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-trans mitting medium. Reflections corresponding to the simple cubic (s.c.) p hase of C60 are identified at 2.5 GPa. The powder data agree well with calculated profiles for an ordered structure based on Pa3BAR symmetry and are consistent with a long-range orientational ordering transitio n under pressure identical to that occurring at 255 K and ambient pres sure; the onset of the transition occurs below 0.5 GPa.