Reorientational Dynamics of Solid C70 Probed by Positive Muons

The dynamics of muonium (Mu) adducts to C[sub 70] in the crystalline state have been studied using zero-field muon spin relaxation (ZF-[mu]SR) and avoided level crossing muon spin resonance (ALC-[mu]SR) 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 MuC[sub 70] radicals which are characterized by a prolate hyperfine matrix with components D[sub xx] = -6.8(2) MHz, D[sub yy] = -3.7(2) MHz, D[sub xx] = 10.5(2) MHz is well-described by a pseudostatic model with a temperature-dependent order parameter. In contrast, MuC[sub 60] shows a spherically isotropic diffusive or jump reorientation mechanism. C[sub 70] 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 C[sub 70]. Above 390 K, the reorientational motion is essentially isotropic and the avoided level crossing resonance line disappears. 48 refs., 9 figs., 2 tabs.