Pressure-induced polymerization of fullerenes: A comparative study of ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}$

To investigate the polymerization of fullerenes, solid ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}$ have been subjected simultaneously to high pressures and temperatures, with pressures up to 7.5 GPa and temperatures up to 800 °C. X-ray-diffraction measurements indicate that the fcc solid ${\mathrm{C}}_{60}$ transforms to an orthorhombic structure consisting of a polymerized linear chain of ${\mathrm{C}}_{60}$ molecules. The associated changes in the intramolecular vibrational modes have been probed through Raman and infrared measurements. In contrast to solid ${\mathrm{C}}_{60}$, under similar conditions, polymerization is not observed in the case of solid ${\mathrm{C}}_{70}$, although there is a structural transition from the initial hcp structure to a rhombohedral structure. This lack of polymerization in ${\mathrm{C}}_{70}$ is discussed in terms of the structure of solid ${\mathrm{C}}_{70}$ under high pressure and temperature conditions, coupled with the topochemical features of the ${\mathrm{C}}_{70}$ molecule. © 1996 The American Physical Society.