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, fcc 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 K associated with degrees of molecular orientations for the hexagonal lattice. The fcc 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.