Pressure dependence of intramolecular and intermolecular mode frequencies in solid oxygen determined by Raman studies

Solid molecular oxygen was investigated in a diamond anvil cell by Raman spectroscopy in the region of 6--300 K with pressures up to 14 GPa. Characteristic spectral features in the different crystalline phases of oxygen allow conclusions to be drawn about the p-T phase diagram. The splitting of the vibrational Raman line in {γ}-O₂ is explained by a vibration-vibration resonant transfer between molecules on different sites in the lattice. In addition, the effect of pressure on intermolecular and intramolecular forces, which causes a pressure dependence of mode-Gr\"uneisen parameters, is investigated. Finally, anharmonic contributions to the potential are calculated from independent measurements of ({∂}{ν}/{∂}p)T and (d{ν}/dT)ₚ, allowing the separation of the anharmonicities into pure volume and multiphonon contributions.