Pressure-induced orientational order in C₆₀ 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 Pa3̄ 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.