Light absorption at low temperatures in nominally pure material was investigated. Absorption broad bands peaking at about 1.23, 1.53 and 2.26eV (quenched samples) were observed in reduced samples. Additional UV illumination in the band-gap region at low temperatures gave rise to broad light-induced absorption bands in the same region: at about 1.14, 1.48 and 2.17eV. States which were active in an approximately 2eV absorption region exhibited a long-term metastable behaviour. The observations were explained in terms of O-related intrinsic defects (doubly and singly negatively charged interstitial O ions, as well as doubly and singly positively charged O vacancies). The proposed model included pair dipole-type intrinsic defects which consisted of O interstitial ions and O vacancies and exhibited a metastable behaviour. The latter dipole defects could be treated as being charge transfer vibronic excitons localized on pair defects.

Oxygen-Related Intrinsic Defects and Charge Transfer Vibronic Excitons Localized on Such Defects - Optical Manifestation of Long Term and Metastable States in Pure KTaO3 Crystals. V.S.Vikhnin, A.Gubaev, S.E.Kapphan: Physica Status Solidi C, 2004, 1[11], 2844-7