrapped in the lower portion of the growth tube

in the region of the ring seal. While this appears to be a simple

glassblowing problem, requiring an expanding taper at this point,

our glass shop was not successful in this endeavor. Further difficulties were encountered with charge material occasionally getting into

the growth zone during filling and with restrictions imposed on the

thermal profiles of the oven by the geometry of the ampoule.

Notwithstanding all of the above, this method produced high

quality boules, often with well developed vapor growth faces on the

tops of the boules . While the samples produced in the small ampoules

were relatively strain free, the larger size boules were quite

strained, probably because of a lack of annealing after growth. The

growth geometry used did not automatically produce annealing, and

before the annealing problem became obvious, a change in growth

geometry was initiated.

Several generations of ovens were manufactured to perform the

crystal growth in an inverted Bridgman geometry. The last design

is the only one shown and is described by figure 4(a). In this oven,

crystal growth occurs by moving the tube upward through the furnace.

The furnace has been designed so that the hot zone is uniform with

a temperature of 390 °C with an abrupt gradient of 10 °C in going to

the lower temperature top zone followed by a relatively long top

zone to provide annealing. Actual growth conditions in the tube

correspond to pressures of approximately 1.2 atm. so that no problem

in scale-up is anticipated. Growth rates of 2 mm/day have been

successful, requiring growth runs of approximately two weeks for

a charge of 50-60 gms. The furnace temperature profile is shown in

figure 4 (b) .

5
A.

c/eysTAL

Fig. 1. Early crystal growth ampoule geometry.

Material is transported from upper container to

bottom of tube as ampoule passes through temperature

gradient.

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Fig. 2. Improved early furnace design showing temperature profile,

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Fig. 4. (a) Final furnace design for lower temperature growth

(Bridgman) . (b) Temperature profile of final furnace.

9
Two differently shaped ampoules have been used for crystal growth

in the inverted Bridgman furnace and are shown in figures 5 (a) and

5(b). Most of the crystal growth runs were made in the 5(a) geometry,

but near the end of the contract we switched to the 5 (b) geometry

to attempt to avoid the small -angle grain boundaries found in many

samples. In this geometry, the narrow neck should select out one crystallite from the initial starting growth for a seed for the final

large growth section. This technique has proved successful in the

initial runs, but there is an indication that somewhat slower growth

rates would be more favorable. Typical finished boules are shown in

figures 6 and 7

.

In all of the growth runs, a residue has remained at the bottom

of the tube. If the same material is multiply regrown, the amount

of residue continuously decreases from run to run and the finished

crystals become progressively less yellow, with two regrowths being

sufficient to produce material which is completely colorless. The

first residue occasionally contains free mercury, but the major portion

of the residue is a black fluffy material. The black residue is

amorphous to X ray, and a qualitative analysis performed on this

residue showed the presence of iron and silver.

In conclusion, a crystal growth technique has been developed

which is suitable for scaling up for production of large size

boules. No quantitative measurements of impurities have yet been

attempted. Further work should be performed to optimize the crystal

growth conditions. In addition, there should be quantitative considera

tion of the impurity problems and strategies developed to lower the

effects of any undesirable impurities.

B. Sample Preparation

Samples are removed from the fused silica ampoules by scratching

and breaking the glass- far away from the grown crystal and the samples

then simply shaken ovit of the tube. In the case where the geometry

of figure 5(b) was used, a second break is made in the lower neck and

both crystal and glass are cracked off here.

Examination of the crystals by eye is performed next to see whether any cracks are present. This examination is quite deceptive

as small cracks near the surface often appear to go deep into the

crystal due to the large index of refraction of the material.

Inspection of the boules between crossed polarizers is generally

worthless because of the curved surfaces.

The next step in sample preparation is X-ray orientation of the

samples. Our technique has been to use back-reflection Laue photograph

From numerous disheartening experiences, we have found that, in handlin

the whole boules, great care must be taken during heating and cooling.

