Phase transition from C 60 molecules to strongly interacting C 60 agglomerates at hydrostatic high pressures

Abstract

C 60 molecules were studied at static pressures up to 30 GPa (300 kbar) at ambient temperature, using Raman and absorption spectroscopies. C 60 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 C 60 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 sp 3-hybridized carbon atoms. These transitions are explained in terms of π-electron rehybridization between C 60 molecules, which occurs at substantially decreased intermolecular distances.