Michio Midorikawa

Ferroelectricity in Pb3(VO4)2 Crystal

Dielectric and pyroelectric measurements were carried out for Pb 3 (VO 4 ) 2 crystals. In the lowest temperature phase (the phase III) the pyroelectric charge was detected, and its sign changed according to the polarity of poling field applied beforehand to specimens. The dielectric hysteresis loops were also observed at 60 Hz. In this way it has turned out that the phase III is ferroelectric.

Optical and Dilatometric Studies of KCaCl3 and RbCaCl3 Crystals

Phase transitions in KCaCl 3 and RbCaCl 3 were investigated by means of optical and dilatometric measurements. It was suggested from these investigations that on heating from room temperature both crystals transform from the orthorhombic phase to the tetragonal phase and then to the cubic phase. This transition sequence is in contrast with the one observed in other perovskite trichlorides such as CsPbCl 3 , CsSrCl 3 and RbCdCl 3 which have two different orthorhombic phases.

Dilatometric and Pressure Studies of Phase Transitions in CsSrCl3

The dilatometric measurements for CsSrCl 3 by means of differential transformer, and measurements of pressure dependence of three transition points were carried out. It has turned out that the pressure coefficients of three transition points are 10.1, 11.0 and 9.2 deg/kb. The expressions for spontaneous strains in each phase and for pressure coefficients of the transition temperatures are phenomenologically derived.

Pressure Effect on the Ferroelectric Phase Transiton in Li2Ge7O15

The effect of hydrostatic pressure on the ferroelectric phase transition in Li 2 Ge 7 O 15 has been investigatied by the dielectric and Raman scattering measurements. The phase transition point T c shows a linear dependence whose pressure derivative is positive (d T c /d P =1.46 K/kbar). In the ferroelectric phase a soft mode has been observed which evolves from underdamped to overdamped behavior as the pressure is reduced from high pressure side to 0.7 kbar at 291.5 K.

Thermal Expansion and Pressure-Temperature Phase Diagrams of Pb3(VO4)2 and Pb3(PO4)2 Crystals

The experimental results of dilatation and the temperature-pressure phase diagrams for Pb 3 (VO 4 ) 2 and Pb 3 (PO 4 ) 2 crystals are presented. The pressure coefficients of transition temperatures between the trigonal and the nonpolar monoclinic phases d T 1 /d p and between the nonpolar and polar monoclinic phases d T 2 /d p for Pb 3 (VO 4 ) 2 were -12.1 and 20.2 deg/kbar, respectively. The intermediate monoclinic phase II of Pb 3 (VO 4 ) 2 vanishes at a triple point of 3 kbar and 65°C. The d T 1 /d p for Pb 3 (PO 4 ) 2 was -9.8 deg/kbar, but no triple point was observed. The entropy change estimated for Pb 3 (VO 4 ) 2 on the basis of obtained data suggests that the transition mechanism in this crystal is likely to be of the order-disorder type.

Group-Theoretical Analyses of the Structural Phase Transitions in Lead-Orthovanadate Pb3(VO4)2 Crystal

Successive phase transitions in lead-orthovanadate Pb 3 (VO 4 ) 2 are group-theoretically analyzed. The trigonal-to-monoclinic (nonpolar) transition is considered to be induced by an irreducible representation at the F point of the prototypic trigonal phase. The polar monoclinic phase is induced by simultaneous condensation of two representations at two different points both on the Brillouin zone boundary of the nonpolar monoclinic phase, which are located inside the Brillouin zone of the trigonal lattice. The reasons why the monoclinic (nonpolar)-to-monoclinic (polar) transition in Pb 3 (VO 4 ) 2 is not of the second order are pointed out.

Dielectric Properties of Rubidium Nitrate under Hydrostatic Pressure

The relative permittivity ( e r ) and dielectric conductivity (σ) of RbNO 3 single crystals were measured at 1 MHz under hydrostatic pressures over its two phase transformations; II-III (219°C at l aim) and III-IV (164°C at 1 atm). The value of e r has a large peak at the II-III phase transition pressure, and 1/ e r obeys the Curie-Weiss law on pressure in the phase III. This behavior of e r is elucidated by the phenomenological theory for the first order antiferroelectric phase transition. The II-III phase boundary was determined by the dielectric measurement. At the IV-III phase transition temperature ( T c ), the changes in e r and σ seen in the e r vs. T and σ vs. T curves get smaller with increasing applied-pressure. The behavior of e r and σ is explained by taking account of the rotation of NO 3 - ions being hindered by the pressure.

Effect of Hydrostatic Pressure on Ferroelectricity of KNO3.

The pressure dependences of spontaneous polarization and dielectric constant of KNO 3 single crystals were measured under hydrostatic pressure up to 3,000 kg/cm 2 . At 130°C, the spontaneous polarization is almost independent of pressure, keeping up the magnitude of 10.4 µC/cm 2 . The dielectric constant in paraelectric phase, phase I, obeys the Curie-Weiss law. The Curie constant C is, however, dependent on pressure. It can be expressed as C ( p )=5.63×10 3 -0.68×10 3 p (°K) with p in 10 3 kg/cm 2 . The temperature difference T c ( p )- T 0 ( p ) also depends on the pressure, and can be expressed as \begin{aligned} T_{c}(p)-T_{0}(p){=}89.7-11.2p(^{\circ}\text{K}) \end{aligned} These results are discussed from a phenomenological theoretical point of view.

Phase transitions in R2 SnCl6-type crystals (R = NH3 C2H3, NH3C2H5 and N(CH3)4)

Abstract The thermal dilatation in (NH3 ·CH3) SnCl6, (NH3 · C2H5) SnCl6 and [N(CH3)] SnCl6 was measured, and as the results it has turned out that (NH3 6·C2H5) SnCl6 and [N(CH3)4]2 SnCl6 undergo the first order transitions at 128 K and 158 K, respectively. The low temperature phases of (NH · C2H5) SnCl6 and [N(CH3)4]2 SnC16 are found to be monoclinic and tetragonal, respectively, No phase transition was observed in (NH3 ·CH3)2 SnCl6 down to 77 K.

Successive phase transitions in KMgCl3

Abstract Crystalline KMgCl 3 undergoes phase transitions at 296, 224, 212 and 142°C. Observations under a polarizing microscope showed that the transition sequence is the same as that known in CsPbCl 3 . The room temperature phase of KMgCl 3 is found to be monoclinic from the fact that the extinction position varies as temperature changes, though it has been reported to be orthorhombic.