Calculations of off-centre displacements of divalent substitutional ions in CaO, SrO and BaO from model potentials

The problems of representing the polarizability of oxygen in potential models for oxides are discussed. Simple shell-model interionic potentials for MgO, CaO, SrO, BaO, MnO, FeO, CoO and NiO are proposed and the parameter fitted to the dielectric constants and lattice spacings. The oxygen polarizability and interactions are taken to be the same in all cases. Model predictions of cohesive energies, elastic constants and phonon dispersion are tested against experiment and found to be reasonably satisfactory for such an oversimplified model. Defect energies are calculated for Mg 2+ , Mn 2+ , Fe 2+ , Co 2+ and Ni 2+ substitutional impurities in CaO, SrO and BaO host crystals. Off-centre minima displaced in ⟨111⟩ directions are found in all cases in SrO and BaO, although SrO: Mn 2+ is marginal: in CaO all the impurities studied remain on-centre. For systems which have been investigated experimentally these predictions show good agreement.