PHASE-TRANSITION STUDY OF C-70 CRYSTAL AT AROUND 340-K BY SINGLE-CRYSTAL X-RAY-DIFFRACTION

Structural changes of C-70 crystals were investigated in the temperatu re range between 295 K and 575 K by a single-crystal X-ray diffraction using a precession camera. The crystal structure is based on a deform ed hcp with axial ratio c/a=1.85 at room temperature showing a remarka ble stacking fault along the c axis. Contraction of the c axis and a s light expansion of the a axis take place at the temperature around 340 K, and as a result, the structure becomes close to an ideal hcp struc ture with c/a=1.65. Hysteresis of a wide temperature range is also obs erved with the change of lattice constants. During phase transition, t winning by(-1 0 2) often occurs by the combination of 90 degrees rotat ion and shuffling of C-70 ellipsoid molecules. The mechanism of the tw in formation is also discussed.