Role of vacancies in the structural stability of α-TiO: a first-principles study based on density-functional calculations

Abstract

Many transition-metal monoxides formally having an fcc rock-salt structure exhibit a relatively high degree of vacancies, in particular, the most stable structure of stoichiometric titanium monoxide corresponds to a monoclinic phase, α-TiO, showing 15% vacancies. The role of such vacancies on the stability of the solid has been the subject of speculations for the last 30 years. We report in this work a theoretical study of the electronic structure of α-TiO based on generalized gradient approximation density functional calculations. Analysis of electron distribution changes induced by the creation of defects on the ideal rock-salt structure allows us to clarify the significant function played by both O and Ti ordered vacancies that work together on stabilization of the material. Stabilization of the monoclinic phase with respect to the cubic one involves a noticeable repolarization of the Ti 3d orbitals that simultaneously contribute to two cooperative mechanisms, (i) enhancement of the Ti-Ti bonding interactions through and around the oxygen vacancies, and (ii) electrostatic stabilization resulting from electron density accumulation in a oxygen vacancy (cation environment) together with electron density depletion in titanium vacancy (anionic environment)