PRESSURE-DEPENDENCE OF THE ORIENTATIONAL ORDERING IN SOLID C60

Pressure studies of the molecular orientational ordering in solid C60 have exhibited three principal effects: (1) The simple-cubic (sc) to face-centered-cubic (fcc) ordering transition temperature (T(c) = 249 K) increases with pressure at a rate of 10.4 +/- 0.2 K/kbar corresponding to dlnT(c)/dlna0 = -22.8, where a0 is the lattice parameter; (2) pressure greatly reduces orientational fluctuations of the C60 molecules in the ordered low-temperature sc phase; and (3) the results suggest the presence of two ordering transitions instead of one near 249 K. The implications of these results are discussed.