Energetics of aluminum vacancies in gamma alumina

We present results for total-energy simulations of the energetics of vacancy occupation of \ensuremath{\gamma} alumina using an empirical potential that allows the ionic valence to vary according to the local environment and includes Coulombic electrostatic interactions among the anions and cations. Our model predicts that although the minimum enthalpy configuration has all aluminum vacancies from the spinel structure in octahedral sites, the energetic cost of a modest fraction of tetrahedral sites is small. We furthermore use our model to explain the differences between previously reported simulations on this question in terms of their constraints on structural and charge relaxation.