Abstract
New functional applications of some ferroelastic crystals are expected in the optoelectronics and related fields. Thermal phase transition of ferroelastic Ba2TiGe2O8 has been investigated with polarized microscopy and the X-ray diffraction method at high temperatures. Ba2TiGe208 undergoes phase transtion at about 870°C from the ortho rhombic low-temperature form (mm2) into tetragonal high-temperature form (4mm). At 550°C, stress-strain hysteresis loop was observed and it was confirmed that Ba2TiGe2O8 is a ferroelastic crystal belonging to the species 4 mmFmm 2. The birefringences perpendicular to a, b and c axes of the orthorhombic low-temperature form were measured versus temperature: the crystals become smoothly uniaxial at about 870°C.
The average stru cture of Ba2TiGe2O8 at room temperature was determined to elucidate the mechanism of thermal phase transition and the propagation of the twin planes. The structure is built up of [Ge2O7]6- double groups linked together with TiO5 square pyramids to give fiat sheets perpendicular to the c-axis. These flat sheets are bonded together by the Ba2+ ions. There exist two independent [Ge2O7]6- double groups. The observed electron density distribu-, tion strongly suggests that in a half of [Ge2O7]6- group the bridging Ge-O-Ge bond angles on the (001) projection deviate about 10° from 180°, yielding two possible orientations for these groups. A long period ordering of the orientation of the groups appears to be responsible for the long period superstructure of the low-temperature form.
