NEW PHASES OF C-60 SYNTHESIZED AT HIGH-PRESSURE

The fullerene C-60 can be converted into two different structures by h igh pressure and temperature. They are metastable and revert to pristi ne C-60 on reheating to 300 degrees C at ambient pressure. For synthes is temperatures between 300 degrees and 400 degrees C and pressures of 5 gigapascals, a nominal face-centered-cubic structure is produced wi th a lattice parameter a(o) = 13.6 angstroms. When treated at 500 degr ees to 800 degrees C at the same pressure, C-60 transforms into a rhom bohedral structure with hexagonal lattice parameters of a(o) = 9.22 an gstroms and c(o) = 24.6 angstroms. The intermolecular distance is smal l enough that a chemical bond can form, in accord with the reduced sol ubility of the pressure-induced phases. Infrared, Raman, and nuclear m agnetic resonance studies show a drastic reduction of icosahedral symm etry, as might occur if the C-60 molecules are linked.