New phases of C₆₀ synthesized at high pressure

The fullerene C₆₀ can be converted into two different structures by high pressure and temperature. They are metastable and revert to pristine C₆₀ 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, C₆₀ transforms into a rhombohedral structure with hexagonal lattice parameters 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 C₆₀ molecules are linked.