Phonons in LiC6 and in heavy alkali metal-graphite intercalation compounds

The authors measured the longitudinal (00q) and transverse (q00) modes in LiC6 by inelastic neutron scattering, including the observation of a very high energy phonon branch, tentatively assigned to an optic mode. Analysis of the longitudinal (00q) modes by a one dimensional shell model yields force constants which are considerably larger than those of the heavy alkali-metal stage 1 compounds. Also, the interlayer shear constant C44 is found to be distinctly larger than in the other stage 1 compounds. Yet, LiC6 still shows an ω approx. q2 dispersion of the transverse branch characteristic for layered materials. Low temperature measurements on KC24 revealed a mode splitting in the longitudinal acoustic (00q) branch, which persists at temperatures above the alkaki order-disorder phase transition at 123 K. The splitting may be caused by an Einstein-like in-band mode, which seems also to hybridize the (q00) transverse branch. (Auth.)