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Equation of state and phase transitions in AgGaS2 and AgGaSe2
Pergamon
J. Phw. Chem Soitds Vol. 56. No. 314. pp 4X1-484. 1995 Copyright ‘f; 1995 Elsevier Science Ltd Printed in Great Brimn. All rights reserved 0022.3697195 $9.50 + 0.00
0022-3697(94)00239-8
EQUATION
OF STATE AND PHASE TRANSITIONS AgGaS, AND AgGaSe,
T. TINOCO,_F A. POLIAN,I_ J. P. IT&t iPhysique
des Milieux
fCentro
de Estudios
IN
E. MOYA? and J. GONZALEZ-:
Condenses,
Universite P. & M. Curie, B77. 4 Place Jussieu, F-75252 Paris. Cedex 05, France de Semiconductores, Universidad de Los Andes. M&da 5101, Venezuela
Abstract-AgGaS, and AgGaSe, crystallize in the chalcopyrite structure. Their room temperature phase diagrams have been studied by X-ray absorption spectroscopy at the GaK edge and by X-ray diffraction up to 35 GPa. Crystallographic transitions were observed at 5, 12 and 16.5 GPa for AgGaS, and 2.6, 5. 8 and I8 GPa for AgGaSe,. The transitions are not StructuralIy reversible. A Murnaghan equation of state fitted to the experimental data in the chalcopyrite phase gives, maintaining B’ constant equal to 4. a bulk modulus of 65 GPa for AgGaSe, and 90 GPa for AgGaS,. The symmetry of some of the new phases has been determined. KeJ~or&: diffraction,
A.
C.
semiconductors,
high
pressure,
C.
XAFS
(EXAFS
The diffraction
1. INTRODUCTION
Silver gallium
disulfide
~miconductors
crystallize 142d). their
I-III-VI,
in the chalcopyrite Like
binary
and silver gallium diselenide
of the
many
materials linear
and
family
structure
other
(space group
analogs
they
have
long
non
linear
optical
that
of
been
the
II-VI
studied
for
properties
troscopy
have
been
realized
and X-ray diffraction
by
Raman
by different
samples
K edge.
pressure
of the lattice
neighbor
distances
authors
hydrostatic modulus which
pressure.
(EDX) and X-ray the evolution
parameters
around
3. RESC’LTS AND DISCUSSION
3.1. AgGaSez For
AgGaSe?
the tetragonal
phase
0 and
has
been
2.6 GPa.
The
volume has been fitted with a first-order equation
of state [7]
with
under quasic = 2 -c/a
distortion
have
been deduced.
2. EXPERIMENTAL The samples were grown by the Bridgman method. A membrane diamond anvil cell [5] has been used as high pressure generator with silicone oil as a pressure transmitting medium. The pressure was determined using the linear ruby scale (dE./dP = - 3.65 A GPa at the LURE
Murnaghan
between
V = u% and bulk
B,. as well as the parameter characterizes
mode
the chalcopyrite
by EDX
and of the first
the gallium,
The volume
took place at the energy
spec-
(XAS) in the dispersive to observe
at the
were utilized.
experimental
spectroscopy
were performed
DW 11 on the wiggler beam
[I].
energy dispersive absorption
X-ray
a Si [ 11I] polychromator. The data were treated using the CDXAS software [6]. In both cases, finely ground
observed
at the Ga
C.
station at the Ga K edge (10,367 eV) using
but there has been no agreement between the various results [2-41. We have employed two techniques: X-ray diffraction
station
line. The XAS experiments dispersive
High pressure studies on these ternary ~miconductor compounds
XANES).
experiments
energy dispersive are
and
’ ). Both experiments were performed synchroton facility (Orsay-France).
