Spin-lattice relaxation and ¹³C NMR line shape at multiaxial reorientation of molecules in fullerite C₆₀

Molecular dynamics in the fcc-sc phases of fullerite C-60 (C-13 NMR line shape and spin-lattice relaxation time T-1) is described in terms of discrete orientational states of molecules. Molecular reorientations in the low temperature orientationally ordered sc phase are assumed to take place about molecular symmetry axes. Transition into the high- temperature disordered fcc phase is modeled by additional molecular reorientations about C-4 symmetry axes of the cubic unit cell. At low temperatures, the NMR line shape functions are demonstrated to depend significantly on the model of molecular reorientations in contrast with T-1. In the vicinity of the phase transition at 260 K, T-1 changes dramatically for all models, unlike the NMR line shape. The model of relatively fast molecular jumps about a single C-3 symmetry axis along with slow reorientations about the other axes is found to adequately reproduce the T-1 experiment and qualitatively reflects the temperature changes in NMR line shape