Measures of ionicity of alkaline-earth oxides from the analysis ofab initiocluster wave functions

We present an analysis of the M-O chemical bonding in the binary oxides MgO, CaO, SrO, BaO, and Al₂{O}₃$ based on ab initio wave functions. The model used to represent the local environment of a metal cation in the bulk oxide is an M${O}₆$ cluster which also includes the effect of the lattice Madelung potential. The analysis of the wave functions for these clusters leads to the conclusion that all the alkaline-earth oxides must be regarded as highly ionic oxides; however, the ionic character of the oxides decreases as one goes from MgO, almost perfectly ionic, to BaO. In Al₂{O}₃$ the ionic character is further reduced; however, even in this case, the departure from the ideal, fully ionic, model of ${Al}³⁺$ is not exceptionally large. These conclusions are based on three measures, a decomposition of the ${M}q+$-${O}^{q{{-}}}$ interaction energy, the number of electrons associated to the oxygen ions as obtained from a projection operator technique, and the analysis of the cation core-level binding energies. The increasing covalent character along the series MgO, CaO, SrO, and BaO is discussed in view of the existing theoretical models and experimental data.