A theoretical investigation was made of how the formation of oxygen vacancies and the addition of niobium and chromium atoms as dopants modified the varistor properties of TiO2. The calculations were carried out at the HF level using a contracted basis set, developed by Huzinaga et al., to represent the atomic centers on the (110) surface for the large (TiO2)15 cluster model. The change of the values for the net atomic charges and band gap after oxygen vacancy formation and the presence of dopants in the lattice were analyzed and discussed. It was shown that the formation of oxygen vacancies decreased the band-gap while an opposite effect was found when dopants were located in the reduced surface. The theoretical results were compared with available experimental data. A plausible explanation of the varistor behavior of this system was proposed.

An ab initio Study of Oxygen Vacancies and Doping Process of Nb and Cr Atoms on TiO2 (110) Surface Models. Sambrano, J.R., Andrés, J., Beltrán, A., Sensato, F.R., Leite, E.R., Stamato, F.M.L.G., Longo, E.: International Journal of Quantum Chemistry, 1997, 65[5], 625-31