Defect structure, electrical properties and transport in barium titanate. III. Electrical conductivity, thermopower and transport in single crystalline BaTiO 3

Abstract

Both electrical conductivity and thermopower were studied for undoped BaTiO 3 single crystal in the temperature range 1090–1310 K under controlled gas atmosphere corresponding to the n-p type transition range. Reciprocals of the exponent of p(O 2) determined from electrical conductivity ( n δ ) and thermopower ( n s ) were determined. The parameters n δ and n s assume about −4 and −4·1 for the n-type regime. Both n δ and n s assume about 5·3 in the p-type regime. These values were interpreted in terms of the defect models involving oxygen vacancies as pre-dominant defects in the n-type regime and Schottky-type defects in the p-type regime. The mobility ratio of electronic carriers was determined. Experimental data indicate that the small polaron (hopping mechanism) model rather than the band model is valid in the n-type regime. Present experimental data do not allow a conclusion on the transport mechanism in the p-type regime. It was shown that the width of the forbidden gap for undoped BaTiO 3 is the following function of temperature: E g(T) = E 0−βT where E 0 = 2·9 eV. The parameters β assumes 3·2 × 10 −5 and 2·47 × 10 −4 for the hopping and the band models, respectively.