PRESSURE-INDUCED ORIENTATIONAL ORDERING IN C-60 SINGLE-CRYSTALS STUDIED BY RAMAN-SPECTROSCOPY

Raman spectra of C-60 single crystals have been measured as a function of hydrostatic pressure up to 15 GPa at room temperature. Several mod es exhibit a reversible softening with pressure in the range 0-0.4 GPa . This behavior is associated with the orientational ordering phase tr ansition from the fee to the partially ordered sc phase. New Raman lin es appear above 0.4 GPa, some of them exhibit a softening up to 2.5 GP a. Abrupt changes in the slope of a number of phonons are observed aro und 2.5 GPa. Also a redistribution of intensity among A(g)(2) and H-g( 7) modes, typical of mode interaction, takes place at pressures exceed ing 2.5 Cpa. These effects are connected to the phase transition from the partially ordered sc phase to the rotation free orientationally or dered sc phase of C-60 occurring at 2.5 GPa at room temperature.