10
Fig. 5. (a) Early inverted Bridgman furnace ampoules,

(b) Final ampoule design for seed selecting growth.

11
Fig. 6. Early boule of diameter approximately mm grown in tube of type 5(a).

12
Fig. 7. Late boule grown in tv±»e of tube 5(b) dimensions

as shown on photograph.

13
Our technique had been to place the boule on top of a brass X-ray

goniometer sample holder, whose surface was covered with glass, on a cold "hot-plate" and then heat the hot-plate and glue the sample using

glycol phthalate. Invariably this technique produced cracked boules.

While it is possible that the cracking was caused by the presence

of small-angle grain boundaries in the crystals, the unevenness of

heating was clearly undesirable. We finally arrived at the method

of placing the sample and holder in the center of a large cold oven

and slowly ("^ 1 hour) bringing the temperature up to 100 °C; the oven

was then shut off, the door opened, and the mounted sample

allowed to cool slowly in the oven ("^ 2 hours) . The sample was then

mounted on the X-ray goniometer.

All of the X-ray pictures were taken with a copper target tube

operating at 35 kV and 15 ma at 3 cm distance using Kodak* screen type

film with a camera containing a fluorescent screen. A typical

photograph for orientation required exposures of 30 minutes. In

taking these photographs , it was important not to damage the surface

of the crystal, because, with a material as soft as mercurous chloride,

this often would greatly degrade the X-ray photograph. The best

photographs were obtained from the vapor grown faces . If X-ray

photographs had to be taken of cut faces, it was necessary first

to heavily etch the surface (see below) . When using the screen

type film, X-ray fluorescence produced considerable fogging. This

problem could be alleviated by reducing the tube voltage to '^^ 25 kV

and increasing the exposure time to approximately 45 minutes, but

generally the fogging was not severe enough to prevent sample

orientation

.

The X-ray pictures shown in figures 8, 9, and 10 were taken

specifically for reproduction and were taken under different conditions. In figure 8, we show the c-axis (001) in an exposure of

60 minutes with screen film. This particular photograph was taken

on a vapor grown face , which is in part responsible for its high

quality. Figures 9 and 10 were taken on etched cut faces and show

the a-axis (100) and (110) -directions , respectively. These photographs

were taken with Kodak* Medical No-Screen Film at 25 kV and 15 ma with

exposures of approximately two hours. The presence of some broadened

spots is not indicative of grain boundaries or strain but is a manifestation of large thermal diffuse scattering. The correctness

of the indexing of the X-ray spots has been verified by an X-ray

precession photograph of one of the planes. Without the precession

pattern, the indexing of the (100) and (110) patterns is somewhat

ambiguous

.

After the crystals have been oriented, they are cut on the X-ray

goniometer. Our early attempts were to cut the samples using a diamond wafering saw with kerosine as a lubricant. This invariably

produced fracturing of the samples . We then switched to an abrasive

slurry wire saw which was successful but quite slow. Using a 10-mil

wire and 600-mesh boron carbide abrasive water-glycerine (1:4)

slurry, cutting through the diameter of a boule (17 mm) required

14
Fig, 8. Laue X-ray photograph of [001] surface.

15
Fig. 9. Laue X-ray photograph of [100] face.

The direction of the c-axis is as shown.

16
Fig. 10. Laue X-ray photograph of [110]

surface. The direction of the c-axis is as shown.

17
approximately 20 minutes. Preliminary experiments indicated that

a hot platinum wire could be used as a saw but this technique was not pursued.

Cut samples were then mounted on a brass polishing holder

using glycol phthalate. With large samples, for safety we have resorted

to mounting and dismounting in our oven. With smaller samples, the

conventional technique of heating on a hot-plate was satisfactory.

Dismounting was accomplished in the same manner as mounting with one

change. The temperatures involved for the mounting glue we used

were high enough to produce thermal etching of polished surfaces.

