Pressure induced phase transitions in C60 single crystals

Raman spectra of C60 single crystals, in the pentagon pinch mode frequency region, have been measured as a function of hydrostatic pressure up to 7.2 GPa at room temperature. The softening of the Ag(2) and Hg(8) modes in the pressure region up to 0.4 GPa indicate the orientational ordering phase transition from fcc to sc structure. The pressure dependence of the intramolecular Hg(7) and Ag(2) phonon frequencies and their integrated intensities shows the existence of an interaction between these modes above 2.5 GPa, which is connected with the phase transition to the rotation-free orientationally ordered simple cubic phase of C60 occurring at 2.5 GPa at room temperature.