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 be en used to study the electron-excess gap states in rutile TiO2 which r esult from the formation of oxygen vacancies in the bulk and ar the su rface of an isolated slab of infinite {110} planes. These states are f ound to be insulating and occur at similar to 1-2 eV above the O 2p ba nd 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 essen tially spin-paired. On the other hand, gap states associated with thes e defects are largely spin-polarised states corresponding to excess el ectron 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 re distribution is essentially at the surface Ti atoms. For the oxygen-de ficient {110} surface with an entire plane of missing oxygen atoms, an antiferromagnetic spin arrangement is found consisting of localised s pins on the surface titanium atoms. (C) 1997 Elsevier Science B.V.