Pressure dependence of the external mode spectrum of solid ${\mathrm{C}}_{60}$

Using inelastic neutron scattering we have investigated the external mode spectrum of solid ${\mathrm{C}}_{60}$ in the orientationally ordered phase. Hydrostatic pressures 1 bar $<P<\mathrm{}9\mathrm{}$ kbar and temperatures $180\hspace{0.5em}\mathrm{K}<T<\mathrm{}335\mathrm{}$ K were covered. The entire density of states shows a pronounced pressure dependence. In accordance with theory1 translational phonons have larger pressure coefficients $(dħ\omega /dP\approx 0.2$ meV/kbar) than librons $(dħ\omega /dP\approx 0.06$ meV/kbar). Anharmonic frequency shifts become smaller upon increasing the pressure. It is conjectured that they are related to the orientational disorder in the system arising from competing H and P orientations. A direct link between libron softening and the order-disorder phase transition cannot be established.