where ambient
Y. is the volume. pressure
B,, the bulk
and B ’ its pressure
modulus
derivative
at also
at P = 0. The best fit to the experimental
data gives
B,, = 65 + 10 GPa
8’ = 4. In
maintaining
the 2.6-5 GPa range, additional
constant
peaks appear
along
with the chalcopyrite peaks, showing the appearance of a new phase coexisting with the chalcopyrite one. The new phase could not be indexed. due to the small number of peaks and to the fact that. over the whole prcssurc range, the chalcopyrite form is still present. In the 5- lOGPa range, all the peaks could be indexed in an orthorhombic structure with Z = 2. The parameter variations ranges are (I = 8.96-8.73 A,
482
T. TINOCO et al. AgGaSez
00
00
0
1
;;j 0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
P (GPa) Fig. I. Variation
of the lattice
parameters
a, b, c and of the volume
b = 4.81-4.74 8, and c = 3.39-3.35 A. The [112] chalcopyrite reflection disappears around 8 GPa, so the phase is pure only between 8 and 10GPa. A tetragonal phase is observed between 10 and 18 GPa, with a = 3.70-3.60 A and c = 9.38-9.12 A. Above 18 GPa, only three broad peaks remain, which cannot be indexed. Figure 1 summarizes the EDX results. The Ga-Se distance has been studied under pressure up to 35 GPa by XAS [8] at the GaK edge.
2.42 ,
I
AgGaSez 0 000
0
0 0
Oco 0
0
0 0 0
per molecular
unit in AgGaSez.
Two phase transitions are observed by EXAFS, at 5 and IOGPa, with a systematic increase of the pseudo Debye-Waller factor, Aa’ (Fig. 2). It means that the transition observed by EDX around 2.6 GPa does not affect the local order around the gallium atoms. Between 0 and 5 GPa, the bulk modulus of the Ga-Se bond (do,_,/3 aP/&&,_,) is approximatively I 15 GPa, i.e. much less compressible than the volume. Above 5 GPa, the quality of the fits decreases and an attempt was made to fit the data with two shells of neighbors, the first shell with four selenium atoms and the second one with two silver atoms. There is not yet a convincing conclusion, and new experiments should be made, especially at the Se K edge. The transition pressures are comparable to those of Arora et al. [3]. Using Raman spectroscopy, they have seen three phase transitions up to 16 GPa at 3, 5.1 and 8.3 GPa.
3.2. AgGaSz 2.37O--i
oB05 -
0.004 t N^ 0.003 “b, 3
vv vvv
0B02 0.001 0.000
V
v
v
vv
4
v
v
vvFIv
dp
i
-0.0011 0
W
I 5
I 10
I 15
I 20
I 25
I 1 30
Pressure (GPa) Fig. 2. Variation of the first neighbor distance and of the pseudo Debye-Wailer factor as a function of pressure in AgGaSe?
For AgGaS, the chalcopyrite phase is stable up to 5 GPa, with a bulk modulus of 90 f 10 GPa maintaining constant BA = 4. Between 0 and 5 GPa, the tetragonal distortion t = 2 -c/a increases from 0.21 to 0.25. Between 5 and IOGPa a new phase appears which coexists with the chalcopyrite one. The new peaks appear around 5 GPa, but the chalcopyrite peaks are visible up to 16 GPa, so the new structure was not identified. In the 15-25GPd range, all the peaks have been indexed in an orthorhombic phase with a = 3.73-3.60 A, b = 3.61-3.56 8, and c = 11.59-I 1.54A (Fig. 3). Figure 4 shows the variation of d,, s and of the associated pseudo Debye-Waller factor as a function
Equation
483
in AgGaS, and AgGaSe,
of state and phase transitions
AgGaSz
1
15 -
Chalco
+ y
12 0
000
000
00
8
888
800
00
11 -
6.0 5.5 (zoOa -
0
000
00
0
5.0,;
I
3.5 3.0 o
I
I
I
I
2
4
6
8
III 10 12
14
I
I
I
I
I
16
18
20
22
24
P (GPa) Fig. 3. Variation
of pressure
obtained
very low pressure distance
of the lattice
the EXAFS
from
distance
pressure, decreases poorer
i.e. the
increases
up
the quality of the fits so a six-fold
nation scheme is used, and the first neighbor
coordidistance
ified structures
results
increase.
It is interesting
to examine
the variation
those
obtained
copper
of the DW).
Here,
pressures,
P = 0.5 GPa,
where the value of the DW parameter
as 0. At low pressure,
the DW decreases,
on
on the copper
compounds,
up to 15 (CuGaS,) there
structure.
the
the
present
chalcopyrites compounds.
the chalcopyrite
At these
to the NaCI
the stability range
phase is much smaller,
and there
AgGaSa
2.35 2.34 2.33 -
6
1.
2.32i-
'$ 2.31 'o -?
2.300~
. . 0 0
2.29t
0
4
8
12
16
20
24
0
4
8
I2
16
20
24
P (GPa) Fig. 4. Variation
of the first neighbor
distance and of the pseudo pressure in AgGaS.
