Phonons in LiC6

We have measured the longitudinal [001] and the out-of-plane transverse [100] phonon modes of Li${\mathrm{C}}_6$ by inelastic neutron scattering, including the observation of very-high-energy phonon groups, tentatively assigned to optic modes. The phonon branches yield interlayer force constants and elastic moduli $C_{33}$ and $C_{44}$ which are considerably larger than those of the heavy alkali-metal stage-1 compounds. Yet, Li${\mathrm{C}}_6$ still shows an $\omega \sim q^2$ dispersion of the transverse basal-plane mode, characteristic for layered materials.