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Phase transitions in an ionic conductor CuTeBr crystal
Mat. Res. BuU., Vol. 17, pp. 1467-1470, 1982. Printed in the USA. 0025-5408/82/121467-04503.00/0 C o p y r i g h t (e) 1982 Pergamon Press Ltd.
PHASE TRANSITIONS IN AN IONIC CONDUCTOR CuTeBr CRYSTAL
Michio Midorikawa and Yoshihiro Ishibashi Synthetic Crystal Research Laboratory, Faculty of Engineering, Nagoya University Furo-cho, Chikusa-ku, Nagoya 464, Japan (Received June 7, 1982; Refereed)
ABSTRACT The dilatometric and pressure studies were carried out for CuTeBr crystal. The P-T phase diagram was constructed within the range of 3kbar. A triple point was found at 70 °C and 0.3 kbar, beyond which a new phase appears.
Introduction Crystals having the chemical formula CuTeX (X = CI, Br, I) are known to be ionic conductor based on a disordered arrangement of Cu cations (i). Their basic structure belongs to the body-centered tetragonal lattice (2), and is composed of four-fold spirals of Te atoms and tetrahedra formed by X atoms (1-4). The space group for the disordered phase (phase I) is D ~ -141/amd (5), and the unit cell contains 16 formula units. On the ot~er hand, t~e room temperature phase (phase II) belongs to the space group ~ -Fddd (5), and has an enlarged pseudo~ tetragonal unit cell containings 96 formula units: lattice parameters are a ~ ~ a ~ , b # /~aT and c @ 3cT, where a~ and cT refer to the tetragonal cell in the phase I. In CuTeBr the I-If phase transition occurs at about 70 °C at which discontinuities in electric conductively (1,5), lattice parameters (5) and absorption coefficient (6) have been observed. However, there are few reports about other physical properties related to the phase transition. The results of investigations of dilatometric properties and the P-T phase diagram performed on CuTeBr will be presented. 1467
1468
M. MIDORIKAWA, et al.
Vol. 17, No. 12
Experimental Procedures and Results a)
'_o
CuTeBe crystals were prepared by the Bridgman method with a fused quartz ampoule in which equimolar raw materials, Te and CuBr, were sealed after evacuation. Single crystal blocks having dimensions of several millimeter could be cut out from as-grown crystals. The direction of tetragonal c-axis in the specimen was determined from the cleavage planes.
-I
I
I
I
I
I
I
I
b)
30
5.0
. 60 . . TO
,'o
80
TEMPERATURE
I 0'0
IlO
eC
FIG. I The thermal expansions of CuTeBr along a) parallel and b) perpendicular to the tetragonal c-axis.
II0
ioo P 9¢ 80
j/
,,, 70 re
.~ 60 ~ 50 40 30 i I
20
PRESSURE:
FIG. The P-T p h a s e
i
i
2
3
kbor
2
d i a g r a m o f CuTeBr.
Thermal expansion along directions parallel and perpendicular to the tetragonal c - a x i s were measured with a differential transformer type displacement meter. As i s shown i n F i g . 1 the spontaneous strains which occur at the transition a l o n g t h e two d i r e c t i o n s are of opposite sign to each other. A temperature hystetesis o f a b o u t 6 °K was seen. Pressure dependences of transition temperatures were investigated by a DTA method in a high pressure furnace, utilizing Ar gas. When pressure was applied beyond 0.5 kbar, the peak of DTA signal was split, indicating the existence of two transitions and the split increased with increasing pressure. The P-T phase diagram obtained in this way is presented in Fig. 2. It is seen that there exists a triple point at about 70 °C and 0.3 kbar, and a new phase designated as phase III becomes stabilized with increasing pressure.
Vol. 17, No. 12
-3.5
120 I00
CuTeBr CRYSTAL
80
60
",...
-4.0 E ?-4.5 c: w
"C
40
20
O) *°'o,,.
"'~',,,o,,,•.~ % •• m
b -5.0
~
b)
0
"-..
0 0 Q
oe
-5,5
m 0
-6.0
-6.5
L
~6 I/T
FIG.
xlO 3
K "1
1469
The phase transitions under pressure were also confirmed by the measurement of electric conductivity along the c-direction. The conductivity measurements were carried out in another high pressure apparatus, utilizing silicon oil, at several frequencies of applied electric field• The carbon paint was used for electrodes. An example of the results is shown in Fig. 3, in which the conductivity obtained at ambient pressure is also presented for comparison.