After trying various coatings for the exposed polished surfaces, we found that glycol phthalate dissolved in acetone provided suitable

protection. Most other coatings which we tried were not easily

cleaned from the surface later; the material is too soft to allow any

riibbing of a polished surface. Soaking of finished samples in clear

acetone left the sample apparently perfectly clean.

Because of the possibility of thermal damage during polishing,

all of our polishing and grinding has been done with water lubricant.

For rough grinding, ordinary 400-grit wet-type silicon carbide or aluminum oxide grinding paper has been very satisfactory. Final polishing was accomplished by use of a water suspension of Linde* A aluminum

oxide grinding powder on a standard rotating-lap polishing machine

(Uni-pol* , manufactured by Geoscience) . Polishing is rapid but we have

not examined the possibility that some of the polishing abrasive or iron from the probing ring became imbedded in the sample. No

imbedding was visible, however, under a 100-power optical microscope.

A very high finish could be obtained in this manner, but we have

not attempted to characterize the polish itself.

For measurements such as X-ray measurements from cut surfaces,

we have foiind it necessary to etch the surface to remove damage.

We have attempted to use a number of acids as well as solutions of

mercuric nitrate. The only successful etch we have found is hot

aqua regia. We have not done controlled etching experiments, i.e.,

weight loss, etc., but slowly boiling aqua regia acts as a good

chemical etch. Typical times required for fresh slow boiling aqua

regia are 1/2 to 5 minutes depending on the depth of damage required

to be removed. Chemically polished samples appear quite shiny to

the eye, but the flatness of surfaces has been degraded.

The cracking we have observed upon minor thermal shock could

be related to the presence of small-angle grain boundaries in the

samples produced to date. These boundaries are only observable by

polarized light on polished samples so that in cases where cracks

have been produced there is no simple way to determine the cause.

18
C. Optical Properties

Measurements have been made at room temperature of the optical

absorption of polished single-crystal samples from several different

boules. Over the region 0.35 ym to 2.6 ym a Gary* 14R spectrophotometer

was used, and over the range 2.5 ym to 25 ym a Perkin-Elmer* 337 grating

monochromator was used. The absorption data are presented in figure 11.

Siibstantial dichroism is evident at both ends of the spectrum. The

results of the dichroic absorption should be considered preliminary,

as the effects of imperfect polarizers as well as possible small

misorientation of the crystals have not been considered. With

dichroisms as large as observed in this case, corrections undoubtedly

will have to be made to the data.

In the transparent region of the spectr\am, we have used the

spectrum of one of the most colorless boules. The reflection

correction used was not calculated from the index of refraction, but represented the apparent reflection loss for two samples of two different thicknesses. That is, the same optical density was observed for two different

thickness samples in the transparent region; in general, this type of

correction is more satisfactory for high reflectivity materials where

surface effects are often significant.

On the samples which were not colorless, no distinct absorption

bands were observed. The absorption in these samples could best be

characterized as a tail on the ultraviolet absorption edge. This

absorption is most likely caused by the same impurity which caused

the black residue at the bottom of the growth tubes.

No studies of luminescence were performed on any of these samples,

though the literature indicates that a number of luminescent centers

are often present in mercurous chloride.

Initial measurements of index of refraction were attempted by

measurement of front- to-back interference fringes, but difficulties

in producing suitable thin samples led us to perform index measurements by a different technique.

For this preliminary investigation, an accurate knowledge of the

index of refraction at all wavelengths was not required, so we decided

to make measurements only at important laser wavelengths. A sample

was prepared in the form of a 30° prism with the c-axis oriented

parallel to the refracting edge of the triangular prism. This geometry

was chosen so that both indices of refraction could be measured on a single sample. The sample was placed on a rotation stage and the

index measured by the standard technique of minimum deviation.

19
0.2 0.4 0.6 0.8 1.0 2.0 4.0 6.0 8.0 10 20

X {/j,m)

Fig. 11. Absorption data normalized for reflection losses for a

1-iTun thick sample of mercurous chloride. The incident light was parallel to a [110] axis.

