X-RAY DIFFUSE SCATTERING AND INTERMOLECULAR INTERACTIONS IN SOLID C₆₀. R. Moret, P. Launois and S. Ravy, Laboratoire de Physique des Solides, URA CNRS 02, Université Paris-Sud, 91405 Orsay, France

Orientational ordering phenomena control the temperature-pressure phase diagram of solid C₆₀. At present, their understanding is imperfect because microscopic models fail to describe the interactions between the fullerene molecules in details.

Above the T_o= 259K orientational ordering transition the molecules do not rotate freely in the cubic crystal-field and significant short-range orientational correlations are present. They produce radial and azimuthal modulations of the diffuse scattering intensity which have been measured carefully in the first halo (Q~ 3.3 Å^-1) by single crystal X-ray diffraction. Intensity maxima at the special points X (100), L (1/2,1/2,1/2) and ( (000) of the Brillouin zone together with some "extra" scattering have been identified¹. The X-point maxima can be considered as precursor effects of the low temperature Pa phase, while the other diffuse scatterings may reveal the existence of competing phases. By comparing the observed intensity distribution to that calculated using a microscopic mean-field theory (with the formalism of the symmetry adapted functions, up to the l=12 terms) we evaluate different models of intermolecular potential. Van der Waals-type intermolecular interactions are found to account for the main features of the observed diffuse scattering². Elaborate models of intermolecular potential are being tested.

Below T_o, the remaining X-ray diffuse scattering is analyzed to clarify the orientational order of the nearly degenerate configurations of the C₆₀ molecules ("penta" and "hexa") which coexist in the Pa phase.

¹ P. Launois, S. Ravy and R. Moret, Phys. Rev. B52, 5414 (1995).

² S. Ravy, P. Launois and R. Moret, submitted.