It was recalled that the non-stoichiometry of the main structural phase of rutile and its natural n-type conductivity was due to oxygen deficiency, which was explained in terms of native defects, i.e., oxygen vacancies and titanium interstitials. While the latter was unambiguously identified by electron paramagnetic resonance and attributed to a shallow donor, the oxygen vacancy lacks such identification, in fact, any identification at all. In this work, two charge states of the oxygen vacancy, i.e., the singly VO+ and doubly VO0 occupied charge states with S = 0.5 and S = 1, respectively, were identified by electron paramagnetic resonance under blue light excitation in fully oxidized rutile. It was shown that the vacancy was a very shallow donor (2.8meV) and that both charge states suffered different strong lattice relaxations depending upon charge.

Identification of Two Light-Induced Charge States of the Oxygen Vacancy in Single-Crystalline Rutile TiO2. Brandão, F.D., Pinheiro, M.V.B., Ribeiro, G.M., Medeiros-Ribeiro, G., Krambrock, K.: Physical Review B, 2009, 80[23], 235204