Li₄C₆₀:  A Polymeric Fulleride with a Two-Dimensional Architecture and Mixed Interfullerene Bonding Motifs

Abstract

All known fullerene polymers have interfullerene connections via either [2 + 2] cycloaddition or single C-C bonds. The high-resolution synchrotron X-ray powder diffraction technique was employed here to determine the crystal structure of the Li4C60 fulleride. We find that the ground state of Li4C60 is a two-dimensional polymer with monoclinic crystal symmetry and an unprecedented architecture, combining both the [2 + 2] cycloaddition and the single C-C bridging motifs. The small size of the Li+ cations is crucial in stabilizing the resulting tightly packed polymeric structure.

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

“…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.…”

“…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.…”

High pressure studies of alkali metal doped fullerides A4C60

“…For the structurally very similar compound K 4 C 60 , data from infrared spectroscopy also recently showed the possibility of a phase transition below 250 K [2]. Interest in the A 4 C 60 compounds has continued to increase even further recently, mainly due to the discovery that Li 4 C 60 has a unique, layered twodimensional polymeric structure at room temperature [6]. Recent investigations of the conduction properties of this phase have indicated a very high ionic conductivity with an excitation energy of only 0.23 eV [7].…”

Calorimetric measurements on Li4C60 and Na4C60

“…The structure is based on parallel molecular chains which are internally bonded by covalent intermolecular [2+2] cycloaddition bonds and cross-linked by single covalent bonds (see Figure 1). Except for the single bonds in the a direction 5 and the resulting rotation of the cycloaddition-bonded chains, this structure is very similar to that of the 2D pseudo-tetragonal polymer of C 60 1,2,6 . Na 4 C 60 has only single covalent intermolecular bonds in both directions 7,8 , which because of the molecular geometry results 7 in a monoclinic I2/m structure with a lattice angle of 78°.…”

“…Li 4 C 60 and Na 4 C 60 are two compounds of particular interest since both have unique, two-dimensionally (2D) polymerized layered lattice structures near room temperature. Li 4 C 60 forms a quasi-tetragonal structure with I2/m space group where the lattice angle is about 90.5 degree and the difference between the a and b lattice parameters is very small 5 . The structure is based on parallel molecular chains which are internally bonded by covalent intermolecular [2+2] cycloaddition bonds and cross-linked by single covalent bonds (see Figure 1).…”