Low band gap and ionic bonding with charge transfer, threshold in the polymeric lithium fulleride Li(4)C(60)

We demonstrate the growth of crystalline Li(4)C(60) films. The low-energy electron diffraction pattern of the films indicates the formation of polymer chains in the plane of the surface, consistent with the reported crystal structure. Electron energy loss and photoemission spectra identify the Li(4)C(60) polymer as a low band gap semiconductor, with a relatively strong coupling of electrons to low-frequency stretching modes of the polymer bonds and alkali phonons. No evidence is found for hybridization between the Li- and fullerene-derived electronic states. Instead, a partial charge transfer takes place, which is the same for different Li concentrations. This result rationalizes the stability of the polymer phase over a wide range of stoichiometries.