An ab initio Hartree-Fock study of the electron-excess gap states in oxygen-deficient rutile TiO2

Ab initio spin-unrestricted periodic Hartree-Fock calculations have been used to study the electron-excess gap states in rutile TiO2 which result from the formation of oxygen vacancies in the bulk and ar the surface of an isolated slab of infinite {110} planes. These states are found to be insulating and occur at similar to 1-2 eV above the O 2p band edge. It is shown that while the O 2p band undergoes small changes in both width and shape as a result of these defects. it remains essentially spin-paired. On the other hand, gap states associated with these defects are largely spin-polarised states corresponding to excess electron density trapped at bulk vacancies and local Ti 3d(1) and Ti 3d(2) configurations in both the bulk and at the surface. In the bulk the lowest energy slate is one in which the majority of the excess charge is localised at the vacant oxygen site, whereas al the surface the redistribution is essentially at the surface Ti atoms. For the oxygen-deficient {110} surface with an entire plane of missing oxygen atoms, an antiferromagnetic spin arrangement is found consisting of localised spins on the surface titanium atoms. (C) 1997 Elsevier Science B.V.