STRUCTURE, DYNAMICS AND ORDERING TRANSITION OF SOLID C-60

Solid C60 forms a face-centered-cubic structure at room temperature, in which the molecules exhibit dynamic orientational disorder. At 255 K the molecules develop orientational order via a first order phase transition, lowering the space group symmetry to Pa3BAR in a simple cubic lattice. Even in the ordered phase, short-range disorder persists to the lowest temperatures. The Pa3BAR structure is stabilized by both Lennard-Jones atomic pair potentials and by Coulombic forces arising from excess charge in the double bonds. The low temperature dynamics are primarily characterized by jump rotational diffusion between equivalent orientations superimposed on small amplitude librational oscillations. The behavior at the phase transition is strongly influenced by trace impurities and solvent. Similar effects are seen in C60O and in C70.