The electronic structure of an isolated O vacancy in SrTiO3 was investigated with a variety of ab initio quantum mechanical approaches. In particular, pure density functional theory approaches were compared with the Hartree-Fock method, and with hybrid methods where the exchange term was treated in a mixed way. Both local cluster models and periodic calculations with large super-cells containing up to 80 atoms were performed. Both diamagnetic (singlet state) and paramagnetic (triplet state) solutions were considered. It was found that the formation of an O vacancy was accompanied by the transfer of 2 electrons to the 3d(z2) orbitals of the two Ti atoms along the Ti-V-Ti axis. The two electrons were spin coupled and the ground state was diamagnetic. New states associated with the defect center appear in the gap just below the conduction band edge. The formation energy computed with respect to an isolated O atom in the triplet state was 9.4eV.

Electronic Structure of a Neutral Oxygen Vacancy in SrTiO3. D.Ricci, G.Bano, G.Pacchioni, F.Illas: Physical Review B, 2003, 68[22], 224105