GROUND-STATE AND PHASE-TRANSITIONS IN SOLID C-60

A simple model is developed to describe the intermolecular interactions in solid C60. The model predicts correctly the observed ground-state structure Pa3BAR and the first-order transition to the high-temperature fcc phase. The calculated transition temperature T(c) approximately 270 K and its pressure dependence dT(c)/dP = 11.5 K/kbar agree very well with recent experiments. Below T(c), there exist nearly degenerate orientations which are separated by potential barriers of order approximately 300 meV, leading to a glassy behavior with T(g) approximately 90-130 K. It is suggested that similar orientational disorder exists in K3C60 and other fullerides.