20
In our case, we used linearly polarized laser beams polarized

at 45° to the c-axis. In this geometry, the light beam is split into

two beams of orthogonal polarization upon entering the crystal. A

fine adjustment of the alignment of the c-axis of the crystal with the

prism rotation axis was made using this geometry. With the incident

laser beam polarized at 45° to the c-axis, a linear polarizer was placed between the laser and the crystal. This polarizer was then

rotated to 0°, or 90° to the c-axis. When perfect alignment was

obtained, extinction was obtained for one of the emergent beams for

each polarizer orientation.

The prism angle was measured using a 632.8 nm He-Ne laser by

reflection from both of its faces back upon the laser head. Minimum

deviation measurements were then made at 632.8 nm, 1150 nm and

10.6 ym, using two different He-Ne lasers and a CO2 laser, respectively.

For the measurements at 632.8 nm visual detection was employed. At

1150 nm an image converter was employed, and at 10.6 ym, liquid crystal

thermal imaging material was used. The results of the measurements

are reported in Table II. While the errors are large, further refinement is easily possible. One source of error in these measurements

was crystal imperfection which tended to smear out the laser beams.

This was most apparent on the extraordinary ray, and care was taken

to select a good portion of the crystal for the measurements.

Table II. Indices of refraction and birefringence of mercurous

chloride measured at important laser wavelengths.

Wavelength e o An

.6328 ym (He-Ne) 2.65 ± 0.05 2.00 ± 0.02 .65

1.15 ym (He-Ne) 2.47 ± 0.07 1.91 ± 0.03 .56

10.6 ym (CO2) 2.45 ± 0.15 1.90 ± 0.07 .55

Further measurements of index are contemplated soon, contingent

on preparation of a suitable prism.

Calorimetric absorption measxirements at 10.6 ym have been performed

at AFML on one sample to date. This was not one of the colorless

samples, but nonetheless, absorption was found to be only 0.02 cm"-^.

Further measurements are contemplated including measurements in

polarized light as an aid to determining whether the absorption is

intrinsic or extrinsic.

21
III. Suggestions for Further Developmental Work

A. Crystal Growth

As has been pointed out in the text, the only method of crystal

growth used has been in sealed, evacuated ampoules. With the exception

of a few growth runs near the end of the contract period, no attempt

was made to seed the crystal growth. In all of the growth runs to

date, a problem existed in that small-angle grain boundaries were

found in most crystals. A simple reduction in growth rate could very

well solve this problem, but since it occurred only when growth was carried out in large diameter ('v. 17 mm) tubes, optimization of growth

rate requires a decision on the size crystals desired. This optimization

could probably be carried out best by a commercial contractor who

would have a large number of growth furnaces available. As we do

not have a large nxamber of growth furnaces at NBS, it would appear

inappropriate for this facet of further development to be carried out

at NBS.

Other approaches to crystal growth should be attempted, in

particular, methods in which seeded growth might be more feasible.

Various vapor transport methods are available in addition to the

inverted Bridgman methods used in this investigation. An attractive

possibility would be transport in gradient furnace using a suitable

carrier gas. NBS has previous experience with similar techniques in the

production of ultra-high-purity sapphire. The investigation would involve

appropriate choice of carrier gas(es) as well as all of the conditions

of growth. A reasonable expectation would be that higher purity

material could be produced and that improved physical perfection at

higher growth rates could be achieved.

An important extension of the work reported here would be to

examine the other mercurous halides, all of which are reported to

have the same crystal structure as mercurous chloride. On the basis

of the index of refraction measurements reported here, and the corre- spondences which could be drawn from the known restrahlen bands of these

materials, these other mercurous halides also should possess large

birefringence. The same crystal growth techniques should be

suitable for growth of these compounds, requiring only determination

of suitable growth temperatures. The extension of the work to

these other isostructural compounds would be useful in that the

window regions for each of them would be different, possibly providing

a superior polarizer material for certain regions of the spectrum.

