Inelastic neutron scattering study of the external vibrations in single crystalC60

The external vibrations in solid ${\mathrm{C}}_{60}$ were studied by means of inelastic neutron scattering. The translational spectrum extends to 6 meV at room temperature and hardens by 5% on cooling below ${\mathit{T}}_{\mathit{s}}$. Narrow librational excitations were observed up to temperatures very close to ${\mathit{T}}_{\mathit{s}}$ at energies around 2.4, 3.6, and 4.6 meV. Calculations based on a van der Waals potential give about the correct frequency range for the translations, but frequencies considerably too low for the librations. Bond-charge models give better results only for an unrealistic choice of parameters.