OPTICAL AND X-RAY STUDIES ON THE LOW-TEMPERATURE PHASE-TRANSITION OF BA2NANB5O15

Low-temperature phase transitions of Ba2NaNb5O15 with non-stoichiometr ic composition were investigated using a polarization microscope and x -ray diffractometer. Both apparatuses were equipped with a specially d esigned low-temperature cryostat which enabled us to measure the biref ringence down to 4 K and x-ray reflections down to 20 K. Ferroelastic domain structure was observed to persist even at 4 K. Birefringence De lta n(ab) and pure shear x(12) show a peak at about 200 K and drop ste eply at 110 K but do not vanish below 110 K. These facts indicate that the phase below 110 K is not re-entrant to the tetragonal 4 mm phase. The quite long relaxation process observed in birefringence and the a ppearance of a new reflection in the x-ray diffraction experiment indi cate that the quasi-tetragonal region appears at 110 K and develops wi th long relaxation time.