Pressure-induced transformation and phonon modes of the two-dimensional rhombohedral polymer of C₆0: A Raman spectroscopic study

The Raman spectra and stability of the two-dimensional rhombohedral (2D-R) polymeric phase of C₆₀ have been studied as a function of pressure up to ∼30 GPa at room temperature. The pressure dependence of the phonon frequencies is rather smooth and reversible to P<|15 GPa. At higher pressure the initially sharp and distinct Raman peaks become very broad and diffuse and the material undergoes an irreversible transformation to a new rather disordered phase. The intensity of the A_g(2) pentagonal pinch (PP) mode rapidly decreases in the pretransitional pressure range and vanishes in the new phase. The high-pressure phase is recovered at normal conditions but is metastable and transforms under heating to a mixture of pristine and dimerized C₆₀ as was evident by their Raman spectrum. The quenching of the PP mode, the retention of the molecular cage, and the irreversibility of the transition are the indications that the new high-pressure phase may be formed by random covalent bonding between C60 molecular cages in adjacent polymeric sheets of the 2D-R polymer of C₆₀.