Lattice dynamics in second-stage bromine-graphite intercalation compound

We have studied by coherent inelastic neutron scattering the phonon spectrum of a second-stage bromine-graphite intercalation compound. The intercalate sample was obtained from highly oriented pyrolytic graphite. We have observed the almost complete low-energy ($c$ direction as well as in the layer plane. We have observed three branches polarized along the $c$ axis which present energy gaps along the $c$ direction. The acoustic branch propagating in the layer plane contrary to the optic ones is very sensitive to the intercalate. The main character of the modes polarized in the layer plane is the dispersionless optic branch along the $c$ direction at 1 THz. A phenomenological Born—von Kármán force-constant model with nonaxial symmetry, developed in order to interpret our results, explains them very well.