The influence of metals on the phase transformations of fullerenes at high pressure and high temperatures

Phase transformations in crystalline fullerenes C60 and C70 with additions of two groups of metals (7 at. % each): carbide-forming (V and nichrome Ni20Cr80) and non-carbide-forming (Ag and Ni) at a high pressure of 8 GPa and high temperatures of 500-1100 degrees C have been investigated by X-ray diffraction and Raman spectroscopy. The measurements were carried out ex situ after a thermobaric treatment. Two groups of phase transformations were found: crystalline fullerenes into amorphous graphite and the formation of carbides VC, V2C and Cr3C2. The addition of V and nichrome stabilizes fullerene molecules and raises the temperature of transformation of crystalline fullerene into amorphous graphite by hundreds of degrees, while the addition of Ag and Ni does not have such an effect. Thus, the described effect is explained by chemical interaction. Carbides in systems with V and nichrome are formed mainly in the interaction of metal with amorphous graphite, and not with crystalline fullerene. The result is explained on the basis of a comparison of the interaction of metal atoms with fullerene molecules and with individual carbon atoms.