Defect energies for pure corundum and for corundum doped with transition metal ions

Defect energies have been calculated for pure corundum and for corundum doped with Mg, Mn, Fe, Co, Ni and Ti using the Mott-Littleton procedure as embodied in the CASCADE code. The sensitivity of the results to the potential and the need to allow for relaxation of the perfect crystal either by relaxing basis strains before energy minimization or by a correction procedure are discussed. Formation energies indicate that both Schottky defects and anion Frenkel defects are to be expected in the intrinsic material. The cation-doped crystals display defect clustering and this facilitates solution of impurities. Activation energies for anion diffusion are in good agreement with experiment, but it seems to be impossible to rationalize all the experimental transport data with the calculations.