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Transport properties of BaTiO3−δ under high pressure
ELSEVIER
Physica B 237-238 (1997) 16-18
Transportproperties of BaTiO3_6 under high pressure T. N a k a n i s h i a,*, N. M r r i a, ¥ . A k i s h i g e h, O. N a k a o c, A. K u r o s a k a c a Institute for Solid State Physics, University of Tokyo, Minato-ku, Tokyo 106, Japan b Faculty of Education, Shimane University, Nishikawatsu-cho, Matsue 690, Japan c Advanced Technology R&D Center, Fujikura Ltd., Koto-ku, Tokyo 135, Japan
Abstract
We have measured the electrical resistivity of doped ferroelectric single crystal BaTiO3_6 under high pressure up to 8.0 GPa in the temperature range between 4.2 and 295 K. By applying pressure above 6.5 GPa metallic conduction was observed down to 4.2K. A T2-dependence of resistivity was found in the pressure-induced metallic phase below 100K, suggesting that electrons in the Ti 3d band of BaTiO3_6 have a nature similar to strongly correlated electron systems.
Keywords: BaTiO3_6; Transport properties; Electrical resistivity; High pressure
For the last 10 years, since the discovery of high-Tc superconducting Cu-based oxides, there have been extensive studies on properties of conducting transitionmetal oxides. However, there are only a few reports on the low-temperature transport properties of doped ferroelectric oxides even though many ferroelectric materials are transition-metal oxides. For example, the low-temperature transport properties of doped ferroelectric BaTiO3 have been studied by only a few authors [1]. A related material, doped paraelectfic SrTiO3 which is reduced or Nb-doped, has been intensively investigated and it is well known that, by suitable doping, SrTiO3 exhibits metallic conduction [2] and becomes superconducting below 1 K [3]. To our knowledge, however, no previous high-pressure study on the conducting properties of doped BaTiO3 has been carried out. Single crystals of BaTiO3 were grown by the TSSG method at the Advanced Technology R&D Center of Fujikura Ltd. and the as-grown crystals were reduced in a hydrogen gas flow at 1300°C for 4h. By reduction treatment, the amber-colored as-grown crystals were changed to dark gray. Electrical resistivity * Corresponding author. 0921-4526/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved PII S 0 9 2 1 - 4 5 2 6 ( 9 7 ) 0 0 0 2 5 - 2
under pressure up to 8.0 GPa was measured using a cubic anvil apparatus in the temperature range between 4.2 and 295 K. Nearly hydrostatic pressure was generated in a Teflon cell filled with a pressure-transmitting medium, a 1:1 mixture of Fluorinert FC70 and C77, and the force applied to the apparatus was controlled in order to perform the measurements at constant pressure during cooling and heating processes. Ultrasonicsoldered Indium and Au paste were used as electrodes for ohmic contact. Fig. 1 shows the result of the temperature dependence of DC resistivity of BaTiO3_~ single crystal at various pressures. At ambient pressure, abrupt changes in resistivity are observed at each phase transition temperature. These abrupt changes in resistivity at phase transitions have also been observed in Nb-doped BaTiO3 single crystal, as recently reported by C. ~Gillot et al. [1]. The temperature dependence of resistivity exhibits metallic conduction in the tetragonal and orthorhombic phases, while semiconducting in the rhombohedral phase. With increasing pressure, the resisitivity decreases and the sharp changes at the phase transitions, observed at ambient pressure, become broad. Above 6.5 GPa, the semiconducting phase was completely suppressed and metallic
17
T. Nakanishi et al. / Physica B 237-238 (1997) 16-18
1.5GPa
10-1
........................~ . *
E
10"1
o
10-2
2.5GPa 3.0GPa 3.5GPa 4.0GPa
10.2 o
~ [i.i'.12"i['i"ii".~'[
& 10-3
6.0GPa 10-3
10.4
6.5GPa 7.0GPa . 8.0GPa
/ 10
100
10-4 10
100
Temperature [K] Fig. 1. Temperature dependence of electrical resistivity for hydrogen reduced BaTiO3_~ at 1 atm and at various pressures.
