Anatase single crystals were grown by means of chemical vapour transport reaction. As-grown crystals of a blue colour were heat-treated in an O atmosphere. The optical absorption spectra indicated a change in colour from blue, via yellow, to colourless; depending upon the O annealing treatment. The blue colour was attributed to free carrier absorption. The yellow colour arose from an O vacancy which was a colour centre.

Absorption Spectra of Anatase Single Crystals Heat-Treated under Oxygen Atmosphere. Sekiya, T., Ichimura, K., Igarashi, M., Kurita, S.: Journal of the Physics and Chemistry of Solids, 2000, 61[8], 1237-42