REORIENTATIONAL DYNAMICS OF SOLID C-70 PROBED BY POSITIVE MUONS

The dynamics of muonium (Mu) adducts to C-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(70) 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, MuC(60) shows a spherically isotropic diffusive, or jump reorientation mechanism. C-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-70 Above 390 K, the reorientational motion is essentially isotropic and the avoided level crossing resonance line disappears.