Debye-Wailer
factor
with In the
phase is stable
transition
In the silver compounds,
of the chalcopyrite
and ident-
are summarized
or 13 GPa (CuGaSez).
is a direct
of the
constant, This is a
pressures
for both compounds
obtained
of the Debye-Walter (DW) parameter (it should be noted here that one measures only a relative value
is taken
its value increases.
to compare
to a density
at
where
silver
nevertheless
is taken
of the crystal (decrease
Then the value is almost
in Table 1. It may be interesting
corresponding
point
unit in AgGaS,
signature of a phase transition. The stability range, transition
increases,
the reference
per molecular
an ordering
up to 12 GPa,
is normal,
the pressure
and poorer,
indicating
by less than 0.015 A.
to 12 GPa. Above this pressure, becomes
At
static disorder).
the behavior when
analysis.
the first neighbor’s
(P < 2 GPa)
is stable or increases
At higher
a, b, c and of the volume
parameters
as a function
of
484
T. TINOCO et al.
Table 1. Stability ranges, transition pressure, and identified structures of AgGaS, and AgGaSez. The a, /3, y and 6 phases are not yet identified
Compound AgGaSe,
AgGaS,
Pressure range (GPa) O-2.6 2.65 5-10 lo-18 18-25 o-5 5-12 12-16.5 16.5-30
Transition pressure (GPa) 5 IO 18
5 12 17
effects, this may be due to the position u which in these crystals,
In both samples the stability range of the chalcopyrite phase is much smaller than in the copper compound.
Structure chalcopyrite chalcopyrite + a orthorhombic tetragonal B chalcopyrite chalcopyrite + y orthorhombic 6
are several phases between the chalcopyrite and the six-fold coordinated structure. Besides the ionic size (parameter
explored by energy dispersive X-ray diffraction and by energy dispersive X-ray absorption spectroscopy.
of the anion
In AgGaS, the first transition occurs around 5 GPa and in AgGaSe, around 2.6 GPa. The bulk moduli are obtained from a fit of the results with a Murnaghan equation of state, maintaining B’ = 4. B, (AgGaS,) = 90 GPa and $ (AgGaSe,) = 65 GPa. In AgGa&, three phase transitions are observed, at 5, 12 and 16.5 GPa. Only the phase between 12 and 16.5 GPa could be indexed with an orthorhombic structure. In AgGaSez, four phase transitions occur at 2.6, 5, 10 and 18 GPa and the structure succession is chalcopyrite, unknown, orthorhombic, tetragonal and unknown. In both compounds the transitions are irreversible.
is not equal to
the ideal value of 0.25).
The combination of EXAFS and EDX has allowed the complete determination of the structure in the chalcopyrite samples where two edges are accessible (CuGaS, or CuGaSe,) [9] but the Ag K edge (25,514 eV) is too high in energy for the LURE and the L edges are out of reach (too low) because of the absorption of the diamond anvils. So only the determination of dAgmse is possible at the Se K edge, and this experiment will be done in the near future, but not dAgms. At decompression, the initial chalcopyrite phase was never recovered. The EDX spectra show a disordered structure. 4. CONCLUSIONS The room temperature phase diagram of the silver chalcopyrites AgGaS, and AgGaSe, has been
Acknowled~emenls-One of us (T.T.) wishes to thank the CONICIT and the Universidad de Los Andes (Venezuela) for the maintenance grant for her stay in France.
REFERENCES 1. Boyd D., Kasper H. M., McFee J. H. and Storz F. G., IEEE J. Quant. Electron. QE-8, 900 (1972).
2. Qadri S. B., Skelton E. F., Webb A. W., Wolf S. A., Elam W. T. and Rek Z., in High Pressure in Science and Technology, (Edited by C. Homan, R. K. McCrone and E. Whalley). North Holland, New York, p. 25 (1984). 3. Arora A. K., Sakuntala T. and Artus L., J. Phys. Chem. Solids 54, 381 (1993). 4. Carlone C., Olego D., Jayaraman A. and Cardona M., Phvs. Rev. B22. 3877 (1980). 5. LeToullec R., Pinceaux J. P. and Loubeyre P., High Press. Res. I, 77 (1980).
