Inelastic neutron scattering study of the intermolecular vibrational modes of Ba4C60

We present inelastic neutron scattering measurements for the Ba4C60 superconductor. The translational part of the phonon density-of-states is fairly broad compared to that of A3C60 (A=K, Rb) and extends to ∼17 meV. A lattice dynamical analysis using a phenomenological Born von Kármán model indicates stronger interactions between the intercalated Ba ions and the C60 anions than those in alkali metal fullerides. The results are in accordance with the strong hybridisation of the Ba 5d and C60 t1g orbitals proposed by synchrotron X-ray diffraction measurements and ab initio calculations.