Neutron diffraction study of the polymeric structure of Na2RbC60

High-resolution neutron powder diffraction is used to authenticate that the Na2RbC60 fulleride undergoes on cooling an incomplete structural phase tra nsition from a primitive cubic to a monoclinic phase (space group P2(1)/a, lattice constants at 2.5 K: a = 13.707(2)Angstrom, b = 14.466(2) Angstrom, c = 9.366(1) Angstrom, beta = 133.64(1)degrees). Very short interfullerene C-C contacts are identified, providing evidence for the presence of bridgin g bonds whose distances refine to similar to 1.7 Angstrom. The CC bonds are inclined to the c axis by similar to 7.7 degrees and lie within the be pla ne. The stability of the ground state polymeric structure and the kinetics of the monomer --> polymer transformation are influenced by the charge stat e of the fulleride ions, the steric crowding associated with the dopant alk ali ions and the relative orientational state adopted by neighbouring fulle rides in the precursor monomer phase.