  The two-dimensional polymer structure and lattice dynamics of the superionic
conductor Li4 C60 are investigated by neutron diffraction and spectroscopy. The
peculiar bonding architecture of this compound is definitely confirmed through
the precise localisation of the carbon atoms involved in the intermolecular
bonds. The spectral features of this phase are revealed through ab-initio
lattice dynamics calculations and inelastic neutron scattering experiments. The
neutron observables are found to be in very good agreement with the simulations
which predict a partial charge transfer from the Li atoms to the C60 cage. The
absence of a well defined band associated to one category of the Li atoms in
the experimental spectrum suggests that this species is not ordered even at the
lowest temperatures. The calculations predict an unstable Li sublattice at a
temperature of 200 K, that we relate to the large ionic diffusivity of this
system. This specificity is discussed in terms of coupling between the low
frequency optic modes of the Li ions to the soft structure of the polymer.
