Shozo Sawada

Phase transitions in K2ZnBr4 and K2CoBr4

Abstract The phase transition sequences in K2ZnBr4 and K2 CoBr4 were investigated by means of dielectric constant measurements and DTA (Differential Thermal Analysis).

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.

Phase transitions at low temperatures in A2BX4-type halide substances

The temperature and frequency dependences of the dielectric constants of A2BX4-type halide dielectrics, K2ZnBr4, K2CoBr4, K2ZnI4, Rb, MnI4, Rb2ZnI4 and Rb2CoI4, all of which take crystal structure of Sr:GeS4-type at room temperature, were investigated. It was found that there is a common nature in the temperature dependence of the dielectric constants, namely, all substances show a ferroelectriclike dielectric anomaly with X-type shape below room temperature but this is followed by a stepwise anomaly with frequency dispersion, which occurs at a little lower temperature than the λ-type dielectric anomaly.

Dielectric properties of Cs2BX4 (B=Zn, Co; X=Cl, Br, I)

Abstract Crystal growth was carried out by the Bridgman method for six substances of the Cs2 BX 4-type with β-K2SO4 structure, where B=Zn, Co and X=Cl, Br, I. The dielectric constants were measured to investigate their phase transitions. It has been shown that Cs2ZnI4 and Cs2CoI4 exhibit clear first order transitions at 93K and 51K, respectively, whereas the other four substances exhibit a broad dielectric anomaly at low temperatures.

Dielectric properties of ferroelectric Rb2CoBr4

Abstract Single crystals of Rb2 CoBr4 were grown by the Bridgman method. DTA and the dielectric constant measurements revealed that the normal-incommensurate transition point T i is 60°C and the lock-in transition (Curie temperature T c) is—81°C (on heating). The temperature dependence of spontaneous polarization P s, coercive field E c, and antiferroelectric polarization P AF were measured for a single-crystal sample down to liquid nitrogen temperature.

Phase transitions in Rb2 BI4 (B=Mn, Fe)

Abstract We performed crystal growth of two iodide compounds, Rb2FeI4 and Rb2MnI4 by Bridgman method. In Rb2FeI4, a first-order transition point around 400K was found. In Rb2MnI4 other than the α–β transition around 500K already reported, one phase transition point at 105K in the a-modification and two phase transition points at 190K and 515K in the β-modification were found.

AFM observation of domains of Wo3

Abstract The surface morphology of WO3 crystal was investigated by atomic force micro-scope(AFM). Three types of surface images due to the orthorhombic, monoclinic and triclinic distortions were obtained and crystallographic data such as orthorhombic axial ratio, monoclinic and triclinic angles were estimated from the images.

Time dependent phase transition in K2ZnBr4

Abstract K2ZnBr4 undergoes the first order phase transition at around 470 K from the low temperature α-state with Sr2GeS4 structure to the high temperature β-state with β-K2SO4 structure. The metastable β-state was realized at low temperatures by rapid cooling and the recovering process toward the essentially stable α phase was investigated by measuring dielectric constants as a function of time.

Piezoelectric constants of ferroelectric Li2Ge7O15

Abstract The temperature dependences of the piezoelectric constants d 31, d 32 and d 33 of ferroelectric lithium heptagermanate Li2Ge7O15 were measured by a quasi-static method. All three constants show marked anomalies at the ferroelectric Curie point of 283.5K. The relation among the piezoelectric constant, dielectric constant and spontaneous polarization was discussed.

Dielectric properties of Tl2ZnCl4

Abstract The differential thermal analysis and the dielectric constant measurement were performed for the Tl2ZnCl4 crystals prepared by Bridgman method. It is found that the crystal have two phase transition points around 90°C and-100°C, respectively. Furthermore, a ferroelectric D-E hysteresis loop was observed in the temperature region below −75°C.