Toshirou Yagi

A gigantic photoinduced dielectric constant of quantum paraelectric perovskite oxides observed under a weak DC electric field

A gigantic enhancement of dielectric constant by UV light irradiation is reported for the first time in the quantum paraelectric family of perovskite oxides, such as SrTiO 3 and KTaO 3 , under a weak DC field. The enhancement has not been observed in ferroelectric SrTi 18 O 3 induced by substitution of 18 O for 16 O. The present result suggests a new class of light-switching phenomena controllable externally.

Brillouin Scattering Studies of the Uniaxial Ferroelectric Phase Transition in TGS and TGSe

The Brillouin spectra of the ferroelectric triglycine sulfate (TGS) and its isomorphous selenate (TGSe) crystals have been observed in three cases of orientations of the longitudinal or quasi-longitudinal phonon vector q ; (1) q // c -axis, (2) q // b -axis and (3) q deviates 5° from b -axis in the (100) plane. The temperature dependence of both the frequency shift and the spectral width shows a remarkable anisotropy near T c . In the case (1) the temperature at which the width becomes maximum shifts to the lower temperature than T c which has simultaneously been determined by an accompanied measurement of dielectric constant. The polarization relaxation time is determined from this temperature shift using the phenomenological relation. In the case (2) and the case (3), the decrease in the anomaly is explained as the depolarization field effect of the long range dipole-dipole interaction which is characteristic in the ferroelectric phase transition.

Isotope Effects on the Central Peak Phenomena in KD3(SeO3)2

A central peak and a soft acoustic shear mode associated with C 44 in potassium trideuterium selenite KD 3 (SeO 3 ) 2 has been studied by light scattering. The central peak of a highly deutrated specimen is less divergent near its T tr (=28.85±0.05^° C ) than that of the other specimen with T tr =23.8^° C . Possible origin of the central peak in KD 3 (SeO 3 ) 2 is proposed.

Brillouin scattering study on relaxor ferroelectric Pb(Zn1/3Nb2/3)O3

The 90° Brillouin scattering experiment has been performed on a Pb(Zn1/3Nb2/3)O3 single crystal. The frequency shift and linewidth of the c33 pure longitudinal acoustic mode and the c44 pure transverse mode show bending anomalies near Tm=397 K, at which the low-frequency dielectric constant e has a maximum value. In addition to the acoustic modes, a relaxational mode is found in the form of a central peak. The relaxation time τ of the mode has a temperature dependence similar to that of e and shows a broad peak anomaly around Tm. The temperature dependence of τ can be analyzed by a modified superparaelectric model.

On the Dielectric Anomaly of the Deuterated Dicalcium Strontium Propionate, Ca2Sr(C2D5CO2)6

The deuterated dicalcium strontium propionate crystal (DDSP) was synthesized in order to investigate the role of motion of propionate ions in the phase transition of the ferroelectric dicalcium strontium propionate (DSP). The temperature dependence of the dielectric constant e c at 1 kHz shows an anomaly at 279.5 K ( T C ) indicating a negligibly small isotope effect on T C . The temperature dependence is well described by the Curie-Weiss law e c -e 0 = C ( T - T C ) -1 with e 0 =3.7 and C=56.2 K above T C . The ferroelectricity of DDSP was confirmed by the observation of the dielectric hysteresis loop below T C . The negligibly small isotope effect on T C suggests a secondary role of the motion of the propionate ions in the mechanism of the ferroelectric phase transition of DSP.

Brillouin Scattering Study of the Ferroelastic Phase Transition of BiVO4

The elastic instability in the ferroelastic phase transition of BiVO 4 was investigated by the observation of the Brillouin scattering spectra as a function of temperature around the transition temperature. From the temperature dependence of the observed spectra in two different scattering geometries it turns out clear that the softening of elastic stifiness constant c 11 - c 12 , not of c 66 , causes the ferroelastic transition. The elastic softening is discussed in connection with the domain structure.

Elastic Instability in the Ferroelastic Phase Transition of TlH2PO4 Studied by Brillouin Scattering

Temperature dependence of the acoustic modes of TlH 2 PO 4 has been observed by the Brillouin scattering around its higher phase transition point at 357 K ( T tr ). The elastic constant c 55 obtained by the angular-dependent spectra shows softening with the Curie-Weiss law in both phases. The phase transition is concluded to be a ferroelastic one induced by an instability of the transverse acoustic mode which corresponds to the shear strain x 5 . The angular dependence of the spectra reveals a typical monoclinicity below T tr . The two-dimensionality of hydrogen-bonded network does not affect the dynamical property of the phase transition mechanism.

Brillouin Scattering Study of the Successive Phase Transitions in K2SeO4

The Brillouin scattering spectra of K 2 SeO 4 were observed as a function of temrerature in a ranse from 80 K to 770 K which includes the three successive phase transition points T 1 (745.0 K), T 2 (129.5 K) and T 3 (93.0 K). The Brillouin frectuency shifts Just below the hexasonal-orthorhombic (prototype-normal) transition point T 1 show a softening of the lonsitudinal acoustic (LA) mode propagating along the [100] direction. The index 2 β=0.64 ±0.02 was obtained in a phenomenological treatment of the anomalous part of the elastic stiffness constant C 11 derived from the frequency shift. At the normal-incommensurate transition point T 2 , the LA mode propagating along the [001] direction shows an anomaly of C 33 which is well described in terms of the mean-field theory. Near the incommensurate-commensurate transition point T 3 , the spectra do not show any anomalous temperature dependence.

A Soft Acoustic Mode and a Central Peak in KH3(SeO3)2

Brillouin scattering experiments have been made on KH 3 (SeO 3 ) 2 . The frequency of the transverse acoustic wave with the wave vector q //[010] and the polarization ξ//[001] approaches to zero at the transition temperature with decreasing temperature. The intensity of the Rayleigh line rapidly increases below about –50°C.

Electric Field Gradient and Long Range Order Parameter of Ferroelectric Sodium Nitrite

In order to obtain any relation between an electric field gradient (E.F.G.) at the site of 23 Na nucleus in Sodium Nitrite crystals and spontaneous polarization, the observed E.F.G. along the b -axis in the ferroelectric phase is formally expanded with the long range order parameter S . The E.F.G. in a fictitious perfectly ordered state and in a disordered one are calculated as a function of temperature. The E.F.G. of both fictitious states are calculated in a point charge approximation on the basis of a simplified model. The calculated S from the first order expansion series in terms of the observed E.F.G. and the calculated E.F.G. shows a good agreement with the S obtained from the observed spontaneous polarization except in the vicinity of the transition temperature. An effect of the order-disorder rearrangement of NO 2 - ion is concluded to reflect the temperature dependence of the E.F.G. observed in the ferroelectric phase.