John E. Fischer

Orientational phase transition in Na[sub [ital x]]C[sub 60] (1[lt][ital x][lt]3)

X-ray diffraction and calorimetry data on cubic Na[sub [ital x]]C[sub 60](1[lt][ital x][lt]3) reveal orientational ordering transitions at [ital T][sub [ital m]] above that of pure C[sub 60], e.g., [ital T][sub [ital m]]([ital x]=1.3)=325 K. The ordered phases are the same as in pure C[sub 60]: simple cubic, space group [ital Pa][bar 3], but the orientations in the disordered phase are more restricted. We explain how Na stabilizes the ordered phase to rather high [ital T], while K and Rb do not, in terms of Coulomb interactions between C[sub 60] molecules and and Na ions which we calculate from the local charge density of C[sub 60].

Rotations, Vibrations and Structure in Solid C60: Investigations by Neutron Scattering

In a series of experiments on solid C 60 , the low-energy rotational dynamics of the molecules, the higher-energy vibrational spectra, and aspects of disorder in the static structure, have been studied. The nearly spherical shape of the C 60 molecule, and the occurrence of an orientational phase transition at T c =256 K, make the C 60 solid an excellent system for investigations of orientational dynamics. Coherent quasi-elastic neutron scattering above T c is described quantitatively by a rotational diffusion model. Below T c , inelastic scattering froma soft librational mode is observed between 2 and 3 meV. Models of the strong, temperaturedependent diffuse scattering due to orientational disorder are discussed. Finally, measurements of the density of states for the intramolecular ( E ≥ 33 meV) vibrational modes are also presented.