Pressure-induced polymerization of fullerenes: A comparative study of C-60 and C-70

To investigate the polymerization of fullerenes, solid C-60 and C-70 have been subjected simultarieously to high pressures and temperatures, with pressures up to 7.5 GPa and temperatures up to 800 degrees C. X-ray diffraction measurements indicate that the fee solid C-60 transforms to an orthorhombic structure consisting of a polymerized linear chain of C-60 molecules. The associated changes in the intramolecular vibrational modes have been probed through Raman and infrared measurements. In contrast to solid C-60 under similar conditions, polymerization is not observed in the case of solid C-70 although there is a structural transition from the initial hcp structure to a rhombohedral structure. This lack of polymerization in C-70 is discussed in terms of the structure of solid C-70 under high pressure and temperature conditions, coupled with the topochemical features of the C-70 molecule.