Powder diffraction and inelastic neutron scattering studies of the Na₂RbC₆₀ fulleride

Neutron and synchrotron X-ray powder diffraction and inelastic neutron scattering (INS) studies of Na₂RbC₆₀ in both its polymeric and monomeric phases have been performed as a function of temperature, with particular attention paid to the cooling protocol. The powder diffraction measurements on cooling confirm the slow transformation of the primitive cubic high temperature phase to the monoclinic low temperature phase in the vicinity of 250 K. Rietveld refinements of both the X-ray and neutron data show that over half of the sample transforms to the polymer in the temperature range 180-200 K, this fraction rising to 64(2)% at base temperature. On heating, the percentage of the monomer phase increased from 230 K at the expense of the polymer, with a full transformation occurring by 277 K. A final phase transition to the disordered fcc structure occurs over a range of temperatures from 299 to 317 K. Study of the time evolution of the monomer→polymer transformation at 180 and 200 K allowed us to extract an estimate of the activation barrier to interball C-C bond formation as 0.16(2)eV, comparable to the magnitude of the reorientational potential in the precursor monomer phase. INS spectra in the temperature range 100 to 320 K confirm the reduction of symmetry from the primitive cubic phase through splittings of the intramolecular vibrational modes. The polymeric nature of interfullerene bonding in Na₂RbC₆₀ is also confirmed via direct observation of excess scattered intensity in the 8-25 meV region of the generalised phonon density-of-states (GDOS).