6. San Miguel A., XAFS VlIf, Berlin, August 1994, to be published. 7. Murnaghan F. D., Am. J. Math. 49, 235 (1937). 8. Itie J. P., Phase Transifions 39, 81 (1991). 9. Moya E., Polian A., ItiC J. P., Tinoco T. and Gonzalez J., unpublished.
J. Phw. Chem Soitds Vol. 56. No. 314. pp 4X1-484. 1995 Copyright ‘f; 1995 Elsevier Science Ltd Printed in Great Brimn. All rights reserved 0022.3697195 $9.50 + 0.00
0022-3697(94)00239-8
EQUATION
OF STATE AND PHASE TRANSITIONS AgGaS, AND AgGaSe,
T. TINOCO,_F A. POLIAN,I_ J. P. IT&t iPhysique
des Milieux
fCentro
de Estudios
IN
E. MOYA? and J. GONZALEZ-:
Condenses,
Universite P. & M. Curie, B77. 4 Place Jussieu, F-75252 Paris. Cedex 05, France de Semiconductores, Universidad de Los Andes. M&da 5101, Venezuela
Abstract-AgGaS, and AgGaSe, crystallize in the chalcopyrite structure. Their room temperature phase diagrams have been studied by X-ray absorption spectroscopy at the GaK edge and by X-ray diffraction up to 35 GPa. Crystallographic transitions were observed at 5, 12 and 16.5 GPa for AgGaS, and 2.6, 5. 8 and I8 GPa for AgGaSe,. The transitions are not StructuralIy reversible. A Murnaghan equation of state fitted to the experimental data in the chalcopyrite phase gives, maintaining B’ constant equal to 4. a bulk modulus of 65 GPa for AgGaSe, and 90 GPa for AgGaS,. The symmetry of some of the new phases has been determined. KeJ~or&: diffraction,
A.
C.
semiconductors,
high
pressure,
C.
XAFS
(EXAFS
The diffraction
1. INTRODUCTION
Silver gallium
disulfide
~miconductors
crystallize 142d). their
I-III-VI,
in the chalcopyrite Like
binary
and silver gallium diselenide
of the
many
materials linear
and
family
structure
other
(space group
analogs
they
have
long
non
linear
optical
that
of
been
the
II-VI
studied
for
properties
troscopy
have
been
realized
and X-ray diffraction
by
Raman
by different
samples
K edge.
pressure
of the lattice
neighbor
distances
authors
hydrostatic modulus which
pressure.
(EDX) and X-ray the evolution
parameters
around
3. RESC’LTS AND DISCUSSION
3.1. AgGaSez For
AgGaSe?
the tetragonal
phase
0 and
has
been
2.6 GPa.
The
volume has been fitted with a first-order equation
of state [7]
with
under quasic = 2 -c/a
distortion
have
been deduced.
2. EXPERIMENTAL The samples were grown by the Bridgman method. A membrane diamond anvil cell [5] has been used as high pressure generator with silicone oil as a pressure transmitting medium. The pressure was determined using the linear ruby scale (dE./dP = - 3.65 A GPa at the LURE
Murnaghan
between
V = u% and bulk
B,. as well as the parameter characterizes
mode
the chalcopyrite
by EDX
and of the first
the gallium,
The volume
took place at the energy
spec-
(XAS) in the dispersive to observe
at the
were utilized.
experimental
spectroscopy
were performed
DW 11 on the wiggler beam
[I].
energy dispersive absorption
X-ray
a Si [ 11I] polychromator. The data were treated using the CDXAS software [6]. In both cases, finely ground
observed
at the Ga
C.
station at the Ga K edge (10,367 eV) using
but there has been no agreement between the various results [2-41. We have employed two techniques: X-ray diffraction
station
line. The XAS experiments dispersive
High pressure studies on these ternary ~miconductor compounds
XANES).
experiments
energy dispersive are
and
’ ). Both experiments were performed synchroton facility (Orsay-France).
