SolidC70: A molecular-dynamics study of the structure and orientational ordering

Classical constant-pressure molecular-dynamics simulations have been performed for solid ${\mathrm{C}}_{70}$ using a pairwise-additive atom-atom Lennard-Jones potential that gave a good account of the high-temperature rotator phase of solid ${\mathrm{C}}_{60}$. The simulation results indicate that freezing of the rotational motion of ${\mathrm{C}}_{70}$ molecules about the long and short axes occurs at different temperatures. At high temperatures, an orientationally disordered fcc phase is preferred over a hcp lattice. At low temperature, a triclinic phase seems to be favored by the present model, but other structures (space group Pa3, and hcp) also have very competitive potential energies. Estimates of the molecular-reorientation times are given. The librational phonon density of states has been studied. As expected, libration frequencies for rotation about the long axis are lower than that about the short axis.