Structure and dynamics of the fullerene polymerLi4C60studied with neutron scattering

Abstract

The two-dimensional polymer structure and lattice dynamics of the superionic conductor Li 4 C 60 are investigated by neutron diffraction and spectroscopy. The peculiar bonding architecture of this compound is confirmed through the precise localisation of the carbon atoms involved in the intermolecular bonds. The spectral features of this phase are revealed through a combination of ab-initio lattice dynamics calculations and inelastic neutron scattering experiments. The neutron scattering observables are found to be in very good agreement with the simulations which predict a partial charge transfer from the Li atoms to the C 60 cage. The absence of a well defined band associated with the Li atoms in the experimental spectrum suggests that this species is not ordered even at the lowest temperatures. The calculations predict an unstable Li sublattice at a temperature of ∼ 200 K, that we relate to the large ionic diffusivity of this system: low frequency optic modes of the Li ions couple to the soft structure of the polymer.

H and Li dynamics in Li 12 C 60 and Li 12 C 60 H y

“…Furthermore, light alkali and alkali earth (Li, Na, Mg) intercalated fullerides have been received attention also as ion conductors in the recent past, thanks to the ability of the metal ions to diffuse among the C 60 lattice interstices already at low temperature. In particular, Li 4 C 60 , thanks to a peculiar polymeric arrangement of the fullerene units [21,22,23,24], displayed a high Li-ion conductivity of 10 −2 S/cm at room temperature (a value comparable to that observed in liquid electrolytes), with a relatively low activation energy [2]. Similarly, large Mgion conductivity was also observed in the Mg 2 C 60 compound, isostructural to Li 4 C 60 [25].…”

Dynamics & Spectroscopy with Neutrons—Recent Developments & Emerging Opportunities

“…An interesting feature of fullerides is also the possibility of their polymerization at extreme conditions [ 237 , 238 , 239 , 240 ], which opens new synthetic strategies for obtaining carbon allotropes with entirely new physico-chemical properties. Polymerization can also be promoted by the intercalation of a wide range of electron donors (e.g., metals including Li, Na, K, Rb, or Mg), by entering the voids of the host fullerene lattice to form fully fledged solids [ 237 , 241 , 242 , 243 , 244 , 245 , 246 ].…”

“…As an illustration of this line of work, the structure and dynamics of the fullerene polymer Li 4 C 60 was studied with INS spectroscopy by Rols et al [ 246 , 247 ]. Representative results are illustrated in Figure 8 .…”

“…The calculations presented in this work also predict an unstable Li sub-lattice, a finding that is consistent with the aforementioned ionic conductivity. The temperature dependence of INS features depicted in Figure 8 c is also sensitive to the depolymerization transition, characterized by a (rather counterintuitive) transfer of spectral weight to lower frequencies in the monomeric phase [ 246 ].…”

Electronic and ionic conductivities in superionicLi4C60

“…In this respect, the Li 4 C 60 fulleride polymer is of special interest. In the 2D polymeric layers of this compound, fullerenes are connected by single bonds along one direction and by [2 + 2] cycloaddition in the other [9][10][11] . Li 4 C 60 is a superionic conductor 12 with a high ionic conductivity (0.01 S/cm at 300 K).…”