Molecular dynamics inK3C60: AC13NMR study

Measurements of the ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ NMR spectrum, spin-lattice, and spin-spin relaxation rates in ${\mathrm{K}}_3$ ${\mathrm{C}}_{60}$ reveal the existence of two distinct molecular motions of the same ${\mathrm{C}}_{60}$ molecule above 200 K, with respective activation energies of 2600±300 and 6800±500 K. We propose a simple model to explain these data implying fast axial jump rotation of the ${\mathrm{C}}_{60}$ molecules combined with slow jumps around perpendicular axes. The analysis of the spectra indicates an abrupt change in the molecular motion below 200 K suggestive of a phase transition also evidenced by calorimetric measurements.