conduction was observed down to 4.2 K. The reproducibility of these data was confirmed by remeasuring the temperature dependence of resistivity at ambient pressure after the high-pressure measurements. The temperature dependence of resistivity, p, in the metallic phase above 6.5 GPa is re-plotted in Fig. 2 by subtracting the residual resistivity, P0. As seen in Fig. 2, the high-pressure metallic phase shows roughly P --- P0 + .4 T 2 below 100 K. Above 100 K, the temperature dependence deviates from the T2-dependence and approaches a T3-dependence, indicating optical phonon scattering is dominant. This temperature dependence of resistivity is similar to that pointed out by Baratoff and Binnig for Nb-doped SrTiO3 [4]. Additionally, our analysis of reported data for doped paraelectric SrTiO3_~ [2] also shows this type of behavior. Thus, we speculate that the pressure-induced metallic phase in BaTiO3_~ is caused by suppressing the ferroelectric phase. The coefficient of the quadratic term, A, in this metallic BaTiO3_, decreases with increasing pressure having a pressure coefficient of d l n A / P = - O . 2 3 G P a - ] . We have found a connection between this value and those o f the strongly correlated 3d transition-metal oxides for which these data are available; - 0 . 1 5 GPa-1 in Lao.95Sr0.05TiO3.04 [5], and - 0 . 2 0 GPa -1 in V203 [6]. This connection
Temperature [K] Fig. 2. Temperature dependence of resistivity for pressure-induced metallic BaTiO3_~.
10 .7
**
8
BaTiO3 s 10-s
E
"~i 10-9
/La°.95Sro-°iTiO3 °4 10 -Io
10-9
.
i
. . . . . . .
10-s
i
10-7
10-6
A [f~cm/K2] Fig. 3. Pressure derivative, dA/dP, of the coefficient of the quadratic term, A, for metallic BaTiO3_6 and some strongly correlated 3d transition-metal oxides [5, 6]. is illustrated in Fig. 3 which shows that the pressure derivative, d A / d P , of metallic BaTiO3_6 is clearly in line with these strongly correlated 3d transition-metal oxides. This fact suggests that the T2-dependence
18
T. Nakanishi et al./ Physica B 237-238 (1997) 16-18
of resistivity in metallic BaTiO3_~ originates from electron-electron correlations in the Ti 3d band. The authors are grateful to Prof. Y. Ueda of the Institute for Solid State Physics, University of Tokyo for permitting the use of a reduction-treatment apparatus. This work was supported by a Grant-in-Aid for Scientific Research from the Japanese Ministry of Education, Science and Culture.
References [1] C. Gillot et al., Solid State Commun. 84 (1992) 1033. [2] O.N. Tufte and P.W. Chapman, Phys. Rev. 155 (1967) 796. [3] J.F. Schooley, W.R. Hosler and M.L. Cohen, Phys. Rev. Lett. 12 (1964) 474. [4] A. Baratoff and G. Binnig, Physiea B 108 (1981) 1335. [5] Y. Okada et al., Phys. Rev. B 48 (1993) 9677. [6] D.B. McWhan and T.M. Rice, Phys. Rev. Lett. 22 (1969) 887.
Physica B 237-238 (1997) 16-18
Transportproperties of BaTiO3_6 under high pressure T. N a k a n i s h i a,*, N. M r r i a, ¥ . A k i s h i g e h, O. N a k a o c, A. K u r o s a k a c a Institute for Solid State Physics, University of Tokyo, Minato-ku, Tokyo 106, Japan b Faculty of Education, Shimane University, Nishikawatsu-cho, Matsue 690, Japan c Advanced Technology R&D Center, Fujikura Ltd., Koto-ku, Tokyo 135, Japan
Abstract
We have measured the electrical resistivity of doped ferroelectric single crystal BaTiO3_6 under high pressure up to 8.0 GPa in the temperature range between 4.2 and 295 K. By applying pressure above 6.5 GPa metallic conduction was observed down to 4.2K. A T2-dependence of resistivity was found in the pressure-induced metallic phase below 100K, suggesting that electrons in the Ti 3d band of BaTiO3_6 have a nature similar to strongly correlated electron systems.
