Metastable crystalline and amorphous carbon phases obtained from fullerite ${\mathrm{C}}_{60}$ by high-pressure–high-temperature treatment

A number of metastable ordered and disordered carbon phases have been prepared from fullerite ${\mathrm{C}}_{60}$ by heating to different temperatures at 12.5 GPa and subsequent quenching to ambient conditions. We studied the structure, Raman spectra, and hardness of the phases obtained. The increase of synthesis temperature up to 500 °C leads to a gradual three-dimensional polymerization and subsequent formation of a disordered network with a high share of ${\mathrm{sp}}^3$ sites. Further increase of the synthesis temperature leads to the formation of graphitelike clusters and then crystallization of graphite and diamond at $T\sim 900°\mathrm{C}.$ The structure of the polymer phases was identified as fcc, the lattice parameter value decreasing down to $\sim 11.6\AA$ with increase of synthesis temperature. A model of three-dimensional polymerization explaining the observed structural modification is proposed. The model allows one to calculate the share of covalently bonded molecules and describes the change of mechanical properties of polymeric phases in terms of rigidity percolation. Experimental results obtained for strongly polymerized states which favor a polymerization mechanism, other than $[2+2]$ cycloaddition are discussed.