Orientation of C70 molecules in peapods as a function of the nanotube diameter

Encapsulated C-70 molecules packed in single-walled carbon nanotubes display different orientations depending on the nanotube radius. We present x-ray scattering data obtained on a powder of nanotubes filled with C-70 molecules. Analytical expressions for calculating the diffraction diagram taking into account fullerene orientations are developed. The comparison between calculations and experiments allows us to conclude that the change from the lying to standing orientation-corresponding to the molecule long axis parallel and perpendicular to the tube axis, respectively-takes place when nanotubes reach a diameter of about 1.42 nm. Energy calculations are performed using a Lennard-Jones (6-12) potential, leading to a calculated reorientation diameter in good agreement with that determined experimentally.