Itaru Tatsuzaki

Light scattering spectra of polarization fluctuations and models of the phase transition in KDP type ferroelectrics

Abstract Recent light scattering studies in KH2PO4 type hydrogen-bonded crystals are reviewed. Dynamical properties of these crystals have been discussed in terms of the coupled tunneling proton-optical phonon model up to date. Light scattering spectra in several GHz and cm−1 regions with various scattering geometries observed in KH2PO4, KD2PO4 and their mixed crystals, however, revealed that all these spectra are well analysed in terms of a coupled polarization relaxation mode-acoustic phonon system. Since most of the spontaneous polarization is due to shifts of K, P and O ions, relaxation motions of H2PO4 dipoles arc concluded to be the origin of these low-frequency spectra. Models of the phase transition mechanism are discussed on the basis of these results and other recent experimental studies including the explanation of large isotope effects in these crystals.

Study of the Phase Transition in KH3(SeO3)2. I. Acoustic Phonon Softening

The phase transition of KH 3 (SeO 3 ) 2 , was studied by ultrasonic velocity measurements at 5 and 10 MHz together with X-ray diffraction measurement and pyroelectric examination. It was clarified that the crystal below the transition point ( T c ) is nonferroelectric and belongs to the centrosymmetric space group P2 1 / b . All of the elastic stiffnesses c i j were determined front the velocity data. The phase transition accompanies with a large phonon softening only in the c 44 mode. It was shown that the ferroelastic behavior above and below T c can well be explained by a phenomenological theory.

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.

Raman spectral of low-lying longitudinal mode of KH2PO4 and KD2PO4 in ferroelectric phase

Abstract The origin of the low-lying modes of KH 2 PO 4 and KD 2 PO 4 in the ferroelectric phase has been clarified by the z(xy)z Raman scattering experiments. The “ S -mode”, which has been usually assigned to the proton tunneling mode in KH 2 PO 4 at T T c , has been found in the z(xy)z spectrum of KD 2 PO 4 in contrast to the x(xy)y spectrum. It has been found that the frequency of the “ S -mode” of KD 2 PO 4 is higher than that of KH 2 PO 4 . These results have shown that the “ S -mode” is far from the proton tunneling mode nor the proton/deuteron mode at all. From the present Raman spectroscopy, it is concluded that the “ S -mode” is assigned to the libration mode of the PO 4 tertahedrons.

Influence of Temperature on the Pure Nuclear Quadrupole Resonance in Hexachloroethane

The temperature dependences of the frequencies and the widths of absorption lines of pure nuclear quadrupole resonances in hexachloroethane were studied. The change of resonance frequencies with temperature was explained as influences both of a torsional vibration of one end of CCl 3 groups with respect to the other end and of a rotational vibration of those groups as a whole around the C–C axis, the frequency of the latter vibration being changed with temperature owing to a thermal expansion of the lattice. The line widths at half-maximum intensity were about 2 Kc/s at temperatures of 77°K and 90°K, and about 10 Kc/s at 196°K, although no absorption was detected at room temperature. The broadening of the resonance spectrum seems to be explainable by a reorientational motion around the C–C axis with much lower frequency than the resonance one.

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.

Anomaly of the Proton Spin-Lattice Relaxation Time near the Critical Temperature of Dicalcium Lead Propionate Ca2Pb(C2H5COO)6

The precise temperature dependence of the proton spin-lattice relaxation time T 1 is observed in the vicinity of the transition temperature of dicalcium lead propionate (Ca 2 Pb(C 2 H 5 COO) 6 ). Measurements are carried out by using 90°-t-90° pulse method and the boxcar integrator is used to reduce an experimental error of T 1 . The anomalous part of T 1 -1 , ( T 1 -1 ) fluc , seems to display a logarithmic divergence at the transition temperature T c . The interpretation of the anomalous ( T 1 -1 ) fluc is formulated in terms of the time dependent correlation functions in the order-disorder phase transition system described by the Ising operators, and the relation to the recent theory of the critical phenomena is shown. Some possible interpretations of this divergence are discussed.

Light Scattering Spectra Depending on the Direction of Scattering Wave Vector in KD2PO4. I Brillouin Spectrum and Broad Central Component

The Brillouin and Rayleigh scattering spectra have been observed near the transition temperature T c in the paraelectric phase of KD 2 PO 4 as a function of the direction of the scattering vector q in several scattering geometries. The broad central component ascribed to dynamical spectrum of the Debye type relaxational mode by Reese et al. has been studied in both the ab - and the ac - scattering planes. Theoretical formulas are derived to explain q and temperature dependences of the intensity of the broad central spectrum. The frequency of the acoustic mode coupled with this mode is found not to decrease even near T c in the wave-length region of the Brillouin scattering. The q dependence of the Brillouin scattering intensity has been measured and can be accounted for by the fundamental theory mentioned above.

Light Scattering by the Polarization Fluctuation in the Low Concentration Region of DKDP in Mixed Crystals KDP1-xDKDPx

Light scattering spectra of mixed crystals KDP 1- x DKDP x with x =0.08, 0.04 and 0.0 (KDP) are measured by a Fabry-Perot interferometer with a free spectral range of 8 cm -1 . Spectra are analyzed with the Debye type susceptibility of a polarization fluctuation coupled to an acoustic phonon. The relaxation times of an isolated dipole for x =0.08, 0.04 and 0.0 are found to be 0.27, 0.21 and 0.15 in the unit of 10 -12 sec, respectively.

Pressure-Induced Tricritical Nature in Triglycine Selenate

Temperature dependences of dielectric constant and spontaneous polarization in triglycine selenate were measured under hydrostatic pressure. Coefficients of power series expansion of the free energy in respect to electric polarization were calculated front the experimental data. It has been found that the coefficient ξ of the fourth power changes its sign from plus to minus near 5 kbar. This indicates that the tricritical point of triglycine selenate exists at ∼40.6°C and ∼5 kbar.