LUMINESCENCE AND 2ND-HARMONIC GENERATION AT LOW-TEMPERATURE IN REDUCED, PURE AND DOPED KTAO3

In the incipient ferroelectric system KTaO3 the green luminescence and an SHG-intensity at low temperature are found to depend drastically o n oxidation or reduction treatments. The intensity of both signals is varying with the duration of the reduction in parallel over orders of magnitude. This is seen not only for nominally pure samples but also f or Li-doped specimen ([Li]<2%). As-grown samples display a behaviour c orresponding to a slightly reduced state. Only fully oxidized samples provide a fully characterized starting material. Under applied electri c field a contribution of the bulk polarization and the alignment of d ipolar microregions both of oxygen defect-and of intentionally Li-dope d defect origin are observed. The oxygen related defects are discussed on the basis of charge transfer vibronic excitons or oxygen vacancy r elated centers.