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Produced by the NASA Center for Aerospace Information (CASI)
sport, covering the period, Jan. 1, 1982-June 30, 1984

197) ULTEAPRESSUit MATERIALS N86-15137 it FEFort, 1 Jan. 1582 - 30 Jun.

L Univ.) 16 F HC A02/dF A01

CSCL 20L Unclas

G3/76 04951

c^' 'j J^ 14^5' I

NASA Grant No. NAG-2-158 '^ T /`

DEC 1985 w

RECEIVED

^C'Cna-F

Arthur L. Ruoff, Principal Inver

Department of Materials Science and

Cornell University

Ithaca, New York 14853

ULTRAPRESSURE MATERIALS SCIENCE
2

ABSTRACT

During the grant there were three active areas of research at

ultra pressures: Diffraction Studies with the Cornell High Energy

Synchrotron Source (CHESS), Band Gap and Absorption Edge Effects,

Indentor-Anvil Experisients and Theory and Research to Attain Higher

Pressures. In this research grant we were able to extend the range

over which we actually obtained x-ray diffraction data and absorption

edge data to 700 kbars. Using the indentor technique we were able to

attain pressures of 2.1 Mbars. Our results will be described in the

following sections.

DIFFRACTION STUDIES WITH CHESS

Using energy dispersive x-ray diffraction techniques with CHESS,

we are now able to obtain in one hour (often less), in a successful

experiment, a dozen or so diffraction peaks on a high pressure

polymorph to 700 kbars; with earlier techniques we would have obtained

perhaps three or so peaks in an experiment lasting several hundred

hours. The experimental aspects of this technique are now published.)

A summary of results were presented as an invited paper in Bad Honnef,

Germany. 2 These results, along with the diamond indentor-anvil

experiments, were also described at a Special American Chemical Society

Symposium. 3 The work on germanium confirmed the B-Sn tetragonal

structure found by Jamieson. 4 The work on GaAs and GaP led to the

successful determination of the crystal structures of the high pressure

RIM
r

y 10K^

OU i

1

i

i

i

1

0 i-=

0.00

3

polymorphs for the first title. GaAs has a crystal structure which is

an orthorhombic distortion of the sodium chloride structure in which

the cell is stretched along one cube axis and compressed along another

cube axis. GO has the B-Sn tetragonal structure with the Ca and P

atoms randomly located. 5 The high pressure phase of Alb has the same

structure as the high pressure phase of GaAs. 6 This completes the

phase of the work to 250 kbars.

The next phase of the diffraction work involved studies on the

mercury chalcogenides which carried us into the pressure regime of 500

kbars.

The stable form of HgS at atmospheric structure is cinnabar, a

material with a hexagonal structure which is a semiconductor at

atmospheric pressure. At high pressure (130 kbar) this material has a

phase transition (observed by us for the first time). Figure 1 shows

FI

O

? 0N

O

He

a

_ 4 

CP)

• ^^ N OC^OCl)

Hg F i^ j a e7 N a

12.21 24.43 36.64 48.86

CHANNEL

Energy kev

Fig. 1. Diffraction pattern of PgS at 300 kbar.
4

the diffraction pattern of this new phase at 300 kbars. The pressure

sodium is Ch3ON - C2HSOR.

Table 1 shows the experimental d-spacings, the calculated

d-spacing on the assumption that the new structure is a NaCl-type

structure, and the observed and theoretical relative intensities. The

ten well-indexed peaks at high pressure and the good agreement of the

Intensities show that quantitative results are possible at high

pressure. Similar results were obtained at 400 kbars.7

It was known that both HgSe and HgTe transform from the cinnabar

structure to a new phase at high pressure. We showed for the first

time that the new structure in each case is the NaCl-type structure.

Moreover, in each case, we found that at a still higher pressure, the

Table I. The analysis of the diffraction pattern of Fig. 1. NaC1 crystal

structure at 300 kbar based on a=5.070±0.005 A.

Index nergy ev obs cal Iobs cal

111 15.61 2.930 2.927 32 25

200 18.07 2.531 2.535 87 62

220 25.53 1.792 1.793 100 100

311 29.92 1.529 1.529 76 64

222 31.22 1.465 1.464 43 31

400 36.15 1.265 1.268 13 10

331 39.30 1.164 1.163 10 13

420 40.33 1.134 1.134 14 19

422 44.19 1.035 1.035 11 11

333/511 46.88 0.9757 0.9757 3 4
. ag

NaCl-type phase transforms to another crystal structure. 819 A paper on

the structures of the high pressure polymorphs of the mercury

chalcogenides was presented at the IX AIRAPT International High

Pressure Conference held in Albany, NY, July 24-28, 1983 and has been

published in the proceedings. 10 It is also discussed in Huang's Ph.D.

thesis. 11 These results are summarized in Table 2. It is pointed out

that this is a fascinating series of compounds with the crystal

structure sequence being zincblende, cinnabar, sodium chloride, B-Sn,

and distorted CsCl (discussed later) which are respectively semimetal,

semiconductor, excellent metal, metal and (probably) metal.

