The origin of the red luminescence in incipient ferroelectric KTaO3

The red luminescence at 686 nm in KTaO3 has been investigated in pure as-grown, ultra-high purity and doped (Al, Ba, Cr, Fe, Er, Li, Mg, Ni, Rh, V, Cr, Zr) crystals from different growth laboratories. In contrast to earlier results [P. Grenier, G. Bernier, S. Jandl, B. Salce, L.A. Boatner, J. Phys.: Cond. Matt 1 (1989) 2515; P. Grenier, S. Jandl, M. Blouin, L.A. Boatner, Ferroel. 137 (1992) 105] which led to the conclusion that the main center should be an intrinsic defect Ta3++O2− vacancy, we found a strong dependence on any kind of heat treatment (oxidation or reduction) leading to a decreasing or vanishing of the red luminescence after these treatments. The red luminescence was found to be sample dependent, together with a dependence on growth procedure. Therefore, excited metastable defect states which can be quenched in the same manner by reduction or oxidation treatments, have to be discussed as the origin of the red luminescence. The (i) two hole polarons localised near a Ta3+, (ii) Ta3+–O0-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 are discussed as possible centers connected with the red luminescence.