ROTATIONAL-DYNAMICS OF SOLID C-70 - A NEUTRON-SCATTERING STUDY

We report the results of neutron-diffraction and low-energy neutron-in elastic-scattering experiments on high-purity solid C-70 between 10 an d 640 K. Thermal hysteresis effects are found to accompany structural changes both on cooling and on heating. The observed diffuse scatterin g intensity does not change with temperature. At 10 K broad librationa l peaks are observed at 1.82(16) meV [full width at half maximum=1.8(5 ) meV]. The peaks soften and broaden further with increasing temperatu re. At and above room temperature, they collapse into a single quasiel astic line. At 300 K, the diffusive reorientational motion appears to be somewhat anisotropic, becoming less so with increasing temperature. An isotropic rotational diffusion model, in which the motions of adja cent molecules are uncorrelated, describes well the results at 525 K. The temperature dependence of the rotational diffusion constants is co nsistent with a thermally activated process having an activation energ y of 32(7) meV.