Table 2. Summary of the Pressure-Induced Phase Transitions of Mercury

Chalcogenides

Phase I II III IV e V

Structure Z.B. Cinnabar NaCl b.c.t.

HgS ;

II III

130

H Se ^.

II i III ^ IV

g

8 160 280

I II 111 IV V^

10 HgTe 14 8 20 380

100 200 300 400

Pressure Mar

• The ordered compound analog of the P-Sn structure. However,

the descrepencies with the calculated and the theoretical

intensities is such that we can not determine the space group

definitely.
6

We now enter the third phase of the high pressure diffraction

studies which extended the pressure at which diffraction studies are

made to 700 kbars and included studies on Cal and Bars and BaSe. In

CsI we found a surprising transition from the cubic CsCl structure to a

tetragonal structure which appears to occur at constant volume. This

transformation was first reported at the 1983 AIRAPT Conference in July

1983 in Albany, NY. 12 This transformation at 380 kbars is also

discussed further in other papers of ours. 11.13 The equation of state

was measured to 660 kbars. We will talk about the optical properties

of CsI in a later section.

We showed that BaTs and BaSe both transformed from NaCl-type to

CsCl-type at 48 and 60 kbars, respectively. We also obtained the

equation of state of these materials to 410 and 610 kbars,

respectively. These results were presented at the AIRAPT meeting

mentioned previously 14 and elsewhere. 11,15,16 They will be used in the

next section.

BAND GAP AND OPTICAL ABSORPTION EDGE STUDIES

It was noted by Ruoff 17 that for the rare gas solids, the alkali

halides and the alkaline chalcogenides (including oxides), we found a

simple relationship between the optical dielectric constant and the

optical band gap of the form.

K O Eg W) - D(eV), (1)

F

where D - 21.1 eV.
Moreover, it was noted that this relationship holds for xenon as a

function of pressure. As we become able to independently measure

refractive index, n, as a function of pressure (n2 a K0), this

relationship can be tested in more detail. The results at atmospheric

pressure are illustrated in Fig. 2. These results were presented at

the I% AIRAPT Conference. 17

This simple relation along with the Herzfeld criterion provides a

simple model for the band gap versus volume of the closed shell systems

of Figure 2.

Eg

30

2 o RARE GAS SOLIDS

a ° ALKALINE CHALCOGENIOES

20• &ALKALI HALIDES

+s

&

+o

o &

m

0

4o

o °

5 6 0

0 0

0

3

+ 2 3 4 S

n

Fig. 2. Plot of the logarithm of the energy

band gap versus the logarithm of the

refractive index of the rare gas

solids, alkali halides and alkaline

chalcogenides. The dashed line is

the best least squares fit. The

1

solid line is the best least squares

I fit for the case where the slope is

assumed to be -2.

,x
8

The Herzfeld condition is obtained from the Lorentz-Lorenz

equation.

nl-1^R

n_2 +2 V (2)

where R is the molar refraction and V is the molar volume. At zero

pressure

a

n20

+2 i 70 ; (3)

metallization corresponds to n ; • and hence to

R (4)

m

It is assumed that the molar refraction does not vary with pressure, 

i.e., it is an intrinsic property of the atom or compound, so the

Herzfeld condition follows from these two relations:

no - 1 m

n2 + 1 = V ' (S) 0 0

[R. F. Herzfeld, Phys. Rev. 29, 701 (1927)]. Here n o is the refractive

index at zero pressure, V o is the molar volume at zero pressure, and m

is the molar volume when metallization occurs. The theory is strictly

applicable to cubic solids only.

With the same assumption on R we can write

z V

^ - vm (6)
R .1 IMPW

Combining (6) and (7) we have

2 a D(V- ;)/(V+2 =) (7)

This was fitted to our experimental absorption edge data to obtain D

22.7 eV and m/Vo - 0.655 as shown in Figure 3.16

Note that our ES vs. P data was actually combined with our P(V)

data from x-ray measurements mentioned in the previous section to get

the Eg(V) data in Figure 3.

.. 3.0

r

Q.!)

1.0

nn

v; v0 NGCI

FIG. 3. Absorption edge vs volume fraction at 296 K.

The solid line is fitted to Eq. 9).