Keywords: BaTiO3_6; Transport properties; Electrical resistivity; High pressure
For the last 10 years, since the discovery of high-Tc superconducting Cu-based oxides, there have been extensive studies on properties of conducting transitionmetal oxides. However, there are only a few reports on the low-temperature transport properties of doped ferroelectric oxides even though many ferroelectric materials are transition-metal oxides. For example, the low-temperature transport properties of doped ferroelectric BaTiO3 have been studied by only a few authors [1]. A related material, doped paraelectfic SrTiO3 which is reduced or Nb-doped, has been intensively investigated and it is well known that, by suitable doping, SrTiO3 exhibits metallic conduction [2] and becomes superconducting below 1 K [3]. To our knowledge, however, no previous high-pressure study on the conducting properties of doped BaTiO3 has been carried out. Single crystals of BaTiO3 were grown by the TSSG method at the Advanced Technology R&D Center of Fujikura Ltd. and the as-grown crystals were reduced in a hydrogen gas flow at 1300°C for 4h. By reduction treatment, the amber-colored as-grown crystals were changed to dark gray. Electrical resistivity * Corresponding author. 0921-4526/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved PII S 0 9 2 1 - 4 5 2 6 ( 9 7 ) 0 0 0 2 5 - 2
under pressure up to 8.0 GPa was measured using a cubic anvil apparatus in the temperature range between 4.2 and 295 K. Nearly hydrostatic pressure was generated in a Teflon cell filled with a pressure-transmitting medium, a 1:1 mixture of Fluorinert FC70 and C77, and the force applied to the apparatus was controlled in order to perform the measurements at constant pressure during cooling and heating processes. Ultrasonicsoldered Indium and Au paste were used as electrodes for ohmic contact. Fig. 1 shows the result of the temperature dependence of DC resistivity of BaTiO3_~ single crystal at various pressures. At ambient pressure, abrupt changes in resistivity are observed at each phase transition temperature. These abrupt changes in resistivity at phase transitions have also been observed in Nb-doped BaTiO3 single crystal, as recently reported by C. ~Gillot et al. [1]. The temperature dependence of resistivity exhibits metallic conduction in the tetragonal and orthorhombic phases, while semiconducting in the rhombohedral phase. With increasing pressure, the resisitivity decreases and the sharp changes at the phase transitions, observed at ambient pressure, become broad. Above 6.5 GPa, the semiconducting phase was completely suppressed and metallic
17
T. Nakanishi et al. / Physica B 237-238 (1997) 16-18
1.5GPa
10-1
........................~ . *
E
10"1
o
10-2
2.5GPa 3.0GPa 3.5GPa 4.0GPa
10.2 o
~ [i.i'.12"i['i"ii".~'[
& 10-3
6.0GPa 10-3
10.4
6.5GPa 7.0GPa . 8.0GPa
/ 10
100
10-4 10
100
Temperature [K] Fig. 1. Temperature dependence of electrical resistivity for hydrogen reduced BaTiO3_~ at 1 atm and at various pressures.
conduction was observed down to 4.2 K. The reproducibility of these data was confirmed by remeasuring the temperature dependence of resistivity at ambient pressure after the high-pressure measurements. The temperature dependence of resistivity, p, in the metallic phase above 6.5 GPa is re-plotted in Fig. 2 by subtracting the residual resistivity, P0. As seen in Fig. 2, the high-pressure metallic phase shows roughly P --- P0 + .4 T 2 below 100 K. Above 100 K, the temperature dependence deviates from the T2-dependence and approaches a T3-dependence, indicating optical phonon scattering is dominant. This temperature dependence of resistivity is similar to that pointed out by Baratoff and Binnig for Nb-doped SrTiO3 [4]. Additionally, our analysis of reported data for doped paraelectric SrTiO3_~ [2] also shows this type of behavior. Thus, we speculate that the pressure-induced metallic phase in BaTiO3_~ is caused by suppressing the ferroelectric phase. The coefficient of the quadratic term, A, in this metallic BaTiO3_, decreases with increasing pressure having a pressure coefficient of d l n A / P = - O . 2 3 G P a - ] . We have found a connection between this value and those o f the strongly correlated 3d transition-metal oxides for which these data are available; - 0 . 1 5 GPa-1 in Lao.95Sr0.05TiO3.04 [5], and - 0 . 2 0 GPa -1 in V203 [6]. This connection
Temperature [K] Fig. 2. Temperature dependence of resistivity for pressure-induced metallic BaTiO3_~.
10 .7
**
8
BaTiO3 s 10-s
E
"~i 10-9
/La°.95Sro-°iTiO3 °4 10 -Io
10-9
.
i
. . . . . . .
10-s
i
10-7
10-6
A [f~cm/K2] Fig. 3. Pressure derivative, dA/dP, of the coefficient of the quadratic term, A, for metallic BaTiO3_6 and some strongly correlated 3d transition-metal oxides [5, 6]. is illustrated in Fig. 3 which shows that the pressure derivative, d A / d P , of metallic BaTiO3_6 is clearly in line with these strongly correlated 3d transition-metal oxides. This fact suggests that the T2-dependence
18
T. Nakanishi et al./ Physica B 237-238 (1997) 16-18
of resistivity in metallic BaTiO3_~ originates from electron-electron correlations in the Ti 3d band. The authors are grateful to Prof. Y. Ueda of the Institute for Solid State Physics, University of Tokyo for permitting the use of a reduction-treatment apparatus. This work was supported by a Grant-in-Aid for Scientific Research from the Japanese Ministry of Education, Science and Culture.
References [1] C. Gillot et al., Solid State Commun. 84 (1992) 1033. [2] O.N. Tufte and P.W. Chapman, Phys. Rev. 155 (1967) 796. [3] J.F. Schooley, W.R. Hosler and M.L. Cohen, Phys. Rev. Lett. 12 (1964) 474. [4] A. Baratoff and G. Binnig, Physiea B 108 (1981) 1335. [5] Y. Okada et al., Phys. Rev. B 48 (1993) 9677. [6] D.B. McWhan and T.M. Rice, Phys. Rev. Lett. 22 (1969) 887.