Interdiffusion in (A,B)O‐type Solid Solutions and the Validity of Darken′s Equation

Relations between tracer diffusion coefficients and interdiffusion coefficients for quasibinary oxide solid solutions are derived. It is demonstrated that under certain conditions Darken‐type equations are obtained. By inserting experimentally determined interdiffusion coefficients and tracer diffusion coefficients in the Darken relation for (Co,Mg)O and (Co,Ni)O at 1300°C in air it is shown that the Darken‐type equation cannot be employed for interdiffusion in these systems. This is attributed to deviations from local point defect equilibrium in the diffusion couple, which is contrary to a major assumption in the derivation. Experimentally determined interdiffusion coefficients and tracer diffusion coefficients for Co‐cations are reported for various compositions in (Co,Mg)O at 1300°C in air.