- Email: [email protected]
Metal-insulator transition in thiospinel CuIr2S4
PHY$1CA
Physica B 194-196 (1994) 1077-1078 Nccth-Holland
METAL-INSULATOR TI~SITION 1N THIOSPINEL C u I r 2 S 4 Shoichi NAGATA, Takatsugu tAGINO, Yoshitaka SEKI and Teruo BITOt Department of Applied Materials Science, Muroran I n s t i t u t e of Technology, 27-1Mizumoto-cho, Muroran 050, Japan We have discovered the existence of metal-insulator transition at ~220 K in the new thiospinel CuIr2S 4. The e l e c t r i c a l conductivity, a , for sintered-powder specimens drops abruptly from 2×102 S,cm -1 to 5×10-1S'cm -1 at TM_I-~220 K with decreasing temperature. At temperatures T
3. I~ILTSAND DISCUSSION
1.1NTIC3DUCTION
The X-ray diffraction confirms the spinel
It is well known that magnetite Fe304 undergoes a phase transition (the Verwey transition) accompanied by a sudden change of conductivity at 119 K Eli. We have discovered a metal-insulator transition at TM_I -~
structure as shown in Fig. l. We have obtained single phase specimens. Figures 2 and 3 show the conductivity as functions of T and I / T . The transition is
220 K in a new thiospinel compound Culr2S 4.
slightly sharper on cooling
This compound is one of the thiospinels containing copper [2-3]. An abrupt drop of the conductivity has been observed, by three orders of magnitude, when sintered specimens are cooled through the transition. We present here new experimental results of the
Above the transition the decrease in a with increasing temperature is seen, suggesting that the carriers are not hopping polarons. Attempts have been made to f i t the a vs T dependence in the insulating region to the formula: a = Aexp(-q/kBT).
temperature dependence of conductivity a .
the
electrical
2. EXPERII~TALI~IETHODS Mixture of Cu, Ir and S powders in sealed quartz tubes was calcined at 850 °C for periods of 10 days. Subsequently, the specimens were prepared by sintering in pressed parallelepiped form at 850 °C for 48 hours a f t e r regrinding. X-ray powder diffraction experiments were done. The samples with the dimensions of about 0 . 8 x l . 7 x 4 . 6 mm3 were used for the measurements of a. The conduct i v i t y was measured by a standard dc-fourprobe method. Silver paste was used to fabr i c a t e the electrodes.
than warming.
:g
Culr2S 4
,0
Z
,0
"~ C
i
T_ 10
20
30
t,O
50
: 60
l,t 70
It 80
90
2e/deg.
Fig. 1. X-ray d i f f r a c t i o n pattern of Culr2S 4 at 300 K.
0921-4526/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved SSDI 0921-4526(93)El119-7
1078
102
103 Cu I r2 $4.
102
..
~ .: t
10 aE
Ou I r2 34.
10
$ .
I
1
o 10-1
d9 10-1 10-2 10.2
L I gwl *Jl| el 18|1
10-:3
10-3
lOlL *
!
*1
!
10-4
10 .4.
i
i
10-5 /
I;o
2;0
10-5
300
0
7/K
. . . . . . .
°.
E 1o
.°
-1
the activation energy. Table 1 Activation energy, q, and preexponential factor, A. T q (eY) A (S.cm -1) O. 047 O. 022 0.017
6. 61 O. 76 0.34
the magnetic and thermal properties are in progress. Our detailed experimental r e s u l t s will be reported elsewhere.
1
I0-I
10-2
4. 0
Table 1 shows the values of q and A. The value q is rather smaller than that in other compounds which exhibit the M-I transition. The studies of X-ray analysis below TM_I,
5-
&
310
Fig. 4. Conductivity as a function I/T in the insulating phase. The slope leads to
140
Cu Ir2 $4. 102
210
(1031T)/K
Fig. 2. Electrical conductivity of a sintered sample of CuIr2S 4 as a function of temperature, showing the metal-insulator transition.
103
1'0
~FE~NCES
i
i
l
=
J
i
i
t
2 3 4. 5 6 7 8 9
(103/T)/K
0
-1
Fig. 3. Conductivity of a sintered sample of CuIr2S 4 as a function of 1/T.
1. S. Chikazumi, AIP Proceedings No.29 (1975) 382, and references therein. 2. T. Bitoh, T. Hagino, Y. Seki, S. Chikazawa and S. Nagata, J. Phys. Soc. Jpn. 61 No8 (1992) 3011. 3. Z.Shirane, Z. Hagino, Y. Seki, T.Bitoh, S. Chikazawa and S. Nagata, J. Phys. Soc. Jpn. 6~Nol (1993) 374.
