Neutron diffraction and structural models of RbC60 phases

Pair distribution functions (PDFs), obtained from powder neutron diffraction measurements have been used to construct structural models for the body centered orthorhombic (bco) and fee phases of RbC60. The PDF exhibits small differences, primarily due to the shortened inter-fullerene: distance in the bco phase. The bco-RbC60 system is well fit to a model derived from first-principles quantum molecular dynamics in which the C-C interfullerene distance is 1.57 Angstrom. While the geometry of the interfullerene linkages is similar to a previously proposed model, the present results imply less distortion of the C-60 molecules and larger interfullerene distances. The PDF analysis also indicates significant orientational disorder between chains of linked molecules.