Defect structure, electrical properties and transport in Barium Titanate. IV. Thermopower of polycrystalline BaTiO 3 within the n-p transition range

Abstract

Thermopower ( S) was measured for undoped polycrystalline BaTiO 3 in the temperature range 1023–1253 K over an oxygen partial pressure range of 10 2–10 5 Pa. In this region, BaTiO 3 exhibits p-type properties with 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 energy level of BaTiO 3 ceramics. It is shown that the thermopower data for polycrystalline materials in the n-p transition range involve a substantial grain boundary effect, which results in the correction factor ƒ = S theor S exp . This factor is dependent of temperature. In the case of the studied oxide specimen the correction factor assumes 1·93 and 1·71 for the band model and the small polaron model, respectively. It is demonstrated that, in the n-p transition range, measured thermopower has a complex physical meaning and involves several quantities such as mobilities, concentrations, density of states and kinetic terms of both charge carriers. In contrast to thermopower the Fermi energy is the parameter which, even in the n-p transition regime, can be directly related to the concentration of electronic carriers and, consequently, to the concentration of charged ionic defects. Therefore, the exponent of the p(O 2) dependence of the Fermi energy ( n F) may be used in comparative studies with other electrical properties such as work function. It has been shown that the absolute values of the p(O 2) exponent related to S and E F ( n S and n F, respectively) are essentially the same only when one type of charge carrier is predominant. In the n-p transition regime these two parameters differ considerably.