New Phases of C 60 Synthesized at High Pressure

The fullerene C 60 can be converted into two different structures by high pressure and temperature. They are metastable and revert to pristine C 60 on reheating to 300°C at ambient pressure. For synthesis temperatures between 300° and 400°C and pressures of 5 gigapascals, a nominal face-centered-cubic structure is produced with a lattice parameter a o = 13.6 angstroms. When treated at 500° to 800°C at the same pressure, C 60 transforms into a rhombohedral structure with hexagonal lattice parameters of a o = 9.22 angstroms and c o = 24.6 angstroms. The intermolecular distance is small enough that a chemical bond can form, in accord with the reduced solubility of the pressure-induced phases. Infrared, Raman, and nuclear magnetic resonance studies show a drastic reduction of icosahedral symmetry, as might occur if the C 60 molecules are linked.