Pressure-Induced Orientational Order in C 60 at 300 K

The pressure dependence of the structure of the pure fullerene C60 has been studied by high-resolution synchrotron X-ray powder diffraction at 300 K in a diamond-anvil cell with fluid helium as a pressure-transmitting medium. Reflections corresponding to the simple cubic (s.c.) phase of C60 are identified at 2.5 GPa. The powder data agree well with calculated profiles for an ordered structure based on Pa symmetry and are consistent with a long-range orientational ordering transition under pressure identical to that occurring at 255 K and ambient pressure; the onset of the transition occurs below 0.5 GPa.