Can corundum be described as an ionic oxide?

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Abstract
The electronic structure of bulk corundum(α‐Al2O3) has been studied by using ab initio cluster model Hartree–Fock self‐consistent‐field (SCF) and configuration interaction (CI) wave functions. An array of optimized point charges surrounding the cluster model is used to represent the Madelung field. Different analyses of the energetic chemical bond contributions and of the wave functions have been performed. These include constrained space orbital variations (CSOV), use of projection operator techniques, the decomposition on the ground state SCF wave function into ionic and charge transfer components, and finally, the effect of the electronic correlation. These analyses enabled us to determine the relationship between each chemical contribution to the energy stabilization of corundum and its effect on the corresponding wave function. All these different analyses are consistent and indicate that the oxygen–aluminium bond is highly ionic in nature, with a net atomic charge for Al not less than +2.7 e.