PHASE-TRANSITION FROM C-60 MOLECULES TO STRONGLY INTERACTING C-60 AGGLOMERATES AT HYDROSTATIC HIGH-PRESSURES

C60 molecules were studied at static pressures up to 30 GPa (300 kbar) at ambient temperature, using Raman and absorption spectroscopies. C60 molecules transform reversibly to a semi-transparent phase in the pressure range of 15-25 GPa with a large pressure hysteresis. The high-pressure phase consists of interconnected strongly interacting C60 agglomerates, or networks of fullerenes, whose stability continuously increases with increase of pressure. Above 27 GPa the transition becomes irreversible, and the material recovered from high pressure is metastable and diamond-like at ambient conditions. The Raman spectrum of the high-pressure phase indicates that it contains sp3-hybridized carbon atoms. These transitions are explained in terms of pi-electron rehybridization between C60 molecules, which occurs at substantially decreased intermolecular distances.