The energies of formation of Schottky and Frenkel defects in corundum were calculated, as were the activation energies for cation and anion vacancy hopping, by using the atom-atom potential method which had been formulated by Mott and Littleton. Particular attention was paid to the ability of 2-body potentials to describe the crystal structure and properties, and to the need to allow for the energy change which was associated with the relaxation of basic strains. Several mechanisms of O vacancy hopping were also simulated by using the quantum chemical INDO method, which took account of the covalent contribution of chemical bonding in corundum. The calculations revealed an insignificant increase in the effective charge of the hopping O atom at its saddle-point. This justified the use of atom-atom potentials in this context. The INDO activation energies for vacancy hops revealed the same trend as that found using other computer calculations. Their points of disagreement indicated a failure to optimize ionic displacements completely in the INDO calculations. In accord with other computer predictions, these migration energies were much lower when the hopping ion was allowed to deviate (by 0.03 to 0.04nm) from a straight path. Both types of calculation predicted that the activation energy for vacancy hopping between basic structural O triangles was the factor which limited O migration.

P.W.M.Jacobs, E.A.Kotomin: Journal of Solid State Chemistry, 1993, 106[1], 27-34