The defect chemistry of rutile was studied by using computer simulation techniques. The stable valences of dopant transition metal ions were calculated to be Nb(V), V(III), Fe(III) and Cr(III). It was found that Cr3+, Al3+, Ga3+, V3+ and Fe3+ dissolved preferentially at substitutional sites, with the charge being compensated by O vacancies rather than Ti4+ interstitials. Both were detected experimentally. It was suggested that Nb5+ dissolved preferentially at substitutional sites, with charge compensation being facilitated by Ti(III) ions or Ti vacancies. The solution enthalpy of Nb5+ was reduced upon introducing Al3+ into the lattice. The solution enthalpy was further reduced in the case of bound Al3+/Nb5+ clusters.

Computer Simulation Study of the Defect Chemistry of Rutile TiO2. Sayle, D.C., Catlow, C.R.A., Perrin, M.A., Nortier, P.: Journal of the Physics and Chemistry of Solids, 1995, 56[6], 799-805