Oxygen-related intrinsic defects and charge transfer vibronic excitons localized on a such type defects: optical manifestation of long term and metastable states in pure KTaO₃ crystals

Light absorption at low temperatures in nominally pure incipient ferroelectric KTaO₃ is investigated. In such an absorption broad bands peaking approximately at 1.23 eV, at 1.53 eV and at 2.26 eV (the latter in the quenched sample) are observed in reduced KTaO₃. Additional UV-illumination in the band gap region at low temperature gives rise to broad light-induced absorption bands in the same region: approximately at 1.14 eV, at 1.48 eV, and at 2.17 eV. The states active in ∼2 eV region absorption manifest long-term metastable behaviour. These observations are being considered on the basis of oxygen-related intrinsic defects (double and single negatively charged interstitial oxygen ions as well as double and single positively charged oxygen vacancies). The model proposed includes pair dipole-type intrinsic defects, which consist in oxygen interstitial ions and oxygen vacancies and manifest metastable behaviour. The latter dipole defects can be treated as Charge Transfer Vibronic Excitons localized on the pair defects. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)