Possible nature of the red luminescence in incipient ferroelectric KTaO₃

Abstract

The red luminescence at 686 nm in KTaO₃ is investigated in pure as-grown and 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⁺ + O^2- vacancy, 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. Therefore excited metastable defect states have to be discussed for the origin of the red luminescence. The (i) two hole polarons localized 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.

Key Words:

• KTaO₃
• Luminescence
• Polaron
• Metastable non-equilibrium states