DEFECT STRUCTURE, ELECTRICAL-PROPERTIES AND TRANSPORT IN BARIUM-TITANATE .4. THERMOPOWER OF POLYCRYSTALLINE BATIO3 WITHIN THE N-P TRANSITION RANGE

Thermopower (S) was measured for undoped polycrystalline BaTiO3 in the temperature range 1023-1253 K over an oxygen partial pressure range o f 10(2)-10(5) Pa. In this region, BaTiO3 exhibits p-type properties wi th an influence of the conductivity component resulting from electrons . Experimental data were analysed in terms of both the band model and the small polaron model, resulting in the determination of the Fermi e nergy level of BaTiO3 ceramics. It is shown that the thermopower data for polycrystalline materials in the n-p transition range involve a su bstantial grain boundary effect, which results in the correction facto r f = S(theor)/S(exp). This factor is independent of temperature. In t he case of the studied oxide specimen the correction factor assumes 1. 93 and 1.71 for the band model and the small polaron model, respective ly. It is demonstrated that, in the n-p transition range, measured the rmopower has a complex physical meaning and involves several quantitie s such as mobilities, concentrations, density of states and kinetic te rms of both charge carriers. In contrast to thermopower the Fermi ener gy is the parameter which, even in the n-p transition regime, can be d irectly related to the concentration of electronic carriers and, conse quently, to the concentration of charged ionic defects. Therefore, the exponent of the p(O2) dependence of the Fermi energy (n(F)) may be us ed in comparative studies with other electrical properties such as wor k function. It has been shown that the absolute values of the p(O2) ex ponent related to S and E(F) (n(S) and n(F), respectively) are essenti ally the same only when one type of charge carrier is predominant. In the n-p transition regime these two parameters differ considerably.