Lattice dynamics of first-stage alkali metal graphite intercalation compounds

Phonon dispersion relations for first-stage alkali metal graphite intercalation compounds C₈M (M = K, Rb) are calculated by using the Born-von Kármán force constant model. The characteristic effect of intercalation is revealed by comparing the phonon dispersion relation for C₈M with that for graphite folded into the Brillouin zone of C₈M. Among fifty-four normal modes for C₈M, forty-eight modes are associated with vibrations of carbons and exhibit a definite correspondence with the folded dispersion curves for graphite, The remaining six optical modes, which originate from vibrations of alkali metal atoms, yield a significant perturbation to acoustic modes. Experimental results of the Raman scattering for C₈M are discussed on the basis of the dispersion relations obtained.