The formation energy, geometry, and electronic structure of isolated O and Al vacancies in the bulk, and on the (00▪1) surface, of corundum (α-Al2O3) were investigated by means of periodic calculations performed within the framework of density functional theory, the generalized gradient approximation and large super-cells. The energy cost required to form an O vacancy in the bulk was estimated to be of the order of 10eV, whereas that corresponding to the formation of Al vacancies was found to be at least a 30% larger. The relaxation of the material was rather small for both defects. The removal of an O atom in bulk α-Al2O3 was accompanied by the appearance of an impurity level in the gap, which was a strong indication of electron localization. This was further confirmed by integration of the density of states in the energy interval corresponding to the impurity level and by other theoretical analyses. On the α-Al2O3 (00▪1) surface, the formation of O and Al vacancies exhibited many similarities with the bulk. The energy cost required to form Al vacancies was much larger than that for O vacancies and, in both cases, it was accompanied by rather small atomic displacements. However, there were also significant differences between bulk and surface O and Al vacancies. Thus, the formation energy of one of these point defects in the surface was rather smaller than expected.

Theoretical Study of Bulk and Surface Oxygen and Aluminum Vacancies in α-Al2O3. J.Carrasco, J.R.B.Gomes, F.Illas: Physical Review B, 2004, 69[6], 064116 (13pp)