Reaction fronts formation during oxygen chemical diffusion in oxides

Abstract

We present a theoretical study of the impact of oxygen diffusion in oxide crystals on the dynamic changes of metal dopants in the ionic state, and on the crystal conduction type. The analysis invokes ambipolar diffusion for all participating species: oxygen vacancies, metal dopant, holes and electrons. The species dynamics are demonstrated for an oxide of 8eV energy gap, electron and hole diffusivities being 105 times larger than the oxygen vacancy one, and the dopant energy level being 1.0eV above or 1.0eV below the mid-gap. We obtain linkage between the oxygen chemical diffusivity and the occurrence of a reaction front. The necessary condition is the existence of a sufficiently long interval, where a power dependence of the oxygen chemical diffusivity on the ambient oxygen pressure exists. In our demonstrated system, prominent reaction fronts occur at very low and at very high ambient oxygen pressures. The fronts relate to the reversible MMe×+e′→←MMe′ reaction at the low pressures, and to the reversible MMe′+h•→←MMe× reaction at high pressures, where M marks the dopant metal, and Me marks an oxide lattice constituent metal. In the mid-pressure interval, the oxygen chemical diffusivity is approximately constant, being several orders of magnitude smaller than the oxygen vacancy diffusivity.