Charged oxygen interstitials in corundum: first principles simulations

Combining supercell models and hybrid B3PW exchange-correlation functionals, ab initio simulations on quasi-stable configurations of interstitial O_i ^- ions in α-Al₂O₃ (corundum) crystals and possible migration trajectories have been modelled. We have studied crystalline distortion around migrating O_i ^- including interatomic distances and the effective atomic charges, as well as redistributions of the electronic density. Unlike neutral interstitial atom O_i studied by us previously, migrating O_i ^- ion does not form dumbbells with the nearest regular O_reg ^q- oxygen ions, due to the strong Coulomb interaction with the nearest Al_{reg q+} cations as well as stronger repulsion between O_i ^- and adjacent regular O_reg ^q- ions. We have also estimated the energy barrier for migration between the nearest quasistable configurations of interstitial O_i ^- ion. One of these configurations is an octahedron formed by six nearest O_reg ^q- anions in the centre of which O_i ^- ion is located, unlike a neutral O_i atom which prefers to migrate directly towards one of adjacent O_reg ^q- anions with the dumbbell formation. As the result, the barrier for O_i ^- ion migration in corundum has been found to be by ∼70% smaller than that of O_i atom.