Pressure-induced phase in tetragonal two-dimensional polymeric C₆₀

The behavior of the phonon modes of the tetragonal phase of the two-dimensional polymerized C₆₀ has been studied as a function of pressure, up to 27.5 GPa, at room temperature by means of Raman spectroscopy. Gradual transformation of the material to a new phase was observed in the pressure region 19.0-21.0 GPa. As a result of this phase transformation dramatic changes in the Raman spectrum have been recorded. Namely, the total number of bands was reduced and a number of very strong peaks appeared. The Raman spectrum characteristics provide strong indication that the fullerene molecular cage is retained and therefore the high-pressure phase may be related to a three-dimensionally polymerized C₆₀ phase. The high-pressure phase remains stable upon pressure decrease from 27.5 down to 9 GPa. Further release of pressure leads to the destruction of this high-pressure phase to a highly disordered structure whose broad features in the Raman spectrum resemble those of amorphous carbon.