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Photoconductivity (PC), thermally stimulated conductivity (TSC), photoluminescence (PL), thermoluminescence (TL), and electron spin resonance (ESR) measurements have been made on single crystals of potassium tantalate over the temperature range 4.2-290 K. We revealed two sorts of O[SUP-] shallow hole centers which are responsible for the two temperature regions of PL and PC enhancement: T,70 K and 100-150 K. Both O[SUP-] centers were identified by their ESR spectra. While at low temperatures PL and PC have a rather intrinsic origin, i.e., they do not depend essentially on the sort or quality of crystals, at 100-150 K both quantities strongly depend on the defect content and vanish in well-oxidized crystals. We show that O[SUP-] centers serve as radiative electron-hole recombination centers. Their energy levels are situated at 0.08 and 0.16 eV above the top of the valence band. Measurements of TSC and TL after UV irradiation revealed several glow peaks at temperatures 18-30 K and 65-70 K. There is a good correlation between TSC and TL intensity in different samples as well as after annealing in O[SUB2] and H[SUB2] atmospheres. Because electrons are mobile species in KtaO[SUB3] we attribute both TSC and TL to the thermal ionization of the same shallow donor centers related with isolated oxygen vacancies. The experimental data were treated in a one-trap/one-recombination center model, which takes into account the presence of "thermally disconnected" deep electron traps. [ABSTRACT FROM AUTHOR]

The optical and electrical properties of a set of Hf doped Lithium Niobate crystals with doping level from 2 to 8 mol% are reported. The photoconductivity is found to increase remarkably for Hf concentration equal and above 4 mol%. In contrast, the electro-optical coefficients keep almost constant with the crystal structural properties preserved. The conclusion is drawn that the decrease of the photorefractive response in Hf doped Lithium Niobate recently observed by other authors is mainly related to the increased photoconductivity. [ABSTRACT FROM AUTHOR]

Abstract: Detailed photoconductivity measurements have been performed in nominally pure SrTiO3 in order to elucidate the effect of the antiferrodistorsive cubic–tetragonal phase transition. Small features in the photoconductivity’s temperature dependence in the phase transition region were found using low intensity interband UV or 514 nm light illumination. Such features are associated with a transformation of the defect system controlling the photoconductivity. At the same time, the temperature behavior of the photoconductivity spectral maximum reveals a rather unusual feature which is connected with changes in the absorption band edge structure in the phase transition region. [Copyright &y& Elsevier]

The effect of the antiferrodistorsive phase transition observed at ∼⃒ 105 K on the photo-carriers transport and band structure of nominally pure SrTiO 3 single crystals was investigated. Simultaneously the ab-initio analysis based on the DFT-HF hybrid approach was performed in order to elucidate the atomic and electronic structure changes. It was found that the structural transition play a minor role in photo-charge transport. However, in highly resistive specimens at low UV-light intensities or under visible-light, 514 nm excitation, weak photoconductivity features are observed in the transition region. Besides, in this region the temperature maximum was found for the photoconductivity spectra. Experimental results are discussed and agree well with theoretical predictions. [ABSTRACT FROM AUTHOR]

Observation and first studies of the UV light-induced optical absorption for KTa 1- x Nb x O 3 single crystals (x = 0.004, 0.007, 0.012, 0.07) are reported. Under illumination (300–400 nm, 3.10–4.13 eV) at low temperatures a broad light-induced absorption band was found in NIR region, peaking at 5600 cm - 1 (1.79 μ, 0.69 eV, T = 1.3 K) for x = 0.007. No light-induced absorption was found at room temperature in all Nb containing samples as well as in nominally pure KTaO 3 . Results of detailed studies, including photoconductivity and photoluminescence experiments, are associated with the localization of the charge carriers in the form of electronic polarons at low temperatures. [ABSTRACT FROM AUTHOR]

We made Thermally Stimulated Conductivity (TSC), Thermoluminescence (TL) and Electron Spin Resonance (ESR) measurements on single crystals of potassium tantalate in the temperature range 4.2-290 K. We revealed two sorts of O - shallow hole centers which are responsible for Photoconductivity (PC) and Photoluminescence (PL) enhancement. Both O - centers were identified by their ESR spectra. We show that these centers serve as radiative electron-hole recombination centers. The measurements of TSC and TL after UV irradiation revealed several glow peaks at temperatures 18-30 K and 65-70 K. Both TSC and TL are attributed to the thermal ionization of the same shallow donor centers related with oxygen vacancies. Experimental data were treated in a simple one-trap/one-recombination center model, which takes into account the presence of "thermally disconnected" deep electron traps. [ABSTRACT FROM AUTHOR]