STRUCTURAL CHANGES IN SUCCESSIVE PHASE TRANSITIONS OF PYRIDINIUM TETRAFLUOROBORATE Takashi Nemoto, Yuji Ohashi, Minoru Hanaya and Masaharu Oguni, Department of Chemistry, Tokyo Institute of Technology, Meguro-ku, Tokyo 152, Japan

Using a new diffractometer with the imaging plate, the structural changes of the successive phase transitions have been observed, although the diffraction spots were considerably splitted after the phase transitions.

The PyHBF₄ crystal undergoes first-order phase transitions at 205.2 and 238.9K (Fig. 1.). The significant changes of permitivity were observed and the diffraction spots became broad and splitted after the phase transitions. The intensity data were collected using IP-Weissenberg type diffractometer. Since it was impossible to collect the splitted intensities, the new data reduction programs for the splitted spots have been developed.

The analyzed structures at three phases I, II and III indicate that both the pyridinium ion and the tetrafluoroborate ion are on the 3 symmetry at phase I (Fig. 2.), the inversion centers on molecules disappear at phase II, and the a and b axes become double at phase III.