Note that the D agrees closely with the general D in Eq. (1)

while the fractional volume at metallization is close to that predicted

from the Herzfeld criterion (Eq. (5)) which is Vm/Vo - 0.623. It is a

challenge to theoretical physicists to explain why Herzfeld's

assumption apparently works so well.
10

Figure 4 shows data for Col. 11 Here we have analysed the

data as if it is a direct band gap E 

t

i

t2and indirect E . The cubic to

tetragonal transition mentioned earlier occurs at V/Vm - 0.54. We

noted it occurs without measurable volume change. Note that there is

no noticeable discontinuity in the energy gap either. The extrapolated

Intercepts are at volume fractions of 0.39 and 0.44. The Bernfeld

prediction is at 0.42. The highest pressure point in this data le 710

kbar. The transition pressure is likely to be above 1 mbar.

INDENTOR ANVIL EXPERIMENTS AND THEORY AND RESEARCH

TO ATTAIN HIGHER PRESSURES

Results with the diamond indentor-diamond anvil system have been

described in an invited paper at a special American Chemical Society

symposium. 3 A thorough review was given in the leadoff plenary talk at

the Eighth AIRAPT Conference on High Pressure at Uppsala. 18 This

technique was used in an earlier NASA grant to show that PMKA

(polymethylmethacrylate) was still an electrical insulator at a

pressure of 1 Mbar [K. S. Chan and A. L. Ruoff, J. Appl. Phys. 52, 5395

(1981)].

In this technique it is necessary to produce interdigitated

electrodes of small dimensions. We have succeeded in producing

interdigitated electrodes with a spacing of 1400A and a width of 2000A

(see Figure 5), a considerable improvement over the 0.5 Um electrodes

produced previously.
11 

N

O

46

O

v

CL

a

W

N

N

O

C

0.36 0.52 0.68 0.84 1.00

V/Vo

Figure 4. Optical Band Gap versus fractional volume for Cal assuming

direct (1) or indirect (2) gaps.

WOMAN
M3INAL PAGE

TILITY OF POOR Q

12

i•

1.

J

t

g1

' ' 1

Figure 5

Interdigitated electrode of

palladium on diamond produced

by electron beam lithography.

Spacing 1400A, line width

20001(.

It is important to note that pressures of 2.1 Mbars were

obtained. .'

In our studies of indentors we developed a new method for the

observation of ring cracking in electrical insulators, l ' which nicely

compliments optical observation and provides a method of in situ

measurement even if the sample is not transparent.

We carried out an important analysis (finished during this grant)

invol ,.ing pressure concentrations created by plastic flow of thin discs

between rigid anvils. This gives the maximum attainable pressure and

involves only the compressive flow stress of the crystal (including

strain hardening) and the ospect ratio a/h (where a is the disc radius

and h is the gasket thickness). This gives us a guod physical

understanding of how extreme pressures are created in the diamone.
13

window call. 20 (These attainable pressures are currently limited by

the fact that the diamonds are not rigid so the faces become concave

under the stress. Bence the diamonds touch at the edges while h at the

center is still finite. If the diamonds were rigid and perfectly

aligned. then h could be made arbitrarily small and the pressure

arbitrarily large if the diamonds also had infinite strength.) In

special cases the compresaive flow stress of the sample (or gaukat)

varies strongly with pressure; this problem was also analyzed. This

analysis shows that the pressure present in xenon when Nelson and Ruoff

observed metallization was greater than 1 Mbar rather than the smaller

Hertzian pressure which they assumed. 21 While the obvious concluhio.:

of this analysis is that we had underestimated the pressure, the much

more important conclusion is that the diamond indentor-anvil system is

capable of much higher pressures than thought previously.

Another important paper by other high pressure workers published

recently measured the pressure distribution under spherical tips by

observing the ruby shift of ruby powder through the flat diamond as a

function of radius (M. I. Eremets, E. S. Itskevich. A. M. Shirokov and

E. N. Yakovlav, J. Exp. Tech. Phys. 36 (3). 58 (1982).) The

experimental results were in excellent agreement with our theoretical

analysis. 20 The experimental pressure distribution with a gasket was

of the form P w Po (1-r/a) where a is the contact radius and r is the

distance along the flat from the center of contact. This means that

the maximum pressure. Po . is three times Pay . For the Mertz case (no

gasket or sample) P o a 3 Pav/2. Thus the presence of a plastically

deforming gasket has doubled the maximum pressure. In the diamond

--7 ;T
_—

14

anvil cell, the maximum pressure would be three times the average

gasket pressure. If, in addition, the yield stress of the gasket

material increased strongly with pressure (a case we analyzed in

detail 2U), then the pressure intensification can be much higher. Hence

this analysis greatly increased our understanding of the role of the

gasket in the diamond anvil cell.
15

PUBLICATIONS

1. M. A. Baublitz, Jr., V. Arnold and A. L. Ruoff, "Energy

Dispersive X-ray Diffraction from High Pressure Polycrystalline

Specimens Using Synchrotron Radiation," Rev. Sci. Instrum. 52,

1616 (1981).

