FREE-ENERGY AND ORIENTATIONAL PHASE-TRANSITION IN SOLID C-60

On the basis of a microscopic model, the free energy of the orientationally disordered fcc crystalline phase of C60-fullerite is derived up to fourth order in the rotational density fluctuations. The order parameter variables are symmetry adapted rotator functions which belong to the l = 6 and l = 10 manifolds. They are basis functions of T2g representations of the cubic group. The phase transition to the Pa3 structure is discussed, and is found to be of first order. The orientationally ordered phase is described in terms of condensed rotator functions.