Impurities in a nominally undoped c-domain monocrystal were studied by means of electron spin resonance techniques at temperatures ranging from 4.2 to 700K, before and after illumination. It was found that several 3d-metal paramagnetic impurities (Mn4+, Fe3+ and Ni3+), substituted for Ti4+. Light-induced centers (Pt3+, Ti3+-Vo-A[Pb] and Ti3+-A[Pb]) were also identified. A comparison of the observed temperature dependence of the axial crystal field constant, with calculations that were based upon a Newman model, showed that Mn4+ occupied an off-center position in the O cage, in a manner similar to that of Ti4+, while Fe3+ remained centered at any temperature, with its excess charge being compensated within more distant shells. The temperature dependence of the Ti4+ ion displacements which were deduced from electron spin resonance data was in good agreement with neutron diffraction measurements, but did not confirm X-ray absorption fine-structure data on the Ti4+ off-center position. The Ni3+ center and the light-induced Ti centers had not been previously considered. The Ti3+-Vo-A[Pb] center was imagined to be an electron on a Ti ion nearest to the O vacancy, and a defect on a neighboring Pb site (FA center model). It was concluded that Pt3+ and Ti3+-Vo-A[Pb] were shallow donors.

V.V.Laguta, M.D.Glinchuk, I.P.Bykov, J.L.Maksimenko, J.Rosa, L.Jastrabík: Physical Review B, 1996, 54[17], 12353-60