It was noted that many transition-metal monoxides with a face-centered cubic rock-salt structure exhibited a relatively high fraction of vacancies. In particular, the most stable structure of stoichiometric titanium monoxide corresponded to a monoclinic phase, α-TiO, with 15% of vacancies. The role played by such vacancies in the stability of the solid had been the subject of speculation for some 30 years. Here, a theoretical study of the electronic structure of α-TiO was based upon generalized gradient approximation density functional calculations. Analysis of the electron distribution changes induced by the creation of defects in the ideal rock-salt structure permitted clarification of the significant role played by O and Ti ordered vacancies; that worked together in stabilizing the material. Stabilization of the monoclinic phase, with respect to the cubic phase, involved noticeable re-polarization of the Ti 3d orbitals that simultaneously contributed to 2 cooperative mechanisms. These were: enhancement of the Ti-Ti bonding interactions through and around the O vacancies, and electrostatic stabilization which resulted from electron density accumulation in an O vacancy (cation environment) together with electron density depletion in a Ti vacancy (anionic environment).

Role of Vacancies in the Structural Stability of α-TiO - a First-Principles Study Based on Density-Functional Calculations. Graciani, J., Márquez, A., Sanz, J.F.: Physical Review B, 2005, 72[5], 054117