Modeling of iron diffusion in the iron oxides magnetite and hematite with variable stoichiometry

The vacancy model of diffusion is applied to magnetite and hematite, and mathematical expressions for the iron flux in the lattice-fixed frame of reference, as a function of the defect structure, are presented. The defect structures, i.e. the vacancy content on the different type of sites, and the thermodynamic factors are calculated from the available Calphad type of thermodynamic descriptions for the oxides. Expressions for Fe tracer diffusion coefficients are derived and the relations between mobility and tracer diffusivity are given. The mobilities are fitted by a least-squares optimization to experimental data on tracer diffusion from the literature. For magnetite, an excellent representation of the experimental tracer data is achieved together with a satisfactory description of the sparse chemical diffusion data available. For hematite, the experimental scatter is very large and anomalous large frequency factors and activation energies have been reported. In the present report a compromise is suggested.