General expression for effective diffusivity of foreign atoms migrating via a fast intermediate

In many solids, diffusion of foreign atoms takes place primarily through highly mobile intermediate species that periodically exchange with atoms in the crystalline lattice. The governing reaction-diffusion equations include a diffusion coefficient as well as kinetic parameters describing the exchange of the intermediate species. Yet it is often convenient to model a diffusive process in terms of a single parameter, no matter what the time regime. This communication derives for a delta function initial profile an exact expression for the effective diffusivity that is valid in all time regimes. In the case of semiconductor solids, such an expression can be helpful in the interpretation of dopant diffusion measurements.