Effect of pressure on intermolecular and intramolecular phonons in solid C60

Abstract

The effect of pressure on inter- and intramolecular phonons in solid C60 is studied using a unified model which consists of a tight-binding potential for the intramolecular interaction and a Lennard-Jones and bond charge model for the intermolecular interaction. At various pressures (up to 56 kbar), the phonon dispersion and density of states of solid C60 are calculated in the energy range from 0 to 210 meV. At zero pressure, the intermolecular phonon density of states shows peaks around 2.3 and 3.7 meV, and extends to 7.6 meV. Not only the intermolecular phonon modes but also the intramolecular modes show significant dispersions, especially those modes with energy below 70 meV. Under pressure, the intermolecular phonon spectrum shows strong broadening. The libron modes shift to higher frequencies at a rate of 0.40 cm−1/kbar. The effect of pressure on intramolecular modes manifests in two ways: (i) the lower-frequency modes broaden and shift toward higher energies, and (ii) the higher-frequency modes split and shift upward in energy. Intramolecular modes shift to higher energy at a rate up to 0.88 cm−1/kbar. Most Raman and infrared active modes show strong pressure dependence. Inter- and intramolecular phonon spectra at various pressures are presented and results are compared with experiments.