Neutron scattering and μsr studies of fullerenes and their derivatives

The results of recent neutron scattering and μSR studies on C₆₀and C₇₀fullerenes and their derivatives in the solid state are briefly reviewed. Particular attention is paid to the temperature evolution of the rotational dynamics and the phase transitions accompanying orientational ordering. The potential of muons as spin labels both inside and outside the fullerene cage is also explored. Solid C₆₀shows a first-order phase transition at 260 K from a face-centred to a simple cubic structure. The isotropic molecular reorientations change abruptly to quasi-random jump motion between nearly-degenerate orientations differing in energy by 11.4(3) meV. A transition to an orientational glass state occurs at 85 K. Static disorder dominates the crystal chemistry of C₇₀. The orientational ordering transitions from the high temperature face-centred cubic structure to the low temperature rhombohedral and monoclinic structures are accompanied by severe hysteresis effects. The rotational dynamic behaviour changes progressively on cooling from isotropic to anisotropic to uniaxial reorientations about the unique molecular axis. High pressure diffraction experiments reveal that C₇₀has a smaller compressibility than C₆₀. Disorder effects are also present well in the superconducting state for the potassium fulleride K₃C₆₀. Small-amplitude librational motion of the C^3-₆₀units is evident up to 650 K.