Lattice dynamics of stage-2 alkali intercalates in graphite

The Debye-Waller factor and the phonon dispersion curves are evaluated for stage-2 Rb and Cs intercalates in graphite. The calculations use a density functional approach to construct the effective force constants within an intercalate sheet from its measured liquid structure factor at room temperature and to account for anharmonicity. The sheet is first treated as a two-dimensional system floating inside a smooth transverse confinement: this model is shown to yield a gross underestimate of the mechanical stiffness within the sheet. Pinning by the graphite hounding layers is next included phenomenologically. The calculated phonon dispersion curves in the intercalate are then compatible with measurements of its phonon density of states.