Comments on the defect chemistry of undoped and acceptor-doped BaTiO3

In a recent study of the defect chemistry of undoped polycrystalline and acceptor-doped single crystalline BaTiO3, an attempt was made to obtain defect and thermodynamic parameters without any a priori assumptions, e.g. about the magnitude and temperature dependence of the carrier mobilities. The parameters found for the undoped sample differ significantly from those obtained from the acceptor-doped sample, and even more substantially from those reported from several earlier studies. It is suggested that these discrepancies result from the lack of sufficient data in certain critical regions of temperature and oxygen activity. As a result, the attempt to extract parameters from the equilibrium conductivity by multiparameter curve-fitting has considerable uncertainty. However, the electron and hole mobility products and ratios, mu(n)mu(p) and mu(n)/mu(p), were determined by linear extrapolation of the well-established log-log slopes of +1/4 and -1/4 for the equilibrium conductivities and oxygen diffusivities to their intersection. Both the products and the ratios were found to be independent of temperature, indicating that both mobilities are also independent of temperature. This important conclusion validates the thermodynamic parameters obtained in earlier studies in which it was assumed that the mobilities are temperature independent or nearly so, and that the carrier concentrations are proportional to the conductivities.