Pressure dependence of phonons inRb4C60studied with Raman spectroscopy

Raman spectra of ${\text{Rb}}_{4}{\text{C}}_{60}$ have been measured as a function of pressure up to 8 GPa at room temperature. Intramolecular modes exhibit three remarkable changes around 0.6, 1.7, and 4 GPa. An increase in the pressure slopes of various mode frequencies occurs at 0.6 GPa. Then an overall decrease in the pressure slopes occurs around 1.7 GPa. Finally, we observe the new lines appearing around 4 GPa. These effects are discussed in terms of reduction in the symmetry of the ${\text{C}}_{60}$ molecule and electron hopping.