The other mercury halides have been much less investigated than

the chloride. There also exists the possibility that one of them

has a non-centrosymmetric crystal structure, contrary to literature

reports, and would be useful as a parametric material. Our observation of a low temperature phase transition in mercurous chloride

22
makes it unlikely that all of the mercurous halides do have the same structure at room temperature. If a non-centrosymmetric phase does

exist, the expected large birefringence would easily allow phase

matching to occur and a useful parametric material could result.

If the conditions for growth of the other halides could be

determined, an extension envisioned would be the growth of mixed

halides by codeposition. This would undoubtedly require a substantial

effort, but alloy hardening could occur as is the case in thallous

halides, in the production of KRS-5 and KRS-6. While hardening of the

material is not required for polarizer use, some toughening would be

desirable

B. Chemical and Physical Properties

Chemical purity of the finished crystals could become important

depending upon the particular usage for the material. Crystal growth

has demonstrated a segregation of impurities, as judged by boule

color, but no quantitative chemical analysis for impurities has been

attempted

In any new material , it is advisable to examine possible impurity

effects before considering placing the material in service in a system.

In particular, photolysis, photochromism, stability in various

atmospheres, luminescence, and possibly the effects of ionizing

radiation should be examined. In general, while each of these

phenomena could be intrinsic, impurities often tend to catalyze these

effects. Conversely, doping studies should be conducted to see if

improvement in desired properties can be obtained. These studies

are most easily conducted on a small scale as a modification of a crystal growth run.

Further study into the phase transition observed at 181 K is

warranted to determine the structure and optical properties of the

material in the low temperature phase. If this phase lacks a center

of inversion, mercurous chloride could become an important parametric

material. These studies could be performed by low temperature X-ray

and Raman studies, both of which have been started. The crystals

appear to be undamaged by repeated cycling through the transitions,

which is an obvious and necessary requisite for the material to be

useful in the low temperature phase.

Another study which clearly should be conducted would be the

effect of purity on absorption. This study should be conducted

in conjunction with other indirect measurements of purity, such as visible absorption and luminescence, as these are often more sensitive

than ordinary chemical analysis. Measurements should be performed

using a polarized laser beam in the calorimetric studies so as to

aid in separation of lattice effects.

23
Acknowledgments

We would like to thank Deane Horowitz and Albert Feldman of the

National Bureau of Standards for their aid in the measurements of

refractive index. We would also like to thank G. T. Johnston of AFML

for perfoming measurements of calorimetric absorption at 10.6 ym

on our crystals.

Certain commercial instruments and materials are identified in this

report in order to specify adequately the experimental procedure.

In no case does such identification imply recommendation or endorsement

by the National Bureau of Standards, nor does it imply that the

instrument or material identified is necessarily the best available for

the purpose.

24
Selected References

Barta, V., Kristall u. Technik 5_, 541 (1970), (crystal growth).

Bohiin, A., Rozsival, M. , and Brabee, F., Phys . Stat. Solidi (b) 45 , K85 (1971) , (uv absorption)

Deb, S. K., Phys. Rev. B2, 5003 (1970), (uv absorption).

Dufet, M. H., Bull. Soc . Franc. Mineral. 21_, 90 (1898), (index of

refraction)

Durig, J. R., Lau, K. K. , Nagarajan, G., Walker, M. , and Bragin, J.,

J. Chem, Phys. 50, 2130 (1969), (IR and Raman).

Ejder, E., J. Phys. Chem. Solids 31.' 453 (1970), (absorption and

lijminescence) .

Gmelin's Handbuch der Anorganische Chimie System 34, Vol. B, Sec. 2 (1967) , 8th edition (general reference on properties)

25

<rU.S.Government Printing Office: 1974 — 657-014/146