HIGH-PRESSURE FT-IR STUDY OF SOLID C60

Pressure dependences of vibrational frequencies were measured for four intramolecular F1u modes of solid C60 at hydrostatic pressures to 3.2 GPa at ambient temperature. The mode Gruneisen parameter of the lowest frequency mode, which is characterized as a radial distortion of the pseudosphere C60 molecule, was shown to be a negative value of -0.036. The Gruneisen parameters of three other modes were from 0.067 to 0.089, comparable to the value for an intralayer stretching mode in hexagonal graphite. The softening of the radial mode suggests that besides van der Waals force there exists another attractive force between the adjacent molecules, probably covalent-bond-like interactions arising from the overlapping of pi-orbitals.