Structural stability of the rhombohedral 2D polymeric phase of C₆₀ studied by in-situ Raman scattering at pressures up to 30 GPa

The structural stability of the rhombohedral two-dimensional (2D) polymeric phase of C₆₀ has been studied as a function of pressure up to ∼30 GPa at room temperature by means of in-situ Raman scattering. An irreversible transformation to a new disordered phase was observed at a pressure of ∼15 GPa. The intensity of the A_g(2) pentagonal pinch (PP) mode rapidly decreases in the pre-transitional pressure range while the Raman spectrum of the transformed material becomes very diffuse. The high-pressure phase recovered to normal conditions is metastable and transforms under heating to a mixture of pristine and dimerized C₆₀ as can be seen by their Raman spectra. The retention of the fullerene molecular cage at high pressure and quenching of the PP-mode are the indications that the high-pressure phase may be associated with a random creation of new polymeric bonds between the molecules in adjacent polymeric planes of the 2D-rhombohedral phase of C₆₀.