Structure and dynamics of C-60 intercalation compounds: N2C60 and O2C60

We have studied the dynamics of polycrystalline C-60 intercalated with O-2 and N-2 by inelastic and quasielastic neutron scattering. Complementary neu tron diffraction investigations have been performed for O2C60 The diffracti on pattern of the low-temperature phase can be refined with the structural parameters of crystalline C-60 (space group Pa (3) over bar). All oxygen mo lecules are found on octahedral sites and an expansion of the host lattice is registered with respect to pristine C-60. The samples show a reduction i n the order-disorder transition temperature T-0 of about 20 K with respect to bulk C-60 This is demonstrated by differental scanning colorimetry measu rements and the onset of quasielastic scattering. The fact that quasielasti c intensity is observed appreciably below To indicates the presence of larg e fluctuations in the ordered phase. Above T-0 we find the same quasielasti c response as for pristine C-60 The phonon spectrum of the host is only wea kly perturbed by the presence of the guest molecules. Vibrations of the tra pped O-2 /N-2 molecules give rise to well pronounced peaks at 7 and 8 meV, respectively. The external molecular vibrations show a highly anharmonic te mperature behavior. There are, however, no signs of rapid reorientational j umps of the guest molecules on the ps time scale. Model calculations indica te a predominant alignment of the guest molecules along the [111] direction s combined with appreciable orientational disorder.