where ambient
Y. is the volume. pressure
B,, the bulk
and B ’ its pressure
modulus
derivative
at also
at P = 0. The best fit to the experimental
data gives
B,, = 65 + 10 GPa
8’ = 4. In
maintaining
the 2.6-5 GPa range, additional
constant
peaks appear
along
with the chalcopyrite peaks, showing the appearance of a new phase coexisting with the chalcopyrite one. The new phase could not be indexed. due to the small number of peaks and to the fact that. over the whole prcssurc range, the chalcopyrite form is still present. In the 5- lOGPa range, all the peaks could be indexed in an orthorhombic structure with Z = 2. The parameter variations ranges are (I = 8.96-8.73 A,
482
T. TINOCO et al. AgGaSez
00
00
0
1
;;j 0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
P (GPa) Fig. I. Variation
of the lattice
parameters
a, b, c and of the volume
b = 4.81-4.74 8, and c = 3.39-3.35 A. The [112] chalcopyrite reflection disappears around 8 GPa, so the phase is pure only between 8 and 10GPa. A tetragonal phase is observed between 10 and 18 GPa, with a = 3.70-3.60 A and c = 9.38-9.12 A. Above 18 GPa, only three broad peaks remain, which cannot be indexed. Figure 1 summarizes the EDX results. The Ga-Se distance has been studied under pressure up to 35 GPa by XAS [8] at the GaK edge.
2.42 ,
I
AgGaSez 0 000
0
0 0
Oco 0
0
0 0 0
per molecular
unit in AgGaSez.
Two phase transitions are observed by EXAFS, at 5 and IOGPa, with a systematic increase of the pseudo Debye-Waller factor, Aa’ (Fig. 2). It means that the transition observed by EDX around 2.6 GPa does not affect the local order around the gallium atoms. Between 0 and 5 GPa, the bulk modulus of the Ga-Se bond (do,_,/3 aP/&&,_,) is approximatively I 15 GPa, i.e. much less compressible than the volume. Above 5 GPa, the quality of the fits decreases and an attempt was made to fit the data with two shells of neighbors, the first shell with four selenium atoms and the second one with two silver atoms. There is not yet a convincing conclusion, and new experiments should be made, especially at the Se K edge. The transition pressures are comparable to those of Arora et al. [3]. Using Raman spectroscopy, they have seen three phase transitions up to 16 GPa at 3, 5.1 and 8.3 GPa.
3.2. AgGaSz 2.37O--i
oB05 -
0.004 t N^ 0.003 “b, 3
vv vvv
0B02 0.001 0.000
V
v
v
vv
4
v
v
vvFIv
dp
i
-0.0011 0
W
I 5
I 10
I 15
I 20
I 25
I 1 30
Pressure (GPa) Fig. 2. Variation of the first neighbor distance and of the pseudo Debye-Wailer factor as a function of pressure in AgGaSe?
For AgGaS, the chalcopyrite phase is stable up to 5 GPa, with a bulk modulus of 90 f 10 GPa maintaining constant BA = 4. Between 0 and 5 GPa, the tetragonal distortion t = 2 -c/a increases from 0.21 to 0.25. Between 5 and IOGPa a new phase appears which coexists with the chalcopyrite one. The new peaks appear around 5 GPa, but the chalcopyrite peaks are visible up to 16 GPa, so the new structure was not identified. In the 15-25GPd range, all the peaks have been indexed in an orthorhombic phase with a = 3.73-3.60 A, b = 3.61-3.56 8, and c = 11.59-I 1.54A (Fig. 3). Figure 4 shows the variation of d,, s and of the associated pseudo Debye-Waller factor as a function
Equation
483
in AgGaS, and AgGaSe,
of state and phase transitions
AgGaSz
1
15 -
Chalco
+ y
12 0
000
000
00
8
888
800
00
11 -
6.0 5.5 (zoOa -
0
000
00
0
5.0,;
I
3.5 3.0 o
I
I
I
I
2
4
6
8
III 10 12
14
I
I
I
I
I
16
18
20
22
24
P (GPa) Fig. 3. Variation
of pressure
obtained
very low pressure distance
of the lattice
the EXAFS
from
distance
pressure, decreases poorer
i.e. the
increases
up
the quality of the fits so a six-fold
nation scheme is used, and the first neighbor
coordidistance
ified structures
results
increase.
It is interesting
to examine
the variation
those
obtained
copper
of the DW).
Here,
pressures,
P = 0.5 GPa,
where the value of the DW parameter
as 0. At low pressure,
the DW decreases,
on
on the copper
compounds,
up to 15 (CuGaS,) there
structure.
the
the
present
chalcopyrites compounds.
the chalcopyrite
At these
to the NaCI
the stability range
phase is much smaller,
and there
AgGaSa
2.35 2.34 2.33 -
6
1.
2.32i-
'$ 2.31 'o -?
