Luminicence and optical second harmonic generation by dipolar microregions in KTa_O3

The incipient ferroelectric KTaO3 being cubic down to lowest temperatures, is not expected to yield second harmonic intensity (SHG) due to the inversion symmetry of the lattice. Nevertheless a weak SHG intensity of Nd:YAG laser light is observed, which increases with decreasing temperature. This low temperature SHG is drastically enhanced in KTa0₃samples which have been reduced at 1000°C in H2-atmosphere. Simultaneously the luminescence intensity observed at low temperature (T = 80 K) with an emission at λ_max— 520 nm is increasing strongly upon reduction as well. This increase is not only due to a thin reduced surface layer, but rather uniform throughout the volume of the sample crystal. Oxidation at 1000°C in pure O₂-atmosphere diminishes both, the low temperature SHG- and the luminescence intensity, far below the values of the pure, ’as-grown’ crystal. Sharp luminescence features reported to be at λ = 687 nm in the literature and assigned to Ta³⁺near oxygen vacancies, have not been observed in our crystals, neither in the as-grown, nor in the reduced or oxidized samples. The interpretation is based upon dipolar microregion around a defect core due to oxygen vacancy complexes, locally breaking the inversion symmetry.