Tsutomu Yamakami

Structural Phase Transition of Layer Compound (C2H5NH3)2FeCl4

The sequence of the structural phase transitions of the layer compound (C 2 H 5 NH 3 ) 3 FeCl 4 has been determined with the ultrasonic, dielectric and polarized microscope measurements as following; Phase I ( D 4 h 17 )–(378.8 K)–Phase II ( D 2 h 18 )–(203.5 K)–Phase III–(133.7 K)–Phase IV. The set of the critical exponents obtained with ultrasonic methods are explained by the results of the renormalization group theory on the three dimensional XY model. The ferroelastic domains are observed in the phase II and phse III. The anomaly of he dielectric constant is found near the II-III phase transition point.

Specific heat study of ferroelectric CsH2PO4 and CsD2PO4

For ferroelectric CsH2PO4 and CsD2PO4 the specific heat has been measured using an AC calorimeter. The singularity of the specific heat at the phase transition point is logarithmic. The most characteristic feature of the behaviour is the existence of an anomalous specific heat over the wide temperature region from Tc-60K to Tc+60K. This anomalous specific heat is considered to be due to the one-dimensional fluctuation of the polarisation in these materials. The authors also compare the experimental results with the pseudo-one-dimensional Ising model.

Measurement of the dielectric relaxation of ferroelectric substances by lumped-capacitor time-domain spectroscopy

The dielectric relaxation of ferroelectric CsH2PO4 was measured by lumped-capacitor time-domain spectroscopy in both the paraelectric and ferroelectric phases in the temperature range from 144.8 K to 182.2 K. The time-domain responses from the sample were subjected to time-domain and frequency-domain analyses and the dielectric relaxation parameters were calculated. The results agree with those of earlier studies using frequency-domain measuring methods.

On the Structural Phase Transitions in (C2H5NH3)2FeCl4. I. Brillouin Scattering

The successive phase transitions in the layered perovskite compound (C 2 H 5 NH 3 ) 2 FeCl 4 have been investigated through Brillouin scattering. The I-II phase transition is characterized by a large dynamical critical behavior and a strong Landau-Khalatnikov contribution to the C 66 elastic constant. In the phases III, IV and V non-critical acoustic dispersion has been observed on the C 11 elastic constant. The present results suggest a tetragonal-orthorhombic-orthorhombic phase sequence for the I, II and III phases.

Light scattering studies on the phase transitions in [N(CH3)4]2ZnCl4

Abstract The successive phase transitions in [N(CH3)4]2ZnCl4 have been investigated through Brillouin and Raman scattering. Frequency dispersion of the sound velocity has been found in the incommensurate phase for the [001] La-phonon and the relaxation time is determined as τ = 2.2 × 10−10/(T 4−T) sec with T 1 = 296 K. Neither soft mode nor critical relaxation mode has been observed in the incommensurate phase by Raman scattering.

Study of Ferroelectric CsH2PO4 by the Ultrasonic Velocity and Attenuation Measurements

The velocity V and the attenuation α of the longitudinal ultrasonic waves have been measured accurately near the ferroelectric phase transition temperature. Critical anomalies in the paraelectric phase were represented by Δ V ∼log ( T - T 0 ) and \(\sqrt{\varDelta\alpha}{\sim}\log(T{-}T_{0})\), where T 0 was determined from the simultaneously measured dielectric constant e = C /( T - T 0 ). It was also found that the energy relaxation time τ E varied as τ E ∼( T - T 0 ) -1 . This type of the critical behavior is the same as that of the usual ferroelectrics. Thus it is concluded that the dipole-dipole interaction is dominant in the critical anomaly of the ultrasonic waves of CDP. Analysis of the attenualion based on the dynamical scaling law is also presented.

Brillouin and Raman scattering in [N(CH3)4]2ZnCl4

The normal-incommensurate-commensurate phase transitions of [N(CH3)4]2ZnCl4 have been extensively studied through Brillouin and Raman scattering. Frequency dispersion of sound velocity of the [001] LA-mode has been confirmed in a frequency range of between 1 GHz and 10 GHz in the incommensurate phase. The relaxation time is characterized by [image omitted] and we obtain τ0 = 0.74 × 10-12 sec for T1 = 296 K.Low frequency light scattering spectra below 240 cm-1 have been carefully examined by Brillouin and Raman scattering. Neither soft mode nor critical relaxation mode has been observed in the light scattering spectra in the incommensurate phase.