Thermal/pressure-induced transformation of C60(CF2)

Abstract

Homofullerenes are the class of fullerene derivatives where all fullerene carbon atoms remain in the sp²-state. This is facilitated by cleavage of the skeletal C–C bonds between the adjacent sites of attachment of divalent addends, e.g. certain types of >CX₂ groups. Homofullerenes rarely reveal any important differences in optical and chemical properties compared to pristine fullerenes. Here we report an important novel example of such rare differences: formation of new fullerene polymer phases under thermal and high-pressure treatment as evidenced by the in situ and ex situ studies by means of Raman spectroscopy, HRTEM and XPS. Initially, C₆₀(CF₂) molecules undergo dimerization yielding double-caged multiply linked [C₆₀(CF₂)]₂ dimers. High-pressure treatment of C₆₀(CF₂) with additional shear stress leads to phase transitions observed up to 27 GPa yielding high-pressure phases (IV and V) with partial defluorination. This behavior is qualitatively similar to that of pristine C₆₀, but due to the presence of the CF₂ moiety, the pressures required are 5–8 GPa higher compared to pristine C₆₀. While the shear loads distort the crystal lattice of the C₆₀(CF₂) sample, interplane distances are preserved, indicatively of the lack of long polymeric fullerene chains typical of pristine C₆₀. Most importantly, high-pressure phase V of C₆₀(CF₂) demonstrates a transition at 45 GPa yielding an even harder phase which remains stable under 45–80 GPa loads.