Satoshi Nanamatsu

Crystallographic and Dielectric Properties of Ferroelectric A2B2O7 (A=Sr, B=Ta, Nb) Crystals and Their Solid Solutions

Single crystals of Sr 2 Ta 2 O 7 and Sr 2 Nb 2 O 7 are grown by a floating zone technique. Strontium tantalate belongs to D 2 h 14 or D 2 h 17 space group at room temperature. Ferroelectric phase transition temperature is -107°C. The Curie-Weiss temperature θ 0 and the Curie constant C P are: θ 0 =161 K(=-112°C) and C P =0.84×10 5 K. Spontaneous polarization and coercive field are 1.9 µC/cm 2 and 0.4 kV/cm at -190°C, respectively. Strontium niobate belongs to C 2υ 12 space group at room temperature. Its ferroelectric phase transition occurs at T c =1342±2°C, with θ 0 = T c and C P =1.2×10 5 K, and second phase transition occurs at -156°C. Solid solution Sr 2 (Ta 1- x ·Nb x ) 2 O 7 exists in the entire composition range and its ferroelectric phase transition temperature increases markedly from -107°C to 1342°C, according to the increase of x . Phase transition temperatures of other solid solutions, partially replacing Sr by Ca, Pb or Ba, are also studied on ceramic samples.

A new piezoelectric crystal: Ba2Ge2TiO8

A Ba2Ge2TiO8 single crystal is synthesized by the Bridgman method. The space group of Ba2Ge2TiO8 is determined to be C2v21‐Iba2, where the lattice constants a, b, and c are 12.30, 135.2, and 10.70 A, respectively. The crystal includes a wedge‐shaped domain structure which resembles those observed in Gd2(MoO4)3 and Rochelle salt. Both X and Y plates have a large electromechanical coupling factor in a thickness shear mode, k15 = 0.27 and k24 = 0.31.

Ferroelastic behavior in Ba2Ge2TiO8

A Ba2Ge2TiO8 crystal is found to exhibit ferroelastic behavior. Optical, thermal, and elastic studies indicate that the phase transition occurs at about 810 °C. The wedge‐shaped twin domains disappear above the phase transition temperature. The twinned crystal is detwinned by an exchange of orthorhombic axes a and b under the application of a compressive stress of 250 kg/cm2 at room temperature and of 100 kg/cm2 at 700 °C.