DEFECT STRUCTURE, ELECTRICAL-PROPERTIES AND TRANSPORT IN BARIUM-TITANATE .3. ELECTRICAL-CONDUCTIVITY, THERMOPOWER AND TRANSPORT IN SINGLE-CRYSTALLINE BATIO3

Both electrical conductivity and thermopower were studied for undoped BaTiO3 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(O2) determined from electrical conductivity (n(sigma)) and thermopower (n(s)) were determined. The parameters n(sigma) and n(s) assume about -4 and -4.1 for the n-type regime. Both n(sigma) 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 predominant 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 BaTiO3 is the following function of temperature: E(g) (T) = E0 - betaT where E0 = 2.9 eV. The parameter beta assumes 3.2 x 10(-5) and 2.47 x 10(-4) for the hopping and the band models, respectively.