The Cohesion of Solid Magnesium and Calcium Oxide: The Role of in-Crystal Modification of the Oxide ion and Electron Correlation

The cohesive properties of solid MgO and CaO are investigated using the fully ionic description calculating exactly, after generating the electronic wavefunctions of the ions, those portions of each inter-ionic potential energy function that do not arise from electron correlation. Three major new refinements are introduced. The modifications of each anion wavefunction that arise from the crystalline environment are described using two new methods in which the environmentally generated contribution to the potential energy acting on an anion electron is related to the cation electron density. These methods improve substantially on earlier models, shown to be inadequate for solid oxides, in which the environmental potential energy has the form of that generated by a spherical shell of charge. Density functional theory is used to evaluate the contribution of electron correlation to the oxide-ion rearrangement energy, that is the energy required to convert a free O^- ion to an in-crystal O^2- ion. For any oxide, this correlation term varies appreciably with crystal geometry, as well as differing significantly between different oxides at their equilibrium geometries. The present models for the in-crystal environment of the oxide ion require a new way of deriving the parameters that govern the damping of the dispersive attractions between the ions, this damping originating from ion-wavefunction overlap being too large to neglect. The lattice energies, closest equilibrium cation-anion separations and bulk compressibilities, predicted for both MgO and CaO using all of the improvements presented here, agree very well with experiment. There is no evidence for any significant covalent contribution to the cohesion of these two oxides.