  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 BaC6, 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
CaC6. Additional IXS intensity is observed below 15 meV in both BaC6 and CaC6.
It has been previously suggested that this additional inelastic intensity may
arise from defect or vacancy modes in these compounds, unpredicted 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 large disorder 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.
