Topochemical 3D polymerization of C60 under high pressure at elevated temperatures

Fullerene C-60 monomer crystals were compressed to a face-centered cubic (fcc) phase with a lattice parameter of a = 11.93(5) angstrom and a micro-Vickers hardness of 4500 kg/mm(2) using high-pressure and high-temperature conditions of 15 GPa at 500-600 degrees C. The hardness is compatible with that of cubic boron nitride (c-BN), suggesting the formation of a 3D C-60 polymer. The single-crystal X-ray structural analysis revealed that each C-60 molecule in the polymer was linked to the 12 nearest neighbors by [2+2] cycloaddition between the common pentagon -hexagon (56) edges. However, ab initio geometry optimization and molecular dynamics calculations suggested that the 3D polymer should have a rhombohedral structure with the space group of R (3) over bar containing [3+3] cycloaddition between the pentagons of C-60 molecules within the plane perpendicular to the 3-fold axis. The higher apparent symmetry of fcc was observed as an averaged structure of different orientations of the rhombohedral structure. The R (3) over bar structure can be derived by only a slight rotation of each C-60 unit in the (1111) plane of the fcc structure. The band-structure calculation suggested that the 3D polymer (R (3) over bar) was a semiconductor; the activation energy for the electrical conductivity was experimentally determined to be 0.25 eV at 550 K.