Novel rare gas interstitial fullerenes of C60 with Ar, Kr and Xe

In this paper we report the formation and characterisation of rare gas C-60 interstitial compounds of Ar, Kr and Xe. The materials were produced by hot isostatically pressing (HIP) the fullerene solid at temperatures between 200 and 550 degrees C and under rare gas pressures in the range 170-200 MPa. With this method, we have been able to make rare gas fullerene compounds with stoichiometries of Ar1C60, Kr0.9C60 and Xe0.66C60. Thermal gravimetric analysis (TGA) showed that the HIPed materials contained rare gas after treatment, and gave a method for determining the stoichiometry. TGA also enabled the thermal stabilities of these materials with respect to rare gas loss to be investigated. The structure of the rare gas fullerenes was characterised by both X-ray and neutron powder diffraction. By Rietveld analysis of the diffraction data it has been shown that only the octahedral interstices of the fullerene face centred cubic (FCC) lattice were occupied by the rare gas, and the site occupancy of this site gave a stoichiometry agreeing within 5% of that obtained from TGA. The face centred to primitive cubic orientational ordering phase transition for these rare gas fullerenes was studied using neutron diffraction. The transition temperature was found to decrease as the size of the rare gas increases. This lowering is a result of the net weakening of the C-60-C-60 interaction potential, as the rare gas pushes the C-60 molecules slightly apart; a consequence of not only their size but also a result of their thermal motion (internal pressure) within the interstitial site. Differential scanning calorimetry (DSC) confirmed the transition temperatures obtained from neutron diffraction. In addition, transmission electron microscopy (TEM) and C-13 NMR studies have been performed on these materials and the results are discussed. (C) 1997 Elsevier Science Ltd.