Intermolecular and intramolecular phonons in solidC60: Effects of orientational disorder and pressure

Lattice dynamics of solid ${\mathrm{C}}_{60}$ is studied using a unified interaction model which consists of a tight-binding potential for the intramolecular interaction and a Lennard-Jones and bond-charge model for the intermolecular interaction. Phonon dispersion and density of states of solid ${\mathrm{C}}_{60}$ are calculated in the energy range from 0 to 210 meV. The intermolecular-phonon density of states shows peaks around 2.3 and 3.7 meV, and extends to 7.6 meV. Intermolecular phonons and intramolecular vibrational modes are well separated by a gap of nearly 22 meV. The intermolecular-phonon modes show strong dispersion, so do the intramolecular modes, especially those with energy below 70 meV. The calculated phonon spectrum agrees very well with recent inelastic neutron-scattering experiments. The effects of orientational disorder and pressure on the intermolecular and intramolecular phonons of solid ${\mathrm{C}}_{60}$ are investigated. Intermolecular-phonon modes are sensitive to orientational disordering, and orientational disordering softens libron modes. The pressure dependence of the phonon dispersion and density of states is studied for solid ${\mathrm{C}}_{60}$ in the range of pressures from 0 to 56 kbar. The intermolecular-phonon spectrum shows strong broadening when pressure is applied. The libron modes shift to higher frequencies with a rate of about 0.4 ${\mathrm{cm}}^{\mathrm{-}1}$/kbar. Intramolecular modes broaden or split, and shift toward higher frequencies at a rate of up to 0.88 ${\mathrm{cm}}^{\mathrm{-}1}$/kbar. Most Raman and infrared active modes show strong pressure dependence. Intermolecular- and intramolecular-phonon spectra at various pressures are presented and results are compared with the available experiments.