MOLECULAR-DYNAMICS INVESTIGATION OF ORIENTATIONAL FREEZING IN SOLID C-60

Constant-pressure molecular-dynamics calculations have been carried out to investigate the reorientational behaviour of C60 molecules in the solid phase. Under ambient conditions, an intermolecular potential based on pairwise-additive atom-atom interactions yields a stable fcc crystal in which the C60 molecules are found to undergo rotational diffusion. On cooling to 200 K the molecules freeze into an orientational ordered structure with tetragonal symmetry. Although the presence of a phase transition just below room temperature agrees well with calorimetric, x-ray, and NMR data, the predicted low-temperature structure does not. Possible reasons for this are discussed. The orientational relaxation time and phonon density of states are calculated at several temperatures. The former is in fair agreement with NMR data.