101,233 research outputs found
Structural effects of Li and Nb codoping in a KTaO3 matrix: A Raman investigation
No full textRaman investigations on single crystals of K1-xLixTa1-yNbyO3 were performed to identify structural effects arising from the simultaneous presence of off-centre Li and Nb ions in the incipient ferroelectric KTaO3. Two soft-mode transitions were evidenced at 45K and 30K in a sample with x = 0.0014 and y = 0.024 after both zero-field and field cooling. These temperatures are close to the maximum and to the extrapolated Curie temperature for nonrelaxing permittivity reported in the same system. A sample with x = 0.0028 and y = 0.013 displayed a ferroelectric transition just below 30K only after field cooling, also being consistent with permittivity data
Influence of Cu ions on the photo-transport properties in KTaO3:(Cu,V) single crystals.
No full textPhotoconductivity kinetics, temperature dependence and excitation spectra were measured in the intervals 15-300 K and 2-4.2 eV in KTaO3:(Cu,V) single crystals processed by oxidation/reduction treatments. The kinetics, excited by band-gap excitation, present very slow saturation times, while faster time responses were observed using excitation below the band-gap. The oxidized crystal exhibited higher photoconductivity when Elight > 3 eV. Linear dependence on light intensity was measured, suggesting one center recombination processes. Optical absorption at room temperature was checked as complementary informatio
Local structure studies of and
No full textIn this work we report on the local structure of Ti in SrTiO (STO16) and SrTiO (STO18) investigated in the low temperature range (6–300 K) by extended x-ray absorption fine structure and x-ray absorption near edge structure (XANES) spectroscopy at Ti K-edge and by optical second harmonic generation (SHG). By comparing XANES of STO16 and STO18 we have identified the isotopic effect which produces at T < 100K a noticeable difference in the measured mean square relative displacements (MSRD) of Ti–O1 bonds: while STO16 follow the expected Einstein-like behavior, for STO18 we have measured an increase of MSRD values with decreasing temperature. This is an indication of an increasing off-center position of the Ti atoms in the TiO octahedra
Phase transitions, related properties and possible applications of (K,Li)(Ta,Nb)O3 crystals
No full textDielectric properties of Mn doped SrTiO3
No full textThe dielectric permittivity (100 Hz-1 MHz) and IR reflectivity of SrTiO3:Mn(0.1 at.%) crystals and SrTiO3:Mn(0.1-3 at.%) ceramics prepared maintaining the ratio (Sr+Mn)/ Ti = 1 or (Ti+Mn)/Sr = 1 (A- and B-type ceramics, respectively) were studied. Surprisingly, the magnitude and ε′(T) dependence for the crystals and B-type ceramics with 0.1 at.% of Mn appeared to be nearly the same; the magnitudes rise on cooling and reach ∼3000 at T = 7 K. At the same time, IR spectra of SrTiO3:Mn(0.1 at.%) crystals reveal that TO (transverse optical) phonons yield a magnitude of low frequency permittivity of ∼4000 at T = 5 K, evidencing crystal inhomogeneity. Both crystals, like B-type SrTiO 3:Mn(0.1 at.%) ceramics, reveal faint low temperature relaxations with the Arrhenius activation energies ∼37-40 meV and ∼120-130 meV (P- and T-relaxations). These relaxations emerge in losses only and were tentatively attributed to reorientation of defects associated with MnTi 2+-O--type polaronic centres and Oh 1-D4h18 phase transition related effects, respectively. In reduced crystals P-relaxation strongly enhances, becoming distinctly seen in the real part of the permittivity. More heavily Mn doped A- and B-type ceramics reveal strongly pronounced Arrhenius-type dielectric relaxation with activation energy ∼59-68 meV depending on the Mn concentration
Local Structure Studies of for and by Advanced X-ray Absorption Spectroscopy Data Analysis
No full textStrontium titanate is a model quantum paraelectric in which in the region of dominating quantum statistics the ferroelectric instability is inhibited due to nearly complete compensation of the harmonic contribution into ferroelectric soft mode frequency by the zero-point motion contribution. The enhancement of atomic masses by the substitution of O for O decreases the zero-point atomic motion and low-T ferroelectricity in SrTiO is realized. In this study we report on the local structure of Ti in SrTiO and SrTiO by Ti K-edge extended x-ray absorption fine structure measurements in temperature range 6 – 300 K
Raman study of Cu-doped K-1-xLixTa1-yNbyO3
No full textThe temperature evolution of TO1 Raman spectra was investigated in single crystals of K1.xLixTa1-yNbyO3:Cu with different Li and Nh amounts. Auxiliary observations of TO4 spectra were also performed. in a sample with x = 0.002 and y = 0.025 and containing 190 ppm of Cu, the results indicate a clear-cut transition to an ordered phase around 55 K, in good agreement with previous findings from dielectric measurements in the same crystal. However, no evidence was found of a role of Cu in this structural modification. Samples with x = 0.01 and y = 0.003 revealed features of an order-disorder phase transition around 45 K, irrespective of the presence of Cu
Association of impurities and vacancies in copper doped potassium tantalate crystals studied by EPR
No full texthe high temperature behavior of Electron Paramagnetic Resonance (EPR) signals of Cu2+ centers and Cu2+-Cu2+ exchanged coupled pair centers in potassium tantalate (KTaO3, and related K1-xLixTa1-yNbyO3) 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 Cu2+ ions EPR spectra were found. A model for divalent impurity ions in the potassium tantalate crystal is proposed
- …
