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We present a detailed investigation of structural properties of manganese doped strontium titanate ceramics and Verneuil grown single crystals, the last prepared with Sr deficiency compensated by proper Mn amount. EXAFS, TEM, ESR and Raman techniques, combining local and lattice probes, allows to inspect the local chemical environment and electronic structure of manganese taking into account possible chemical non-homogeneities and/or phase segregation. On the basis of experimental evidences, structural models for Mn insertion in SrTiO3are discussed considering possibility for Mn ions to occupy both Ti4+and Sr2+sites as well as manganese segregation and Mn incorporation related non-homogeneities. [ABSTRACT FROM PUBLISHER]

Micro Raman and static magnetization data on manganese or cobalt-implanted (at 3 or 5 at.%) strontium titanate (Mn,Co):SrTiO3 single crystals are presented. Micro Raman results reveal a cubic perovskite structure at 300 K with no evidence of any secondary phase. All samples show ferromagnetism both at 300 K and at 10 K. Moreover, micro Raman spectra at low temperature reveal the presence of polar ordering, which allow to consider the investigated materials as possible host matrices for multiferroic phenomena. Obtained results are discussed in connection to supplementary outcomes derived by content, optical absorption and electron paramagnetic resonance (EPR) measurements. [ABSTRACT FROM AUTHOR]

Non linear optical, dielectric, magnetic properties of ABO3 single crystal oxides let the implementation of acousto-optic, electro-optic, photonic, spintronic devices. In particular, significant role is played by the ferroelectric and photorefractive materials, where the photo-induced change in refractive indexes requires charge transport and impurities which support charge transfer, space charge formation and localization with accompanying electric field. Besides, ABO3 paraelectric perovskites regularly reveal newel effects connected with charge transport, e.g., photo-induced polar state recently reported for SrTiO3 and KTaO3. In this work we review our recent investigation, pointing out our studies of photo-transport in SrTiO3, Cu-doped KTaO3 and LiNbO3:Hf. [ABSTRACT FROM AUTHOR]

From the analysis of the temperature dependence of the Electron Paramagnetic Resonance spectra for “as grown” and reduced SrTiO3:Mn crystals and ceramics it was proposed that the manganese incorporates into the strontium titanate mainly as small particles of MnO2 or MnO. [ABSTRACT FROM AUTHOR]

The emission and excitation spectra of photoluminescence of the as-grown and reduced SrTiO3:Mn (280 ppm) crystal were studied within wide temperature (12 - 300 K) and spectral (300 - 1600 nm) regions. The emission spectra consist of two overlapping bands. The first one corresponds to the emission of Mn4 + ions substituted in octahedral coordinated Ti4+ sites with zero-phonon line peaking at 723.23 nm at 12 K. The second broad emission band peaking near 870 nm was observed on SrTiO3:Mn crystals for the first time. Temperature behavior of photoluminescence intensity and the origin of this new Mn related emission band are discussed. [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]

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]

In order to understand the manner of incorporation of oxygen vacancies into perovskite structure oxides, KTaO 3 crystals containing Cu 2+ impurity ions were investigated following thermal treatments in atmosphere of H 2 , air, and vacuum. The results of thermal treatments were controlled by EPR. The influence of light illumination on copper centres in these crystals was studied also. The thermal treatment and light illumination effects in EPR spectra of Cu impurity centres are studied for the incipient perovskite-like ferroelectric KTaO 3 single crystals. It was found that thermal treatment at ∼⃒950°C in hydrogen atmosphere and in vacuum strongly depresses EPR signal of both types of Cu 2+ (1) and Cu 2+ (2) axial centres substituting Ta 5+ in the B-sublattice positions. Subsequent high-temperature treatment in air restores EPR signals of Cu 2+ centres, but changes the ratio between Cu 2+ (1) and Cu 2+ (2) centres concentration. These effects, together with UV light illumination effects observed for T [ABSTRACT FROM AUTHOR]

