PRECISE DETERMINATION OF THE MOLECULAR-GEOMETRY IN FULLERENE C-60 POWDER - A STUDY OF THE STRUCTURE FACTOR BY NEUTRON-SCATTERING IN A LARGE MOMENTUM-TRANSFER RANGE

The molecular structure of fullerene C60 has been determined with high precision using neutron scattering over a large range of momentum-transfer values. In the high-temperature plastic phase, at 295 K, the description of the complete structure factor in the 0-20-angstrom range, including Bragg and diffuse intensities, confirms the free reorientation of the C60 spherical molecules. This analysis gives the carbon-carbon bond length within the five-member ring (single bond) equal to 1.4527(7) angstrom, and that connecting five-member rings (double bond) equal to 1.3909(10) angstrom. As the temperature is lowered to 4 K, the structural parameters are extracted from the analysis of the intramolecular contributions to the diffuse intensity in the 6.5-20-angstrom-1 range: the single bond is elongated [1.460(2) angstrom] and the double bond shortened [1.381(3) angstrom], indicating that electrons get more localized on the pi orbitals. The calculation method allows us to consider small distortions from the ideal truncated icosahedral model (oblate or prolate deformations); the method could easily be extended to the study of substituted fullerene molecules.