DIFFUSE-SCATTERING AND ORIENTATIONAL CORRELATIONS IN SOLID C-60

A single-crystal x-ray diffuse scattering study of C-60 is presented. Precession photographs and three-circle diffractometer experiments have been carried out at room temperature, showing that the diffuse halos at Q(1) similar or equal to 3.3 Angstrom(-1) and Q(2) similar or equal to 5.3 Angstrom(-1) are modulated. Comprehensive measurements performed in the first diffuse halo are analyzed in terms of maxima of scattering at the special points X and L of the Brillouin zone, plus some extra scattering. Below the Fm ($) over bar 3m-Pa ($) over bar 3 phase transition where superstructure peaks appear at points X, the L point and extra scatterings are: still observed, though weakened. The scattering modulations reveal orientational correlations of the C-60 molecules. The X-point scattering can be related to Pa ($) over bar 3 pretransitional fluctuations, while the L-point scattering indicates local order corresponding to a phase competing with the Pa ($) over bar 3 one, A comparison with the current models for the orientational ordering in C-60 is presented.