Comparative study of the phonons in nonsuperconducting BaC(6) and superconducting CaC(6) using inelastic x-ray scattering.

The low-energy phonons of two different graphite intercalation compounds (GICs) have been measured as a function of temperature using inelastic x-ray scattering (IXS). In the case of the non-superconductor BaC{sub 6}, the phonons observed are significantly higher (up to 20%) in energy than those predicted by theory, in contrast to the reasonable agreement found in superconducting CaC{sub 6}. Additional IXS intensity is observed below 15 meV in both BaC{sub 6} and CaC{sub 6}. It has been previously suggested that this additional inelastic intensity may arise from defect or vacancy modes not predicted by theory [d Astuto et al., Phys. Rev. B 81 104519 (2010)]. Here it is shown that this additional intensity can arise directly from the polycrystalline nature of the available samples. Our results show that future theoretical work is required to understand the relationship between the crystal structure, the phonons, and the superconductivity in GICs.