Kikuo Ohi

The Photoluminescence Decay of the 2.2 eV-Emission Band in KTaO3 Single Crystal

The decay of the 2.2 eV emission band in KTaO 3 single crystal was measured using an N 2 laser. The decay of the 2.2 eV emission band at low temperature is in good agreement with that of the 2.6 eV emission band. However, the decay of the 2.2 eV emission band differs from that of the 2.6 eV emission band above 70 K. The decay kinetics of the 2.2 eV emission band is explained by the theory of the recombination of holes trapped on two classes of acceptors and electrons trapped on one class of donors.

Optical mode softening in the incommensurate phase of Sr2Nb2O7

Abstract In the incommensurate phase below about 220°C of Sr2Nb2O7, the soft mode, of which frequency decreases toward 215°C has been found by the Raman scattering measurements in the b(cc) b scattering geometry. The level repulsion and the intensity transfer between the soft mode and another low frequency mode are clearly observed. The uncoupled soft mode frequency ωs has been found to be expressed as ωs =A(Ttr−T)β, where β= 0.38 ± 0.02. Pressure dependence of the soft mode has also been measured up to 24kbar. No remarkable pressure dependence has been observed.

Optical absorption and interband faraday rotation in litao3 and linbo3

The optical absorption and the interband Faraday rotation in LiTaO3 and LiNbO3 were measured at room temperature. The optical absorption in both crystals follows Urbachs equation and the effective temperatures defined by this equation are considerably higher than those of the cubic perovskites. The interband Faraday rotations are negative and about one order of magnitude smaller than those of the perovskites. The results are well described by a combination of indirect transition and direct transition which is estimated from the absorption data.

Photoluminescence in KTaO3 single crystal

The photoluminescence was observed in undoped KTaO 3 single crystal under the illurnination of the band-gap light at low temperatures. The emission spectrum at 49 K shows an asymmetric broad band peaked at 2.6 eV. It can fit in with superimposed two Gaussian curves peaked at 2.61 eV and 2.28 eV. As the temperature increases, the intensity of the 2.61 eV emission band decreases steeply. The emission spectrum at 83 K can be fit in with one Gaussian curve peaked at 2.1 eV. The decay of the photoluminescence was measured using a N 2 pulse laser. The decay kinetics of the 2.61 eV emission band can be explained by the theory of the recombination of electrons trapped on donors and holes trapped on acceptors. The temperature dependence of the luminescence can be explained by thermal ionization of holes on acceptors in parallel with the donor-acceptor recombination.

Dielectric and Ferroelectric Properties in the Low Temperature Phase of Strontium Niobate

Temperature dependence of the dielectric constant, D – E hysteresis loops and pyroelectric charges in Sr 2 Nb 2 O 7 have been studied in the temperature range from 4.2 K to 300 K. In addition to the well known ferroelectric transition at 1615 K, another ferroelectric phase transition occurs at T 0 ∼100 K. The dielectric constant along the b -axis shows a large ferroelectric anomaly at T 0 and the Curie-Weiss constant above T 0 is 8.0×10 3 K. On cooling, the direction of the spontaneous polarization P s begins to deviate at T 0 from the original c -axis of the high temperature ferroelectric state and the new b -component of P s appears; the b -component is 2×10 -2 C/m 2 at 4.2 K. Effects of a dc biasing field on the dielectric constant have been examined. With increasing the field strength, the dielectric constant decreases and the transition temperature shifts to the high temperature side. The results are analyzed with the phenomenological theory developed by Devonshire.

ESR Study of Low Temperature Phase Transitions in Se Doped LiKSO4

ESR spectra of γ-ray irradiated LiKSO 4 : Se have been measured. The ESR lines of SeO 4 - in the room temperature phase split into two branches in the intermediate phase from -75°C to -23°C on heating run and the mirror symmetry appears in the a c plane and the b c plane. The principal values and the directions of the g-tensor have been determined at room temperature and at -60°C on heating run. The results indicate that the space group of the intermediate phase is P 31 c . A model of the twin structure is discussed.

Successive Phase Transitions and Their Soft Modes in Ferroelectric Sr2Nb2O7

The soft optic modes responsible for the successive phase transitions of Sr2Nb2O7 have been observed by the Raman scattering measurement. The characteristics of the soft modes are discussed in connection with the macroscopic properties of the successive phase transitions.

Structural Study of La2Ti2O7 by Convergent-Beam Electron Diffraction and Electron Microscopy

A convergent-beam electron diffraction study has revealed that the space group of the room temperature phase of La2Ti2O7 is P21. A dark-field electron microscopy has made clear a twinned domain structure in this phase. C BED patterns obtained at above 1053 K were compatible with the space group Cmc21. An incommensurate phase exists between 993 K and 1053 K. The incommensurate reflections are characterized with the vector =((1/2)-δ)ā0* from the fundamental reflections of the high temperature phase, a0* being the reciprocal lattice parameter of the high temperature phase. The value of δ increases from 0.015 to 0.017 as the temperature is raised. The transition between the room temperature phase and the incommensurate phase is of the first order, and that between the latter phase and the high temperature phase is of the second order.

High Pressure Raman Study of KNbO3–KTaO3 and KNbO3–NaNbO3 Mixed Crystals

High pressure Raman spectra of KNbO 3 –KTaO 3 (KTN) and –NaNbO 3 (KNN) mixed crystals have been measured to study the behavior of the anomalous mode coupling as well as the phase transition and analyzed on the basis of the two-mode coupling model. The anomalous mode coupling has been observed for all samples under pressure. The mode coupling was observed without compression for K(Ta x Nb 1- x )O 3 crystals with x more than 40%. The upper B 1 (TO) mode frequency of these coupling modes decreased with increase of Ta concentration for the KTN crystals, but remained constant independent of the Na concentration for the KNN crystals. Further, Ta doping plays an equivalent role to the pressure effect with respect to the upper B 1 (TO) mode.

Temperature Dependence of Electrooptic Coefficient and Spontaneous Birefringence of Ferroelectric Gadolinium Molybdate

The temperature dependence of electrooptic coefficient z 63 and spontaneous birefringence Δ n xy S is determined by static measurements on a single-domain and mono-phase region of a GMO crystal under the temperature control with the precision of ±10 -2 °C, so that the thermal hysteresis of about 0.1°C is clearly observed at the transition. Results on z 63 are in good agreement with those of Fouseks dynamical measurements within experimental uncertainty. The third power of Δ n xy S decreases linearly with increasing temperature in the neighborhood of the transition point. Temperature dependence of z 63 and Δ n xy S is qualitatively explained by Aizu-Dvořaks theory.