A red luminescence at 686nm was investigated in pure as-grown, ultra-high purity and doped (Al, Ba, Cr, Fe, Er, Li, Mg, Ni, Rh, V, Cr, Zr) crystals from various sources. Unlike previous work, which had concluded that the main centre was an intrinsic Ta3+ + O2- vacancy, a marked dependence upon the heat treatment was found which led to a decrease or disappearance of the red luminescence after such treatments. The red luminescence was sample-dependent, and also depended upon the growth procedure. Excited metastable defect states which could be quenched in the same manner, by reduction or oxidation treatments, had to be considered as being the cause of the red luminescence. Two hole polarons localized near to Ta3+, a Ta3+-O0-centre (vibronic exciton with double charge transfer) and a Ta3+ pair (one in a Ta5+ site and the other in a K+ site) were proposed as possible centres which were connected with the red luminescence.

The Origin of the Red Luminescence in Incipient Ferroelectric KTaO3. V.Vikhnin, S.Eden, M.Aulich, S.Kapphan: Solid State Communications, 2000, 113[8], 455-60