S. A. Basun

Photoinduced phenomena in Sr1-xCaxTiO3, 0 ≤ x ≤ 0.12

Abstract Photoinduced changes of the spectral, temperature and time dependences of the conductivity, the Cr3+ R-luminescence, the electron paramagnetic resonance, the refractive index and of the spatial variation of the linear birefringence are measured in nominally pure SrTiO3 and ferroelectric Sr1-xCaxTiO3 (x=0.007 and 0.12). In both materials a large, long time persisting increase of the photoconductivity for light with energy less than 2.1 eV was found as a result of previous illumination with higher photon energies. This memory threshold coincides with the low energy edge of the strongest extrinsic band in the excitation spectra. Basic mechanisms of extrinsic photoelectronic processes in both materials are proposed. They involve a parallel action of (i) photoinduced charge transfer from the valence to the conduction band via deep energy levels within the gap, (ii) recombination due to Cr3+-Cr4+ centers, and (iii) trapping and light-induced emission processes via relatively shallow traps.

Ferroelectric phase transition induced pseudo-stark splitting in spectra of Li2Ge7O15:Cr3+ crystals

Abstract At D 2h → C 2v ferroelectric phase transition in Li2Ge7O15:Cr3+ crystals the doublet R-lines splitting was observed, which evidences for splitting of whole Cr3+ centers ensemble (in paraphase) into two physically nonequivalent ensembles in ferrophase. It is shown that this phenomenon is connected with complex structure and low triclinic (C 1) point symmetry of Cr3+ centers which possess a permanent electric dipole moment and that R-lines splitting is a pseudo-Stark splitting in internal electric field, induced in ferrophase by polar order parameter.

Ferroelectric transition induced dipole moments in probe ions in Li2Ge7O15 crystals doped with Mn4+ and Cr3+

Abstract In ferroelectric phase of Li2Ge7O15 (LGO) crystals doped with Mn4+ and Cr3+ ions the effect of external electric field on the R-lines in luminescence spectra of probe ions is studied. From the observed linear in field pseudo-Stark splitting of R-lines the existence of phase transition induced electric dipole moments in probe ions was established which are symmetry forbidden in paraelectric phase and appear only in ferrophase. The symmetry analysis of the phenomenon is given. The results allow to conclude that in pure LGO ferrophase the large electric dipole moments perpendicular to ferroelectric axis “c” are induced in octahedral GeO6 sites which are ordered and alternatively opposite in sign in four GeO6 sites included in unit cell which provides the full cancellation of net induced moment in plane ⊤ c .

Anisotropic photoionization as a mechanism of axial iron center alignment in KTaO3

An EPR study has revealed light-induced recharging and optical alignment of the FeTa4+-VO tetragonal complexes in KTaO3. The data on the optical creation and destruction of this center by light of different polarizations and wavelengths are discussed together with similar results obtained for the FeK3+-Oi center. These two centers were established to undergo mutual charge transfer, in which the electron released in the photoionization of the FeK2+-Oi center is trapped by the FeTa5+-VO center. Irradiation by light with a photon energy below 2.05 eV, which is the ionization threshold of FeK2+-Oi, reverses this process. In both cases, the absorption cross section depends on the orientation of the center axis relative to the light polarization vector. As a result, the FeTa4+-VO and FeK3+-Oi tetragonal centers in KTaO3 acted upon by polarized light undergo orientation-sensitive light-induced recharging and the defects with the given charge state are no longer characterized by an equally probable distribution of the orientations of their axes over the three 〈100〉 directions. This mechanism, which does not involve real reorientations of the FeTa-VO and FeK-Oi complexes, gives rise, nevertheless, to the alignment of the centers along (or at right angles to) the light polarization vector.

EPR of a non-Kramers iron ion in KTaO3

The EPR spectrum of the non-Kramers iron ion Fe4+ (S=2) in a KTaO3:Fe crystal appearing after illumination of the sample in the visible has been detected and studied. Because of the large initial splitting (|D|=4.15 cm−1), only transitions within the |±1〈 and |±2〈 doublets are seen experimentally. Superhyperfine structure in the spectrum of a non-Kramers ion in perovskites has been detected for the first time. A structure is proposed for the center responsible for the new EPR spectrum, which represents a complex of a Fe4+ ion substituting for Ta with an oxygen vacancy at the nearest anion site.

Electric field effects in the EPR spectrum of low-spin Ni3+ centers in the KTaO3 crystal

The electric field effects in the EPR spectra of low-spin (S = 1/2) Ni3+ tetragonal centers in KTaO3 single crystals are investigated. It is revealed that the resonance lines are split and the centers are oriented as a result of the interaction of the external field with the electric dipole moment of the center. The dipole moment of the center is determined to be p = 100 D = 21 eÅ. An analysis of the set of experimental data obtained permits one to choose correctly the microscopic models for two nickel centers in KTaO3 crystals among the models discussed in the literature.

Low-symmetry paramagnetic center in a KTaO3: Ni crystal

A new paramagnetic center of orthorhombic symmetry with spin S = 1/2 is revealed in KTaO3: Ni crystals. The well-resolved superhyperfine EPR structure consisting of 15 components indicates preferred interaction of the center with two tantalum nuclei (181Ta, I = 7/2). This center does not decay and its orientation remains unchanged up to room temperature. Possible models of the center are discussed.

Optical alignment of Cu2+ axial centers in KTaO3: Spectral dependence of the effect

The CuTa2+-VO axial centers in crystalline KTaO3 were found to undergo alignment under the action of polarized light. The sign of the effect is shown to change depending on the wavelength of the aligning light. A parallel study of the spectral response of photoconductivity of the same samples led to the conclusion that the alignment of the copper centers is driven not by reorientation but rather by an anisotropic recharging of the centers, which involves both the conduction and valence bands of the crystal. This interpretation was supported by a study of the kinetics of thermal destruction of the copper center alignment.

Elimination of Photovoltaic Induced Instabilities and the Theoretical Evaluation of Material Parameters in Iron-Doped Lithium Niobate Using Contra-Directional Two-Beam Coupling

Two-beam coupling efficiencies are measured experimentally using a contra-directional single incident beam geometry. These results are compared to a theoretical model, which takes into account the presence of dark conductivity. The experimental results were obtained from a crystal where grating-writing instabilities were eliminated. The source of the instabilities and their elimination are discussed.

Dominant Cr3+ centers in LiNbO3 under hydrostatic pressure

Low-temperature luminescence spectra of stoichiometric Cr: LiNbO3 and of congruent Cr, Mg: LiNbO3 were studied. Cr3+ impurity ions preferentially occupy Li+ sites (CrLi) in the LiNbO3 crystal lattice, while Cr3+ ions substituting for Nb5+ ions (CrNb) occur in addition to CrLi centers in codoped Cr, Mg: LiNbO3 crystals. Application of a high hydrostatic pressure leads to a transformation of (dominant in concentration) Cr3+ centers from low-to high-crystal-field centers. Due to a strong pressure-induced blue shift of the 4T2 state resulting in crossing with the 2E state, the replacement of the broad band 4T2 → 4A2 emission by a narrow R-line emission 2E → 4A2 occurs in the luminescence spectra of the samples. This effect of level crossing was observed for the dominant CrLi3+ and CrNb3+ centers at pressures which correlated well with estimations based on the 4T2-2E energy gap (230 and 1160 cm−1) and on the rate of their pressure-induced change (14.35 and 11.4 cm−1/kbar, respectively).