Physica B 194-196 (1994) 1077-1078 Nccth-Holland
METAL-INSULATOR TI~SITION 1N THIOSPINEL C u I r 2 S 4 Shoichi NAGATA, Takatsugu tAGINO, Yoshitaka SEKI and Teruo BITOt Department of Applied Materials Science, Muroran I n s t i t u t e of Technology, 27-1Mizumoto-cho, Muroran 050, Japan We have discovered the existence of metal-insulator transition at ~220 K in the new thiospinel CuIr2S 4. The e l e c t r i c a l conductivity, a , for sintered-powder specimens drops abruptly from 2×102 S,cm -1 to 5×10-1S'cm -1 at TM_I-~220 K with decreasing temperature. At temperatures T
3. I~ILTSAND DISCUSSION
1.1NTIC3DUCTION
The X-ray diffraction confirms the spinel
It is well known that magnetite Fe304 undergoes a phase transition (the Verwey transition) accompanied by a sudden change of conductivity at 119 K Eli. We have discovered a metal-insulator transition at TM_I -~
structure as shown in Fig. l. We have obtained single phase specimens. Figures 2 and 3 show the conductivity as functions of T and I / T . The transition is
220 K in a new thiospinel compound Culr2S 4.
slightly sharper on cooling
This compound is one of the thiospinels containing copper [2-3]. An abrupt drop of the conductivity has been observed, by three orders of magnitude, when sintered specimens are cooled through the transition. We present here new experimental results of the
Above the transition the decrease in a with increasing temperature is seen, suggesting that the carriers are not hopping polarons. Attempts have been made to f i t the a vs T dependence in the insulating region to the formula: a = Aexp(-q/kBT).
temperature dependence of conductivity a .
the
electrical
2. EXPERII~TALI~IETHODS Mixture of Cu, Ir and S powders in sealed quartz tubes was calcined at 850 °C for periods of 10 days. Subsequently, the specimens were prepared by sintering in pressed parallelepiped form at 850 °C for 48 hours a f t e r regrinding. X-ray powder diffraction experiments were done. The samples with the dimensions of about 0 . 8 x l . 7 x 4 . 6 mm3 were used for the measurements of a. The conduct i v i t y was measured by a standard dc-fourprobe method. Silver paste was used to fabr i c a t e the electrodes.
than warming.
:g
Culr2S 4
,0
Z
,0
"~ C
i
T_ 10
20
30
t,O
50
: 60
l,t 70
It 80
90
2e/deg.
Fig. 1. X-ray d i f f r a c t i o n pattern of Culr2S 4 at 300 K.
0921-4526/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved SSDI 0921-4526(93)El119-7
1078
102
103 Cu I r2 $4.
102
..
~ .: t
10 aE
Ou I r2 34.
10
$ .
I
1
o 10-1
d9 10-1 10-2 10.2
L I gwl *Jl| el 18|1
10-:3
10-3
lOlL *
!
*1
!
10-4
10 .4.
i
i
10-5 /
I;o
2;0
10-5
300
0
7/K
. . . . . . .
°.
E 1o
.°
-1
the activation energy. Table 1 Activation energy, q, and preexponential factor, A. T q (eY) A (S.cm -1) O. 047 O. 022 0.017
6. 61 O. 76 0.34
the magnetic and thermal properties are in progress. Our detailed experimental r e s u l t s will be reported elsewhere.
1
I0-I
10-2
4. 0
Table 1 shows the values of q and A. The value q is rather smaller than that in other compounds which exhibit the M-I transition. The studies of X-ray analysis below TM_I,
5-
&
310
Fig. 4. Conductivity as a function I/T in the insulating phase. The slope leads to
140
Cu Ir2 $4. 102
210
(1031T)/K
Fig. 2. Electrical conductivity of a sintered sample of CuIr2S 4 as a function of temperature, showing the metal-insulator transition.
103
1'0
~FE~NCES
i
i
l
=
J
i
i
t
2 3 4. 5 6 7 8 9
(103/T)/K
0
-1
Fig. 3. Conductivity of a sintered sample of CuIr2S 4 as a function of 1/T.
1. S. Chikazumi, AIP Proceedings No.29 (1975) 382, and references therein. 2. T. Bitoh, T. Hagino, Y. Seki, S. Chikazawa and S. Nagata, J. Phys. Soc. Jpn. 61 No8 (1992) 3011. 3. Z.Shirane, Z. Hagino, Y. Seki, T.Bitoh, S. Chikazawa and S. Nagata, J. Phys. Soc. Jpn. 6~Nol (1993) 374.