Pressure dependence of the orientational ordering in solidC60

Pressure studies of the molecular orientational ordering in solid ${\mathrm{C}}_{60}$ have exhibited three principal effects: (1) The simple-cubic (sc) to face-centered-cubic (fcc) ordering transition temperature (${\mathit{T}}_{\mathit{c}}$=249 K) increases with pressure at a rate of 10.4±0.2 K/kbar corresponding to ln${\mathit{T}}_{\mathit{c}}$/d ln${\mathit{a}}_0$=-22.8, where ${\mathit{a}}_0$ is the lattice parameter; (2) pressure greatly reduces orientational fluctuations of the ${\mathrm{C}}_{60}$ 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.