In situ Raman study of ${\mathrm{C}}_{60}$ thin films at high pressures

Raman spectra from a systematic in situ study of ${\mathrm{C}}_{60}$ thin films at nonhydrostatic compression up to 30 GPa are presented in a broad spectral range (200–1800 ${\mathrm{cm}}^{\mathrm{-}1}$). The results show that the superhard fullerite obtained at 22.5 GPa transforms back to one-dimensional (1D) polymeric phase after quenching if no heat is added. This suggests that the most likely structure for the superhard fullerite phase is a three-dimensional polymeric structure of unbroken ${\mathrm{C}}_{60}$ molecules. Nevertheless, depolymerization of superhard fullerite was not observed if a heat treatment was added. In situ Raman spectra from the heated films were identical to those obtained without heating but the absence of a depolymerization transformation suggest a structural change which cannot be observed in the Raman spectra. A thickness effect was also observed for the high-pressure transformation. Higher pressures were required for the transformation of the thinner films. This was attributed to a decreased dimensionality in the thinner films making a 3D polymerization more difficult.