Influence of Cu ions on the photo-transport properties in KTaO3:(Cu,V) single crystals.

Photoconductivity 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

Divalent impurity ions in potassium tantalate crystals: EPR investigations.

Electron paramagnetic resonance experimental results of potassium tantalate single crystals doped by Co2+ ions as well as new results of our continuous studies of KTaO3:Cu single crystals are presented. 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. A new axial centre of Co2+ in the crystal of KTaO3:Co has been studie

Impurity centers and host microstructure in weakly doped SrTiO3:Mn crystals: New findings

Electron paramagnetic resonance (EPR), optical absorption and low-frequency (100 Hz - 1 MHz) dielectric permittivity investigations on Verneuil as-grown and reduced SrTiO3:Mn (0.1% at.) crystals were carried out to establish connection between the structure of Mn doping related centers and the SrTiO3 dielectric properties. At room temperature the EPR spectrum of as-grown crystals evidences isotropic Mn4+ and Mn2+ centers. The Mn2+/Mn4+ ratio increases, according to the EPR spectra, by decreasing the temperature and the signals intensity of both centers strongly decreases approaching T similar to 120 K. Such behavior is attributed to antiferromagnetic interaction in MnO and MnO2 nano-clusters. Also the dielectric permittivity behavior evidences samples inhomogeneity as well as absence of pronounced dielectric relaxation in as grown crystals. In reduced crystals the Mn2+ concentration increases and the dielectric relaxation emerges evidencing a polaronic-related nature of dipole reorientations