Powder diffraction and inelastic neutron scattering studies of the Na2RbC60 fulleride

Neutron and synchrotron X-ray powder diffraction and inelastic neutron scat tering (INS) studies of Na2RbC60 in both its polymeric and monomeric phases have been performed as a function of temperature, with particular attentio n paid to the cooling protocol. The powder diffraction measurements on cool ing confirm the slow transformation of the primitive cubic high temperature phase to the monoclinic low temperature phase in the vicinity of 250 K. Ri etveld refinements of both the X-ray and neutron data show that over half o f 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 percent age 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 t o the disordered fcc structure occurs over a range of temperatures from 299 to 317 K. Study of the time evolution of the monomer --> polymer transform ation 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 ma gnitude of the reorientational potential in the precursor monomer phase. IN S spectra in the temperature range 100 to 320 K confirm the reduction of sy mmetry from the primitive cubic phase through splittings of the intramolecu lar vibrational modes. The polymeric nature of interfullerene bonding in Na 2RbC60 is also confirmed via direct observation of excess scattered intensi ty in the 8-25 meV region of the generalised phonon density-of-states (GDOS ).