High-pressure transformations of ${\mathrm{C}}_{60}$ to diamond and ${\mathit{sp}}^3$ phases at room temperature and to ${\mathit{sp}}^2$ phases at high temperature

Although stable under hydrostatic pressures up to 20 GPa, ${\mathrm{C}}_{60}$ molecules transform to a transparent and insulating material if compressed nonhydrostatically at room temperature. On the other hand, quasihydrostatic loading at higher temperatures yields black, electrically conducting samples. We report here on detailed x-ray synchrotron-radiation and electron-diffraction characterization of these samples. The former transparent samples consist in polycrystalline cubic diamond embedded in a bulk ${\mathit{sp}}^3$ amorphous matrix, while the latter samples correspond to a graphitic amorphous ${\mathit{sp}}^2$ structure.