Rotational dynamics of solid C₇₀ monitored by positive muon spin labels

We report the results of an avoided-level-crossing muon spin resonance study of a muonated radical of C₇₀ between 73 and 310 K. The temperature-dependent averaging of the anisotropic hyperfine interaction of the polarized muon spin label is interpreted in terms of the reorientational dynamics of the fullerene. Uniaxial rotation about the long molecular axis appears likely even at 73 K. Reorientational disorder then sets in gradually at 160 K with a small anisotropy persisting across the phase transition in the vicinity of 270 K into the plastic phase.