Anisotropic Packing and One-Dimensional Fluctuations of C₆₀ Molecules in Carbon Nanotubes

The confinement of a C₆₀ molecule encapsulated in a cylindrical nanotube depends on the tube radius. In small tubes with radius R_T≲7  Å, a fivefold axis of the molecule coincides with the tube axis. The interaction between C₆₀ molecules in the nanotube is then described by a O₂-rotor model on a 1D liquid chain with coupling between orientational and displacive correlations. This coupling leads to chain contraction. The structure factor of the 1D liquid is derived. In tubes with a larger radius the molecular centers of mass are displaced off the tube axis. The distinction of two groups of peapods with on- and off-axis molecules suggests an explanation of the apparent splitting of A_g modes of C₆₀ in nanotubes measured by resonant Raman scattering.