Toshihisa Yamaguchi

New Ferroelectric K2ZnBr4

The phase transition sequence of a K 2 ZnBr 4 crystal was investigated by DTA and dielectric measurements. It was found that the crystal undergoes at least two phase transitions, at -118°C and around 200°C, on heating. Ferroelectricity was observed below the lower transition point.

Time-dependent phase transition in K2CoBr4

The phase-sequence of K 2 CoBr 4 was investigated by measuring the time dependence of dielectric constant. It was revealed that the crystal undergoes a phase transition around 200°C and the high-temperature phase can be brought into the low-temperature region. The time-dependent recovering process from this metastable phase to the original low-temperature phase was observed. A ferroelectric-like behavior was detected.

Incommensurate-Commensurate Transition in K2ZnBr4 and K2CoBr4

The structural phase transitions of K 2 ZnBr 4 and K 2 CoBr 4 are investigated by means of X-ray scattering. The transition from the room-temperature α phase to the high-temperature β phase takes place at about 200°C. It is found that the β phase is an incommensurate one with the modulation wave vector q =(1/3-δ) c * , which is followed by a normal phase of Pmcn above 288°C (Zn-salt) or 282°C (Co-salt). If the β phase is supercooled to below room temperature, then the crystal is a commensurate phase with q =1/3 c * . With increasing temperature, the commensurate-incommensurate phase transition is observed at 5°C in Zn-salt. The transition sequence of normal, incommensurate and commensurate phases is quite common in A 2 X Y 4 -type incommensurate materials, though the α phase is very different from others.

Dielectric Properties of Ferroelectric Rb2CoCl4

Phase transition temperatures of single crystals of Rb 2 CoCl 4 prepared by the Bridgman method were checked by a DTA technique and a dielectric measurement and it was found that the Curie temperature T C is -74°C and the normal-incommensurate transition point T i is 21°C on heating. The thermal anomaly at the Curie temperature T C was observed by the DTA technique with a Ge–Ag thermocouple. The temperature dependence of the dielectric constants in the b and c directions was measured in a temperature range covering these phase transition points and the temperature dependence of P s and E c was determined from the observed 50 Hz D - E hysteresis loop, respectively. It was determined that the ferroelectric axis of Rb 2 CoCl 4 is the a axis in reality.

Experimental Studies on Phase Transitions in Ferroelectric {N(CH3)4}2ZnCl4

Thermal, dielectric and dilatational measurements were performed to study the phase transitions in ferroelectric {N(CH 3 ) 4 } 2 ZnCl 4 . The temperature dependence of triple D - E hysteresis loop in ferroelectric Phase III (3.3∼6.0°C, C 2 v 9 - P 2 1 c n ) was observed in detail. The crystal is paraelectric along the b and c directions. A remarkable contraction of the lattice is seen in Phase V (-112∼-92°C, C 2 h 5 - P 12 1 / c 1). Calculated pressure-coefficients of transition temperatures were compared with the measured ones.

Ferroelectricity in A 2 BX 4-type halide compounds

Abstract A review was given for our search for halide ferroelectrics with the chemical formula A 2 BX 4-type, having structure of β-K2SO4-type and Sr2GeS4-type.

Temperature Dependent Surface Images of BaTiO3 Observed by Atomic Force Microscopy

We used an atomic force microscope (AFM) to study the surface morphology of butterfly-like crystals of BaTiO3. Observations were made while varying the temperature from room temperature to 150°C and the annihilation process of the surface structure due to 90° domain boundaries was detected. The relationship between the surface gradient change at the domain boundaries and the tetragonal distortion was discussed.

Ferroelectricity in K2CoCl4

Single crystals of K 2 CoCl 4 grown from a melt by the Bridgman method undergo phase transitions at -131°C and 181°C, respectively, and show ferroelectricity along the a -axis in the phases below 181°C.

Dielectric and Dilatometric Studies on Ferroelectric {N(CH3)4}2CoCl4

Dielectric properties of ferroelectric {N(CH 3 ) 4 } 2 CoCl 4 were measured in detail, covering its all transition points. The triple D - E hysteresis loop in Phase III (4.6∼7.1°C, C 2 v 9 - P 2 1 c n ) was ascertained. The lattice distortion of this ferroelectric was determined along the three crystallographic axes by a differential-transformer type dilatometer. The pressure-coefficient of transition temperature was discussed.

Ferrielectricity in (NH4)2SO4, (NH4)2BeF4 and K2SeO4

Triple 60 Hz hysteresis loops have been observed in (NH 4 ) 2 SO 4 , (NH 4 ) 2 BeF 4 and K 2 SeO 4 crystals of the (NH 4 ) 2 SO 4 family. From these experimental results and other facts already known, these ferroelectrics are concluded to be really ferrielectric.