Two types of Ti3+-related center were identified by means of electron paramagnetic resonance. These were attributed to Ti3+-VO, and to Ti3+-VO which was associated with Na or K, respectively. This conclusion was based upon the chemical and illumination conditions which led to the detection of the defects. It was also supported by the Ti hyperfine structure which was resolved for Ti3+-VO-Na(K). The symmetry of the centers was broken, in that an electron was localized near to one of the 2 equivalent Ti4+ ions next to VO. The observed Ti hyperfine structure also reflected electron capture at one Ti site. The VO-type centers were compared with isolated Ti3+, representing a free small polaron in crystals that contained Nb5+ and no O vacancies. In crystals that were doped with Na or K, an optical absorption precursor of the fundamental absorption was identified and was attributed to the creation of an exciton which was bound to a VO-Na(K) complex.

R.Scharfschwerdt, A.Mazur, O.F.Schirmer, H.Hesse, S.Mendricks: Physical Review B, 1996, 54[21], 15284-90