Phase transitions in C60·C8H8 under hydrostatic pressure



High-pressure infrared transmission measurements up to 9.5 GPa were carried out on the rotor–stator molecular cocrystal C60·C8H8. Helium served as pressure transmitting medium, which intercalates into the C60·C8H8 lattice. Thus, we investigated the pressure effects and effect of intercalation of helium into the C60·C8H8 lattice. The pressure-induced shift of the vibrational modes of C60·C8H8 shows an anomaly around 3 GPa. This anomaly can be interpreted in terms of the orientational ordering transition of fullerene molecules accompanied by a change in the crystal symmetry, which causes the splitting of the vibrational modes. We compare the value of the critical pressure to that obtained earlier [Thirunavukkuarasu et al., J. Phys. Chem. C 112, 17525 (2008); Phys. Status Solidi B 244, 3857 (2007)].