Defect energies for magnesium oxide and lithium oxide

Abstract

A new crystal potential has been developed for magnesium oxide. This is based on electron-gas calculations of the interaction energy for like and unlike pairs of ions, in the presence of a model crystal field. The calculated potentials are checked by calculating perfect lattice properties: cohesive energy, elastic constants, dielectric constants and some phonon frequencies; adjustments of potential parameters are made to ensure the best fit to these properties. Damping of the long-range dispersion energy is introduced during fitting of the potential and in the calculation of perfect lattice and defect properties.

Similar methods have been used to develop a potential for lithium oxide. The calculated phonon dispersion in Li₂O is in good agreement with experimental results from neutron scattering. The calculated defect energies are compared with experimental values from ionic conductivity and NMR measurements. Mg²⁺ substitutionals and Li⁺ vacancies exhibit a high degree of association, which is responsible for the curvature in the extrinsic region of the conductivity plot.