Neutron spectroscopy of phonons in stage-1 rubidium-intercalated graphite

Neutron scattering measurements have been carried out on the graphite intercalation compound RbC₈. Phonon dispersion curves were measured for c-axis longitudinal modes and for some transverse branches with wave vectors in the basal plane and polarization out of plane. For the latter, there is a strong deviation of the dispersion relation from that of pure graphite. In addition, a partial phonon density of states was determined for intercalate (Rb) vibrations by in-basal-plane inelastic scattering. Our analysis shows that a two-dimensional (2D) Born-von K\'arm\'an model with nearest-neighbor Rb-Rb force constants cannot account for the intercalate mode spectrum. A three-dimensional model in which the Rb-Rb forces are due to an unscreened Coulomb interaction agrees well with the data, implying that conduction-electron screening of the Rb-ion motions is substantially reduced in RbC₈ relative to Rb metal.