Unusual polymerization in theLi4C60fulleride

Abstract

Li4C60, one of the best representatives of lithium intercalated fullerides, features a novel type of 2D polymerization. Extensive investigations, including laboratory x-ray and synchrotron radiation diffraction, 13 C NMR, MAS and Raman spectroscopy, show a monoclinic I2/m structure, characterized by chains of [2+2]-cycloaddicted fullerenes, sideways connected by single C-C bonds. This leads to the formation of polymeric layers, whose insulating nature, deduced from the NMR and Raman spectra, denotes the complete localization of the electrons involved in the covalent bonds.

Low Band Gap and Ionic Bonding with Charge Transfer Threshold in the Polymeric Lithium Fulleride Li4C60

“…Three equivalent monoclinic domains are present, corresponding to the three possible polymerization directions in the hexagonal surface plane (once this direction is fixed, the other is uniquely determined). As the shortest intermolecular distance along the two polymerization directions is very similar, we cannot distinguish between the surface termination with in-plane [2 + 2]cycloaddition chains (corresponding to the (4,0,1) crystallographic plane in the structure identified in ref 20) and that with inplane singly bonded chains (corresponding to the (0,4,1) termination). We also cannot exclude the presence of both types of termination.…”

“…33 The LEED pattern of Figure 1a is instead consistent with the pattern expected for the termination plane of the 2D-polymer phase observed in bulk Li x C 60 (3 < x < 5). 19,20 The 2D polymeric networks in the monoclinic bulk phase form parallel to the (100) direction of the pristine fcc structure. Since the film termination of the pristine multilayer is (111), upon formation of the monoclinic phase in the film the 2D polymer networks are tilted with respect to the surface plane, with one bond direction parallel to the surface and the other lying at an angle of approximately 54.7°with respect to it.…”

“…At intermediate Li concentrations, the aforementioned (monoclinic) 2D-polymer phase is present in the bulk at room temperature, which transforms into a metallic monomer phase at higher temperature. 20,30 In contrast with most alkali fullerides, where stable phases are obtained only for integer stoichiometry and the properties of, e.g., x ) 3 or x ) 4 phases are completely different, the 2D-polymer phase reported by Riccò et al 20 is stable in the bulk over a broad range of (also noninteger) Li concentrations from x ) 3 to x ) 5.…”

“…10 The rectangular polymer networks in Li-doped C 60 were initially thought to consist of [2 + 2]-cycloaddition bonds. 17,18 However, more recent studies [19][20][21] have shown that the bonding motif is mixed, with [2 + 2]cycloaddition bonds in one direction and single bonds in the orthogonal direction.…”

“…Some authors also suggest a partial covalency between the Li and fullerene electronic levels. 17,24,25 For low Li concentrations x ∼ 1, a tendency toward phase segregation was reported, 20,26 also observed in Na fullerides, 27 with the formation of alkali-poor regions and of a polymeric phase tentatively assigned an orthorhombic (one-dimensional (1D)) symmetry. 26 Photoemission studies on Li-doped C 60 films with alkali concentration x e 1 have suggested the coexistence of two phases, 28,29 of which one would be a dimer phase analogous to that observed in the stoichiometric AC 60 fullerides at low temperature (A ) K, Rb, Cs).…”