INTERMOLECULAR AND INTRAMOLECULAR PHONONS IN SOLID C60 - EFFECTS OF ORIENTATIONAL DISORDER AND PRESSURE

Lattice dynamics of solid C60 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 C60 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 C60 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 C60 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 cm-1/kbar. Intramolecular modes broaden or split, and shift toward higher frequencies at a rate of up to 0.88 cm-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.