Masaharu Tokunaga

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.

Review on tunneling model for KH2PO4

Abstract A historical review is given on the proton tunneling model in KH2PO4 type ferroelectrics with reference to the experimental results which have been believed to be evidences of the model. Some of the theoretical results based on the model are shown to be inconsistent with recent dynamical experimental results suggesting the order-disorder behavior of PO4 dipoles.

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.

Critical phenomena in ferroelectric phase transitions I. Experimental observations and qualitative discussion

A review is made of experimental studies of the critical phenomena in ferroelectric phase transitions of the order-disorder type.The static critical indices obtained so far agree well with those given by the mean field theory except in the extreme vicinity of the critical point. Recent observations on the critical x-ray scattering, the critical slowing-down and the anomalies in transport processes are surveyed and their significance is discussed qualitatively.

Two Different Mechanisms of the Curie-Weiss Dielectric Susceptibility in Ferroelectrics

The Curie-Weiss law of the dielectric susceptibility χ in ferroelectrics is shown to be derived from two different mechanisms in the self-consistent phonon approximation: One is the increase in the correlation between dipoles which is related to the magnitude of the electrostatic interaction D . The other is the decrease in the mean square amplitude of the soft mode, which leads to the Curie-Weiss law through the rate of increase 1/ t 0 of an individual susceptibility with the decreasing effective “dipole moment”. The dependence of χ on temperature has been numerically computed to investigate the dependence of the Curie constant on t 0 and D . The ratios of the paraelectric moment to the saturation moment have been evaluated from available experimental data in ferroelectrics. Deviations of these ratios from unity in displacive type ferroelectrics are explained in terms of t 0 .

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.

Two Different Mechanisms of the Curie-Weiss Dielectric Susceptibility in Displacive-Type Ferroelectrics

By the use of the analogy between the self-consistent phonon theory and the self-consistent renormalized spin fluctuation theory on itinerant electron ferromagnets, the Curie-Weiss susceptibility in displacive-type ferroelectrics is shown to consist of two different mechanisms, (a) the decrease in the mean square amplitude of the soft mode and (b) the increase in the correlation between dipoles. The ratio of the paraelectric moment to the saturated moment is calculated from experimental data in typical ferroelectrics. The great deviation of this ratio from unity in displacive-type ferroelectrics is shown to be attributed to the mechanism (a), and the appearance of the relaxation mode in dynamical spectra to (b).

Critical phenomena in ferroelectric phase transitions

The width of the critical region in ferroelectric phase transitions is discussed along the line of the Landau theory, referring to the recent development of the theory of the renormalization group. It is shown that the electrostatic dipole-dipole interaction results in a very narrow critical region even if other short range interactions are not negligible.

Internal modes and the local symmetry of PO4 tetrahedra in K(H1−xDx)2PO4 by Raman scattering

Abstract The v 3 and v 4 internal modes of PO 3− 4 tetrahedra are observable in KH 2 PO 4 even above the transition temperature in the scattering geometry x ( zz ) y contrary to the selection rule of the site symmetry S 4 in the D 2 d space group. Tominaga et al. discussed that the origin of this broken selection rule is due to the momentary site symmetry C 2 of these tetrahedra, and that the mechanism of the phase transition is of the order-disorder type of these distorted tetrahedra. Parallel studies compatible with their results have been carried out on mixed crystals K(H 1− x D x ) 2 PO 4 and the same conclusion has been obtained as to the momentary site symmetry. These internal modes increase in intensity with deuteration, and the origin of this increase is discussed.