Neutron scattering study of the high-energy graphitic phonons in superconducting CaC6

We present the results of a neutron scattering study of the high-energy phonons in the superconducting graphite intercalation compound CaC6. The study was designed to address hitherto unexplored aspects of the lattice dynamics in CaC6, and, in particular, any renormalization of the out-of-plane and in-plane graphitic phonon modes. We present a detailed comparison between the data and the results of density-functional theory (DFT). A description is given of the analysis methods developed to account for the highly textured nature of the samples. The DFT calculations are shown to provide a good description of the general features of the experimental data. This is significant in light of a number of striking disagreements in the literature between other experiments and DFT on CaC6. The results presented here demonstrate that the disagreements are not due to any large inaccuracies in the calculated phonon frequencies.