69 research outputs found

    Photoinduced charge transport in KTaO3:Be

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    We present the first growth of KTaO3:Be (6% at. in the batch) single crystals, together with photoconductivity and thermally stimulated currents studies of this material. It is probably that very small Be2+ ions (ionic radius 0.35 Ă… ) mostly substitute K+(1.64 Ă…) and bounded and/or free electronic states formation including polaronic states. As a results Be-doping effect can strongly influence phase instability and electronic properties of KTaO3. It was found that Be2+ doping leads to appreciable photocurrents already at room temperature under irradiation of the samples with photons even at energies just below the fundamental optical gap. Photoconductivity increases strongly at low temperature. Intense thermally stimulated current peaks between 15 and 100 K are probably due to the presence of shallow hole traps and polaronic centres. The obtained results are compared to past observations in pure KTaO3 and KTaO3:L

    Photoinduced charge transport in KTaO3:Be

    No full text
    We present the first growth of KTaO3:Be (6% at. in the batch) single crystals, together with photoconductivity and thermally stimulated currents studies of this material. It is probably that very small Be2+ ions (ionic radius 0.35 \uc5 ) mostly substitute K+(1.64 \uc5) and bounded and/or free electronic states formation including polaronic states. As a results Be-doping effect can strongly influence phase instability and electronic properties of KTaO3. It was found that Be2+ doping leads to appreciable photocurrents already at room temperature under irradiation of the samples with photons even at energies just below the fundamental optical gap. Photoconductivity increases strongly at low temperature. Intense thermally stimulated current peaks between 15 and 100 K are probably due to the presence of shallow hole traps and polaronic centres. The obtained results are compared to past observations in pure KTaO3 and KTaO3:L

    Nontrivial dependence of dielectric stiffness and SHG on dc bias in relaxors and dipole glasses

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    Dielectric permittivity and Second Harmonic Generation (SHG) studies in the field-cooled mode show a linear dependence of dielectric stiffness (inverse dielectric permittivity) on dc bias in PMN-PT crystals and SHG intensity in KTaO3_{3}:Li at small Li concentrations. We explain this unusual result in the framework of a theory of transverse, hydrodynamic-type, instability of local polarization.Comment: 5 figure

    Divalent impurity ions in potassium tantalate studied by EPR

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    EPR results of potassium tantalate single crystals eloped 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

    A new phase in ferroelectric oxides: The phase of charge transfer vibronic excitons

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    It is shown, by means of Hartree-Fock-type calculations using the intermediate neglect of the differential overlap (INDO) method, that polaronic-type charge transfer vibronic excitons (CTVE) in ferroelectric oxides could lead to the formation of a new phase. The ground-state energy of this phase of strongly correlated CTVE lies within an optical gap of pure crystal, and is characterized by a strong tetragonal lattice distortion, as well as ferroelectric and antiferromagnetic ordering. It is shown also that clusters of the CTVE phase being stabilized by oxygen vacancies could be responsible for the unusually strong optical Second Harmonic Generation (SHG) in nominally pure incipient ferroelectrics, like \chem{KTaO_{3}} (\chem{KTO}) and \chem{SrTiO_{3}} (\chem{STO}), especially in the case of SHG induced by an external electric field. Another experimental manifestation of CTVE is related to a drastic red shift with temperature of the fundamental absorption (FA) edge in ferroelectric oxides \chem{SBN} and \chem{KTO} observed at high temperatures. This effect could be explained by thermal population of a low-lying bottom part of an additional valence band induced by the CTVE phase
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