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 s toichiometry, in agreement with our earlier x-ray analysis. This analy sis shows the Ar to be trapped in octahedral interstitial sites of a f ee lattice with a unit cell of similar to 14.230 Angstrom at 285 K, wh ich is unexpectedly close to that of pure C(60)similar to 14.218 Angst rom. A phase transition from an orientationally disordered fcc structu re 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.