3
Electric conductivities measured at I00 kHz along the c-axis under a) 1 kbar and b) ambient pressures•
Discussion The spontaneous strains along two principal axes are of the same order of magnitude but of opposite sign to each other• This leads to a negligibly small change in the crystal volume at the transition, which is certainly reflected in the flatness of the I-II phase boundary in the P-T phase diagram• The fact that the triple point is located quite close to zero pressure line suggests a possibility that the phase III may be stabilized even at atomspheric pressure if some mechanical stress is applied In this respect it is noteworthy (5) that for powdered specimens the tail of endothermic peak of DTA signal is extended over several ten degree centigrade around the transition temperature. This may be due to the local stress caused in powdered specimens. As for the structure of the phase III under pressure, preliminary X-ray diffraction experiments were carried out with diamond anvils and a rotatory target X-ray source. The powder diffractron photographs seem to show a face-centered lattice with a = aT, c = 3cT. Because of experimental difficulties, however, we could not obtain a definite conclusion, and so structural study of the phase III is left to future work.
1470
M. MIDORIKAWA, et al.
Vol. 17, No. 12
Acknowledgement The a u t h o r s w o u l d l i k e t o t h a n k Dr. M. Senoo o f N a g o y a University for his offer of X-ray diffraction apparatus, and are grateful t o M e s s r s . K. Y u k i h i r a and S. H i r a n o f o r constructing the diamond a n v i l c e l l .
References
i)
U. v. Alpen,
J. Fenner,
J. D. Marcoll
Eleatrochim Acta 2__22,801
2)
A. R a b e n a u , 137 ( 1 9 6 9 ) .
3)
P. M. C a r k n e r and H. M. H a e n d l e r : i_88, 183 ( 1 9 7 6 ) .
4)
J.
H. Rau and G. R o s e n s t e i n :
F e n n e r and A. R a b e n a u :
and A. Rabenau:
(1977). Naturwiss.
J.
Z. a n o r g ,
Solid allgo
56,
State
Chem.
Chem. 4 2 6 ,
(1976) 7.
S)
U. v . A p l e n , J . F e n n e r , B. P r e d e l , A. R a b e n a u a n d G. S c h l u c k e b i e r : Z. a n o r g , a l l g . Chem. 438 S ( 1 9 7 8 ) .
6)
H. R. C h a n d r a s e k h a r 2_~5, 73 (1978).
and L. G e n z e l :
Solid
State
Commun.
PHASE TRANSITIONS IN AN IONIC CONDUCTOR CuTeBr CRYSTAL
Michio Midorikawa and Yoshihiro Ishibashi Synthetic Crystal Research Laboratory, Faculty of Engineering, Nagoya University Furo-cho, Chikusa-ku, Nagoya 464, Japan (Received June 7, 1982; Refereed)
ABSTRACT The dilatometric and pressure studies were carried out for CuTeBr crystal. The P-T phase diagram was constructed within the range of 3kbar. A triple point was found at 70 °C and 0.3 kbar, beyond which a new phase appears.
Introduction Crystals having the chemical formula CuTeX (X = CI, Br, I) are known to be ionic conductor based on a disordered arrangement of Cu cations (i). Their basic structure belongs to the body-centered tetragonal lattice (2), and is composed of four-fold spirals of Te atoms and tetrahedra formed by X atoms (1-4). The space group for the disordered phase (phase I) is D ~ -141/amd (5), and the unit cell contains 16 formula units. On the ot~er hand, t~e room temperature phase (phase II) belongs to the space group ~ -Fddd (5), and has an enlarged pseudo~ tetragonal unit cell containings 96 formula units: lattice parameters are a ~ ~ a ~ , b # /~aT and c @ 3cT, where a~ and cT refer to the tetragonal cell in the phase I. In CuTeBr the I-If phase transition occurs at about 70 °C at which discontinuities in electric conductively (1,5), lattice parameters (5) and absorption coefficient (6) have been observed. However, there are few reports about other physical properties related to the phase transition. The results of investigations of dilatometric properties and the P-T phase diagram performed on CuTeBr will be presented. 1467
1468
M. MIDORIKAWA, et al.
Vol. 17, No. 12
Experimental Procedures and Results a)
'_o
CuTeBe crystals were prepared by the Bridgman method with a fused quartz ampoule in which equimolar raw materials, Te and CuBr, were sealed after evacuation. Single crystal blocks having dimensions of several millimeter could be cut out from as-grown crystals. The direction of tetragonal c-axis in the specimen was determined from the cleavage planes.
