Neutron scattering studies of fullerenes

At low temperature, crystalline C60 adopts a simple cubic structure whose stability is driven by optimisation of the intermolecular electrostatic interactions. Above 85 K, molecular motion is no longer frozen and the molecules shuffle between nearly-degenerate orientations, differing in energy by 11.4(3) meV. At 260 K, a first-order phase transition leads to a face-centered cubic structure, characterised by rapid isotropic molecular reorientational motion. In C70, orientational ordering occurs near 270 K. The phonon spectra of C60, superconducting K3C60 and saturation-doped Rb6C60 measured by inelastic neutron scattering in the energy range 0-200meV reveal substantial broadening in the superconducting fulleride of five-fold degenerate Hg intramolecular modes both in the low-energy radial and the high-energy tangential part of the spectrum. This confirms the predictions of models based on intramolecular phonon-mediated pairing. The electron-phonon coupling strength is distributed over a wide range of energies (33-195 meV) as a result of the finite curvature of the fullerene spherical cage. No evidence is found for any significant electron-librational coupling. (author). 22 refs, 3 figs