We report on the first crystal growth and Raman scattering studies in K 0.992 Rb 0.008 Ta 0.96 Nb 0.04 O 3 (KRTN) and KTaO 3 :Rb (3% mol. in the melt), in which Rb + ions (ionic radius 1.47 Åt) substitute K + (1.33 Åt). The experiments were performed in the range 10–300 K and focused on the behavior of TO 1 and TO 4 modes. The results confirm indications by previous dielectric permittivity data about the ferroelectric nature of the phase transition in KRTN. It appears that a small amount of Rb changes the character of the phase transition induced by Nb from mainly displacive to mainly order-disorder, whereas no evidence of ferroelectricity is revealed in the KTaO 3 :Rb sample. [ABSTRACT FROM AUTHOR]

EPR results of potassium tantalate single crystals doped by Co2+ ions as well as new results of our continuous studies of KTaO3: Cu single crystal have been submitted. Divalent impurity ions substitute for Ta5+ in the octahedron position in the KTO3 crystal and oxygen vacancies are required to compensate the extra charge of Ta5+. The Cu2+ ions associated with oxygen vacancies form only axial centres (included exchange coupled pair centre), the symmetry axes being along cubic directions. New axial centre of Co2+ in the crystal of KTaO3: Co have been studied. [ABSTRACT FROM AUTHOR]

The high temperature behavior of Electron Paramagnetic Resonance (EPR) signals of Cu 2+ centers and Cu 2+ -Cu 2+ exchanged coupled pair centers in potassium tantalate (KTaO 3 , and related K 1 - x Li x Ta 1 - y Nb y O 3 ) single crystals has been studied. EPR signals were observed in the range of 300-1000 K. The dynamic behavior of oxygen vacancies and the influence of its on Cu 2+ ions EPR spectra were found. A model for divalent impurity ions in the potassium tantalate crystal is proposed. [ABSTRACT FROM AUTHOR]

Recently we reported that a very small admixture of Li + dipole impurities into moderately concentrated KTaO 3 :Nb leads to an unusual sequence of ferroelectric-type phase transitions with a reentrant glass phase formation at the lowest temperatures [1] . Here, we present dielectric permittivity and Raman scattering study for nearly the same composition as used in Ref. [1] , but without Li admixture, for KTa 0.982 Nb 0.018 O 3 (KTN1.8). It is found that the presence of strong Li dipole defects is not an essential condition for the reentrant glass formation. Above T C (27 K) KTN1.8 behaves as a conventional soft mode displacive type ferroelectric in paraelectric phase. However, below T C a crossover to the order-disorder polar microregion dynamics and a state at which a reentrant glass phase coexists with long-range ordering occurs. [ABSTRACT FROM AUTHOR]

We studied the soft phonon Raman spectra in samples of K 1-x Li x Ta 1-y Nb y O 3 , with x = 0.0015, y = 0.046 and x = 0.0024, y = 0.01 mol% in order to explore the complex structural effects arising in KTaO 3 matrix, weakly doped by Li ( x < 0.01) and simultaneously moderately doped by Nb (0.01 ≤ y ≤ 0.05). The results provide new evidence that the character of the phase transitions in KTaO 3 moderately doped with Nb, can be markedly modified by the addition of small amounts of Li. In particular K 0.9976 Li 0.0015 Ta 0.954 Nb 0.046 O 3 exhibits a complex scenario of structural transformations, that differs from the known behaviour in KTN close to the multicritical point. [ABSTRACT FROM AUTHOR]

Electron paramagnetic resonance (EPR) of single. Cu 2+ ions and exchange-coupled Cu 2+ -Cu 2+ pairs in KTaO 3 , KTaO 3 :Li:Cu and KTaO 3 :Li:Nb crystals has been analyzed in the temperature range of 300 - 1000 K. The temperature stability of the paired centers was found to be similar to that of single Cu 2+ ions. The influence of oxygen vacancies on the Cu 2+ EPR spectra was studied. [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]

In the present work we report on Raman study of Ba0.77Ca0.23TiO3 crystals in the temperature region between 12 K-300 K. We believed to gain insight on the role of Ca admixture, and search manifestations of the possible low temperature tetragonal-orthorhombic phase transition. A special attention was paid to the temperature evolution of Al and E components of the Γ15 modes. Signatures of the presence of CaTiO3 modes are evidenced at around 100 cm-1. The Raman data give no clear indications of the soft mode driven low temperature phase transitions presence, at least down to 12 K, revealing, however, increasing of disordering at cooling. [ABSTRACT FROM AUTHOR]