Precise determination of the molecular geometry in fullerene ${\mathrm{C}}_{60}$ powder: A study of the structure factor by neutron scattering in a large momentum-transfer range

The molecular structure of fullerene ${\mathrm{C}}_{60}$ 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-A${\mathrm{̊}}^{\mathrm{-}1}$ range, including Bragg and diffuse intensities, confirms the free reorientation of the ${\mathrm{C}}_{60}$ spherical molecules. This analysis gives the carbon-carbon bond length within the five-member ring (single bond) equal to 1.4527(7) Å, and that connecting five-member rings (double bond) equal to 1.3909(10) Å. 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-A${\mathrm{̊}}^{\mathrm{-}1}$ range: the single bond is elongated [1.460(2) Å] and the double bond shortened [1.381(3) Å], indicating that electrons get more localized on the π 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.