The influence of titanium and oxygen vacancies on the chemical bonding in titanium oxide

The three TiO phases, alpha -, beta - and gamma -TiO, with stoichiometric composition, contain about 15% vacancies on both the Ti and the O sublattices. The low-temperature modification alpha -TiO, which is stable below 940 degrees C, crystallises in a monoclinic structure corresponding to the formal composition Ti0.833O0.833. In order to study the influence of the Ti and O vacancies on the chemical bonding in TiO, the related NbO structure, of formal composition Ti0.75O0.75 (Ti3O3), has been chosen as a model for the much more complicated real structure of TiO. This choice is based on the assumption that the model shows the essential effects with less computational effort. A self-consistent LAPW band structure calculation has been performed for the hypothetical TiO having the NbO structure. The calculated density of states and the local partial densities of states are compared with the corresponding data for stoichiometric TiO containing no vacancies, and for a model structure of TiO0.75 containing ordered O vacancies. A characteristic feature in Ti3O3 is the occurrence of an additional band caused by the O vacancies and of three further bands with predominant d character, in the energy region between the 'p band' and the 'd band' of TiO. This effect leads to the disappearance of the band gap between the 'p band' and the 'd band' existing in stoichiometric TiO.