Neutron-diffraction study of the rare-gas interstitial fullerene ArC60

Neutron powder diffraction studies have been carried out between room temperature and 15 K on C-60 powder, which has been hot isostatically pressed (HIPed) under 170 MPa of Ar and at 300 degrees C to give ArC60. Rietveld analysis of the neutron diffraction patterns confirms the stoichiometry, in agreement with our earlier x-ray analysis. This analysis shows the Ar to be trapped in octahedral interstitial sites of a fee lattice with a unit cell of similar to 14.230 Angstrom at 285 K, which is unexpectedly close to that of pure C(60)similar to 14.218 Angstrom. A phase transition from an orientationally disordered fcc structure to an ordered simple cubic structure occurs on cooling between 245 and 250 K. which is lower than found for pure unHIPed C-60 (255-260 K). The Rietveld refinement of the diffraction pattern at 15 K indicates that the Ar does not influence the orientation of the C-60 molecules.