FULLERENES AND FULLERIDES IN THE SOLID-STATE - NEUTRON-SCATTERING STUDIES

Fullerene-60 shows a high molecular symmetry, consistent with the icosahedral point group I(h). At low temperature in the solid state, high-resolution powder neutron diffraction reveals that crystalline C60 (local symmetry S6) adopts a simple cubic crystal structure whose stability is driven by optimization of the intermolecular electrostatic interactions. Above 90 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-centred cubic structure, characterised by rapid isotropic reorientational motion of the molecules. The phonon spectra of pristine fullerene, superconducting K3C60 and saturation-doped Rb6C60 measured by inelastic neutron scattering in the energy range 20-2000 cm-1, reveal substantial broadening of fivefold degenerate H(g) intramolecular vibrational modes both in the low-energy radial and the high-energy tangential part of the spectrum. This provides strong evidence for a traditional phonon-mediated mechanism of superconductivity in the fullerides but with an electron-phonon coupling strength distributed over a wide range of energies (33-195 meV) as a result of the finite curvature of the fullerene spherical cage.