Molecular dynamics and phase transition in one-dimensional crystal of C ₆₀ encapsulated inside single wall carbon nanotubes

One-dimensional crystals of 25% ¹³C-enriched C ₆₀ encapsulated inside highly magnetically purified SWNTs were investigated by following the temperature dependence of the ¹³C NMR line shapes and the relaxation rates from 300 K down to 5 K. High-resolution MAS techniques reveal that 32% of the encapsulated molecules, so-called the C ₆₀ ^α, are blocked at room temperature and 68%, labeled C ₆₀ ^β, are shown to reversly undergo molecular reorientational dynamics. Contrary to previous NMR studies, spin-lattice relaxation time reveals a phase transition at 100 K associated with the changes in the nature of the C ₆₀ ^β dynamics. Above the transition, the C ₆₀ ^β exhibits continuous rotational diffusion; below the transition, C ₆₀ ^β executes uniaxial hindered rotations most likely along the nanotubes axis and freeze out below 25 K. The associated activation energies of these two dynamical regimes are measured to be 6 times lower than in fcc-C ₆₀, suggesting a quiet smooth orientational dependence of the interaction between C ₆₀ ^β molecules and the inner surface of the nanotubes.