2. M. A. Baublitz, Jr. and A. L. Ruoff, "Energy Dispersive X-ray

Diffraction at High Pressure in CHESS," in Physics of Solids

uneer High Pressure, ed. by J. S. Schilling and R. N. Shelton,

North-Holland Publishing Co. (1981), p. 81.

3. A. L. Ruoff, "Megabar Pressures in Submicron Volumes," in High

Pressure as a Reagent and an Environment, edited by R. S. Shane

and E. F. Skelton, American Chem. Society, Washington, D.C., p.

105, 1982.

4. M. A. Baublitz, Jr. and A. L. Ruoff, "X-Ray Diffraction from High

Pressure Ge Using Synchrotron Radiation," J. Appl. Phys. 53, 5669

(1982).

5. M. A. Baublitz, Jr. and A. L. Ruoff, "Diffraction Studies of the

High Pressure Phase of GaAs and GaP," J. Appl. Phys. (Oct. 1982).

6. M. A. Baublitz, Jr. and A. L. Ruoff, "X-ray Diffraction Data from

the High Pressure Phase of AM," J. Appl. Phys. (Dec. 1982).

7. T. Huang and A. L. Ruoff, "Pressure-Induced Phase Transition of

HgS," J. Appl. Phys. 54, 5459 (1983).

8. T. Huang and A. L. Ruoff, "Pressure-Induced Phase Transitions of

HgSe," Phys. Rev. B27, 7811 (1983).

9. T. Huang and A. L. Ruoff, "Pressure-Induced Phase Transitions of

HgTe," Phys. Stat. Solidi, 77, K193 (1983).

10. T.-L. Huang and A. L. Ruoff, "High Pressure Polymorphs of Mercury

Chalcogenides," Published in the Proceedings of the IX AIRAPT

International High Pressure Conf. held at Albany, NY, USA, July

25-28, 1983. Mat. Res. Soc. Symp. Proc. 22, Part III, 37 (1984).

11. T.-L. Huang, "High Pressure Studies on Cesium Iodide and Mercury

Chalcogenides," Ph.D. Thesis, Cornell University, Ithaca, New

York, 14853, May 1984.

12. T.-L. Huang and A. L. Ruoff, "High Pressure X-ray Diffraction

Study of Cesium Iodide," Published in the Proceedings of the IX

AIRAPT International High Pressure Conf. held at Albany, NY, USA,

July 25-28, 1983. Mat. Res. Soc. Symp. Proc. 22, Part III, 11

(1984).

t

r
16

13. T.-L. Huang and A. L. Ruoff, "Equating State and High Pressure

Phase Transition of Cal," Phys. Rev. B, 29, 1112 ( 1984).

14. T. A. Grzybowski and A. L. Ruoff, "High Pressure X-ray and

Optical Studies of BaSe and BaTe," Published in the Proceedings

of the IX AIRAPT International High Pressure Conf. held at

Albany, NY, USA, July 25-28, 1983. Mat. Res. Soc. Symp. Proc.

22, Part III, 43 ( 1984).

15. T. A. Grzybowski and A. L. Ruoff, "High-Pressure Phase Transition

in BaSe," Phys. Rev. B27, 6502 ( 1983).

16. T. A. Grzybowski, "Band Overlap Metallization in BaSe and BaTe,"

Ph.D. Thesis, Cornell University, Ithaca, New York, Aug. (1984).

17. A. L. Ruoff, "Empirical Relationship between Energy Band Gap and

the Refractive Index in closed-Shell Systems," This was

presented at the IY. AIRAPT International High Pressure Conf. in

July 1983. Hat. Res. Soc. Symp. Proc. 22, Part I, 279 ( 1984).

18. A. L. Ruoff, "Pressure of Two Mbars with the Diamond Anvil

Indentor," in High Pressure in Research and Industry, ed. by

C.-M. Backman, T. Johannison and L. Tegner, Arketektpopia,

Uppsala, Sweden ( 1982), p. 108.

19. T. A. Grzybowski and A. L. Ruoff, "New Method of Determining the

Onset of Ring Cracking," Rev. Sci. Instrum. 54, 357 ( 1983).

20. K. S. Chan, T. L. Huang, T. A. Grzybowski, J. J. Whetten, and A.

L. Ruoff, "Pressure Concentrations due to Plastic Deformation of

Thin Films on Gaskets between Anvils," J. Appl. Phys. 53, 6607

(1982).

21. K. S. Chan, T. L. Huang, T. A. Grzybowski, T. J. Whetten and

L. Ruoff, "Pressure Present During Metallization of Xenon," Phys.

Rev. B26, 7116 ( 1982).