Local Defect Equilibria: The Conceptual Difficulties in Treating Solid State Reaction Kinetics

For an AO — BO interdiffusion process a formal analysis is given. The conceptual difficulties in using either structural elements, building units or components in the framework of irreversible thermodynamics are discussed. The matrix of transport coefficients of a n‐component system is evaluated and is transformed into a matrix of diffusivities. The interdiffusion problem has been solved numerically, starting with a set of two coupled diffusion equations. Point defect relaxation during interdiffusion has been taken into account through a quasi‐homogeneous point defect relaxation term. The calculated supersaturation and undersaturation of point defects result in concentration profiles which cannot be normalized with √ t . Experiments with the diffusion couple FeO — MnO demonstrate the theoretically predicted behavior and the influence of the cation vacancy relaxation at the sample surface on the concentration profiles.