PO2 dependence of valence number of Co in LaGaO3 and its influence on partial electronic and oxide ionic conductivity

Partial electronic and oxide ionic conductivity in Co doped LaGaO3 was investigated with a polarization method and it was seen that the partial electronic conductivity became significant with increasing temperature. PO2 dependences of the partial electron and hole conductivities at 1173 K are almost proportional to PO2−1/4 and PO21/4, respectively, in accordance to Wagnerʹs theory. On the other hand, PO2 dependences of electron and hole conductivities deviate from those of Wagnerʹs theory and became much smaller as temperature decrease. The small PO2 dependence of the partial electronic conductivity could be explained by extrinsic electrons or holes originating from oxidation or reduction of cobalt. In order to explain the small PO2 dependence, average valence numbers of Co were measured by redox titration. It was seen that the PO2 dependence of the Co valence number is similar to those of partial electron and hole conduction. Consequently, the small PO2 dependence of partial electronic conduction could be assigned to the small PO2 dependence of the valence number of Co.