Vibrational modes, optical excitations, and phase transition of solid ${\mathrm{C}}_{60}$ at high pressures

We have investigated the effect of pressure on the vibrational modes and optical response of ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}$ fullerene powder from several sources, via Raman spectroscopy and optical reflectivity measurements. Fullerene Raman peaks remain observable up to about 22 GPa, indicating that the fullerenes have stability comparable to that of hexagonal graphite under pressure. The Raman signal of this new phase after pressure release is that of amorphous carbon. We compare the reflectivity spectrum of this phase to that of the high-pressure phase obtained from graphite and conclude that the two high-pressure phases are different.