Possible nature of the red luminescence in incipient ferroelectric KTaO3

The red luminescence at 686 nm in KTaO3 is investigated in pure as-grown an d doped (Al, Ba, Cr, Fe, Er, Li, Mg, Ni, Rh, V, Zr) crystals from different growth laboratories. In contrast to earlier results [1,2] which led to the conclusion that the main center should be an intrinsic defect Ta3+ + O2- v acancy, we found that the red luminescence to be sample dependent, together with a dependence on growth procedure. Moreover a strong dependence on any kind of heat treatment (oxidation or reduction) leading to a decreasing or vanishing of the red luminescence after above treatments, was found. There fore excited metastable defect states have to be discussed for the origin o f the red luminescence. The (i) two hole polarons localized near a Ta3+, (i i) Ta3+-O-0-center (a vibronic exciton with double charge transfer) and at last (iii) a pair of Ta3+, one in a Ta5+ site and the other in a K+ site ar e discussed as possible centers connected with the red luminescence.