UV light-induced absorption in pure KTaO3 at low temperature

Pure KTaO3 with cubic symmetry down to lowest temperatures is a model incipient ferroelectric exhibiting the existence of intrinsic inversion symmetry breaking defects in a wide range of experiments. At low temperature a strong second harmonic generation (SHG) of Nd:YAG laser light is observed, despite the overall cubic symmetry of the crystal. Under excitation with UV-light in the band gap region (about 330 nm (3.7 eV)) a strong green luminescence (GL, at 510 nm (2.43 eV)) appears with excitation maximum near 330 nm (3.7 eV). Both, SHG and GL depend strongly on previous oxidation/reduction treatments, increasing with the reduction. In absorption at low temperature broad bands peaking at 1.2 eV and at 1.5 eV are observed in reduced KTaO3 samples. Additional UV-illumination in the band gap region at low temperature gives rise to broad light-induced absorption bands in the same 1-2 eV region. These observations are being discussed on the basis of polaronic holes, of charge transfer vibronic exitons (CTVEs) and of CTVE clusters interacting with oxygen vacancies. (copyright 2005 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)