2.300~
. . 0 0
2.29t
0
4
8
12
16
20
24
0
4
8
I2
16
20
24
P (GPa) Fig. 4. Variation
of the first neighbor
distance and of the pseudo pressure in AgGaS.
Debye-Wailer
factor
with In the
phase is stable
transition
In the silver compounds,
of the chalcopyrite
and ident-
are summarized
or 13 GPa (CuGaSez).
is a direct
of the
constant, This is a
pressures
for both compounds
obtained
of the Debye-Walter (DW) parameter (it should be noted here that one measures only a relative value
is taken
its value increases.
to compare
to a density
at
where
silver
nevertheless
is taken
of the crystal (decrease
Then the value is almost
in Table 1. It may be interesting
corresponding
point
unit in AgGaS,
signature of a phase transition. The stability range, transition
increases,
the reference
per molecular
an ordering
up to 12 GPa,
is normal,
the pressure
and poorer,
indicating
by less than 0.015 A.
to 12 GPa. Above this pressure, becomes
At
static disorder).
the behavior when
analysis.
the first neighbor’s
(P < 2 GPa)
is stable or increases
At higher
a, b, c and of the volume
parameters
as a function
of
484
T. TINOCO et al.
Table 1. Stability ranges, transition pressure, and identified structures of AgGaS, and AgGaSez. The a, /3, y and 6 phases are not yet identified
Compound AgGaSe,
AgGaS,
Pressure range (GPa) O-2.6 2.65 5-10 lo-18 18-25 o-5 5-12 12-16.5 16.5-30
Transition pressure (GPa) 5 IO 18
5 12 17
effects, this may be due to the position u which in these crystals,
In both samples the stability range of the chalcopyrite phase is much smaller than in the copper compound.
Structure chalcopyrite chalcopyrite + a orthorhombic tetragonal B chalcopyrite chalcopyrite + y orthorhombic 6
are several phases between the chalcopyrite and the six-fold coordinated structure. Besides the ionic size (parameter
explored by energy dispersive X-ray diffraction and by energy dispersive X-ray absorption spectroscopy.
of the anion
In AgGaS, the first transition occurs around 5 GPa and in AgGaSe, around 2.6 GPa. The bulk moduli are obtained from a fit of the results with a Murnaghan equation of state, maintaining B’ = 4. B, (AgGaS,) = 90 GPa and $ (AgGaSe,) = 65 GPa. In AgGa&, three phase transitions are observed, at 5, 12 and 16.5 GPa. Only the phase between 12 and 16.5 GPa could be indexed with an orthorhombic structure. In AgGaSez, four phase transitions occur at 2.6, 5, 10 and 18 GPa and the structure succession is chalcopyrite, unknown, orthorhombic, tetragonal and unknown. In both compounds the transitions are irreversible.
is not equal to
the ideal value of 0.25).
The combination of EXAFS and EDX has allowed the complete determination of the structure in the chalcopyrite samples where two edges are accessible (CuGaS, or CuGaSe,) [9] but the Ag K edge (25,514 eV) is too high in energy for the LURE and the L edges are out of reach (too low) because of the absorption of the diamond anvils. So only the determination of dAgmse is possible at the Se K edge, and this experiment will be done in the near future, but not dAgms. At decompression, the initial chalcopyrite phase was never recovered. The EDX spectra show a disordered structure. 4. CONCLUSIONS The room temperature phase diagram of the silver chalcopyrites AgGaS, and AgGaSe, has been
Acknowled~emenls-One of us (T.T.) wishes to thank the CONICIT and the Universidad de Los Andes (Venezuela) for the maintenance grant for her stay in France.
REFERENCES 1. Boyd D., Kasper H. M., McFee J. H. and Storz F. G., IEEE J. Quant. Electron. QE-8, 900 (1972).
2. Qadri S. B., Skelton E. F., Webb A. W., Wolf S. A., Elam W. T. and Rek Z., in High Pressure in Science and Technology, (Edited by C. Homan, R. K. McCrone and E. Whalley). North Holland, New York, p. 25 (1984). 3. Arora A. K., Sakuntala T. and Artus L., J. Phys. Chem. Solids 54, 381 (1993). 4. Carlone C., Olego D., Jayaraman A. and Cardona M., Phvs. Rev. B22. 3877 (1980). 5. LeToullec R., Pinceaux J. P. and Loubeyre P., High Press. Res. I, 77 (1980).
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