Chemical transport in mixed conductors: Application to the model materials SrTiO3 and ZrO2

The effect of internal ionisation equilibria of dopants on chemical diffusion is discussed in detail and applied to the cases of SrTiO3 and yttria-stabilised ZrO2. In particular, the sensitive dependence of the diffusion coefficient on the position of the impurity level is highlighted. Two major cases are worth distinguishing: One in which the redox-active dopant is in a minority and a second one in which it plays a major role in the equation of electroneutrality. In contrast to the equilibrium case, the concentration of redox-active dopants is of pronounced importance for chemical diffusion in both cases. The significance for applications is also discussed.