The calculation of the chemical diffusion coefficient for p-type oxides doped with aliovalent impurities

Abstract

The definition of the chemical diffusion coefficients, obtained by measunng either changes of weight or of electrical conductivity for a pure oxide, is extended to compounds of the same class containing small concentrations of foreign cations having a valency different from that of the ions of the base oxide For simplicity, the treatment is restricted to oxides of type AO showing p-type semiconductivity and containing defects only in the cation subtattice It is shown that in doped oxides the chemical diffusion coefficients are functions not only of the oxygen activity but also of the concentration of the impurity as well as of the ratio between the gradients of these two vanables The chemical diffusion coefficients corresponding to the exclusive presence of gradients of oxygen activity are considered in particular, but the complications ansing in case of different mobilities of the base ions and of the impurity are also pointed out Numerical calculations of the chemical diffusion coefficients are earned out for doped NiO at 1000°C using a convenient model for the defect structure of this oxide, and showing that the addition of a monovalent impurity should produce a slight decrease of the chemical diffusion coefficients while that of a trivalent impurity should produce a large increase of this parameter Finally the theoretical predictions are compared with the few experimental results available so far and possible reasons for the discrepancies observed in some cases are examined.