PRESSURE-INDUCED PHASE-TRANSITIONS IN C-60 SINGLE-CRYSTALS

Raman spectra of C-60 single crystals, in the pentagon pinch mode freq uency region, have been measured as a function of hydrostatic pressure up to 7.2 GPa at room temperature. The softening of the A(g)(2) and H -g(8) modes in the pressure region up to 0.4 GPa indicate the orientat ional ordering phase transition from fee to sc structure. The pressure dependence of the intramolecular H-g(7) and A(g)(2) phonon frequencie s and their integrated intensities shows the existence of an interacti on between these modes above 2.5 GPa, which is connected with the phas e transition to the rotation-free orientationally ordered simple cubic phase of C-60 occurring at 2.5 GPa at room temperature.