Structure and phase transition of fullerene C76 crystal studied by x-ray scattering

Synchrotron x-ray powder diffraction experiments of a fullerene C-76 crystal have elucidated its structure and successive phase transitions for the first time. Two types of crystal lattices, Ece and hexagonal ones, coexist through the temperature range investigated between 10 K and 550 K. Both precise analysis of diffraction patterns as a function of temperature and structural model calculations reveal successive phase transitions at 140 +/- 10 K and 420 +/- 10 It associated with degrees of molecular orientations for the hexagonal lattice. The fee lattice undergoes a similar phase transition at 140 +/- 10 K. By applying pressure and analyzing lattice constants we also study compression and pressure-temperature phase diagram of this crystal.