-I
I
I
I
I
I
I
I
b)
30
5.0
. 60 . . TO
,'o
80
TEMPERATURE
I 0'0
IlO
eC
FIG. I The thermal expansions of CuTeBr along a) parallel and b) perpendicular to the tetragonal c-axis.
II0
ioo P 9¢ 80
j/
,,, 70 re
.~ 60 ~ 50 40 30 i I
20
PRESSURE:
FIG. The P-T p h a s e
i
i
2
3
kbor
2
d i a g r a m o f CuTeBr.
Thermal expansion along directions parallel and perpendicular to the tetragonal c - a x i s were measured with a differential transformer type displacement meter. As i s shown i n F i g . 1 the spontaneous strains which occur at the transition a l o n g t h e two d i r e c t i o n s are of opposite sign to each other. A temperature hystetesis o f a b o u t 6 °K was seen. Pressure dependences of transition temperatures were investigated by a DTA method in a high pressure furnace, utilizing Ar gas. When pressure was applied beyond 0.5 kbar, the peak of DTA signal was split, indicating the existence of two transitions and the split increased with increasing pressure. The P-T phase diagram obtained in this way is presented in Fig. 2. It is seen that there exists a triple point at about 70 °C and 0.3 kbar, and a new phase designated as phase III becomes stabilized with increasing pressure.
Vol. 17, No. 12
-3.5
120 I00
CuTeBr CRYSTAL
80
60
",...
-4.0 E ?-4.5 c: w
"C
40
20
O) *°'o,,.
"'~',,,o,,,•.~ % •• m
b -5.0
~
b)
0
"-..
0 0 Q
oe
-5,5
m 0
-6.0
-6.5
L
~6 I/T
FIG.
xlO 3
K "1
1469
The phase transitions under pressure were also confirmed by the measurement of electric conductivity along the c-direction. The conductivity measurements were carried out in another high pressure apparatus, utilizing silicon oil, at several frequencies of applied electric field• The carbon paint was used for electrodes. An example of the results is shown in Fig. 3, in which the conductivity obtained at ambient pressure is also presented for comparison.
3
Electric conductivities measured at I00 kHz along the c-axis under a) 1 kbar and b) ambient pressures•
Discussion The spontaneous strains along two principal axes are of the same order of magnitude but of opposite sign to each other• This leads to a negligibly small change in the crystal volume at the transition, which is certainly reflected in the flatness of the I-II phase boundary in the P-T phase diagram• The fact that the triple point is located quite close to zero pressure line suggests a possibility that the phase III may be stabilized even at atomspheric pressure if some mechanical stress is applied In this respect it is noteworthy (5) that for powdered specimens the tail of endothermic peak of DTA signal is extended over several ten degree centigrade around the transition temperature. This may be due to the local stress caused in powdered specimens. As for the structure of the phase III under pressure, preliminary X-ray diffraction experiments were carried out with diamond anvils and a rotatory target X-ray source. The powder diffractron photographs seem to show a face-centered lattice with a = aT, c = 3cT. Because of experimental difficulties, however, we could not obtain a definite conclusion, and so structural study of the phase III is left to future work.
1470
M. MIDORIKAWA, et al.
Vol. 17, No. 12
Acknowledgement The a u t h o r s w o u l d l i k e t o t h a n k Dr. M. Senoo o f N a g o y a University for his offer of X-ray diffraction apparatus, and are grateful t o M e s s r s . K. Y u k i h i r a and S. H i r a n o f o r constructing the diamond a n v i l c e l l .
References
i)
U. v. Alpen,
J. Fenner,
J. D. Marcoll
Eleatrochim Acta 2__22,801
2)
A. R a b e n a u , 137 ( 1 9 6 9 ) .
3)
P. M. C a r k n e r and H. M. H a e n d l e r : i_88, 183 ( 1 9 7 6 ) .
4)
J.
H. Rau and G. R o s e n s t e i n :
F e n n e r and A. R a b e n a u :
and A. Rabenau:
(1977). Naturwiss.
J.
Z. a n o r g ,
Solid allgo
56,
State
Chem.
Chem. 4 2 6 ,
(1976) 7.
S)
U. v . A p l e n , J . F e n n e r , B. P r e d e l , A. R a b e n a u a n d G. S c h l u c k e b i e r : Z. a n o r g , a l l g . Chem. 438 S ( 1 9 7 8 ) .
6)
H. R. C h a n d r a s e k h a r 2_~5, 73 (1978).
and L. G e n z e l :
Solid
State
Commun.