Reorientational dynamics of solid C₇₀ probed by positive muons

The dynamics of muonium (Mu) adducts to C₇₀ in the crystalline state have been studied using zero-field muon spin relaxation (ZF-μSR) and avoided level crossing muon spin resonance (ALC-μSR) spectroscopy. The two techniques are sensitive to complementary ranges of temperature and together give a complete and consistent picture of the molecular motion. The behavior of the MuC₇₀ radicals which are characterized by a prolate hyperfine matrix with components D_xx = -6.8(2) MHz, D_yy = -3.7(2) MHz, D_zz = 10.5(2) MHz is well-described by a pseudostatic model with a temperature-dependent order parameter. In contrast, MuC₆₀ shows a spherically isotropic diffusive or jump reorientation mechanism. C₇₀ is frozen on a time scale of around 30 ns up to 170 K. On further heating, the motion is found to be complex, consisting of a uniaxial rotation part together with a nutational or jumping motion of the unique axis. Anisotropy on the 30 ns time scale persists up to 370 K, well into the face-centered cubic phase of solid C₇₀. Above 390 K, the reorientational motion is essentially isotropic and the avoided level crossing resonance line disappears.