At low temperature, crystalline C 60 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-centred cubic structure, characterised by rapid isotropic molecular reorientational motion. In C 70, orientational ordering occurs near 270 K. The phonon spectra of C 60, superconducting K 3 C 60 and saturation-doped Rb 6 C 60 measured by inelastic neutron scattering in the energy range 0–200meV reveal substantial broadening in the superconducting fulleride of five-fold degenerate H g 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.
Inelastic neutron scattering studies of phonon spectra, and simulations of pressure-induced amorphization in tungstatesAWO4(A=Ba,Sr,Ca, andPb)
