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 transition 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 bridging 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 plane. The stability of the ground state polymeric structure and the kinetics of the monomer --> polymer transformation are influenced by the charge state of the fulleride ions, the steric crowding associated with the dopant alkali ions and the relative orientational state adopted by neighbouring fullerides in the precursor monomer phase.