Point defects and transport in non-stoichiometric oxides: Solved and unsolved problems

First, the basic concepts used in the area of defects and transport in non-stoichiometric oxides are reviewed. Their capabilities and shortcomings are addressed, including the problems of dealing appropriately with point defects at higher defect concentrations and with situations where more than one sublattice is available for one type of ion. As an example of where ideal solution point defect thermodynamics can be applied without much problems, the iron oxide magnetite, Fe3 − δO4, is discussed. The cation diffusion in this material is also reviewed. Then some magnetite-based solid solutions are briefly discussed with regard to their defect chemistry and the transport of cations. Finally, two largely ignored topics are addressed: (i) the influence of the valence state of an ion on its diffusivity, (ii) contributions from near-boundary regions to the variation of the oxygen content of non stoichiometric oxides.