Raman spectroscopy of methane (CH4) to 165 GPa: Effect of structural changes on Raman spectra

We have conducted a Raman study of methane (CH4), a major constituent of the outer planets, at pressures up to 165 GPa. We observe splitting of the principal Raman-active vibrational mode above 45 GPa and a nonlinear dependence of Raman peak position on pressure. A discontinuous change in the pressure derivative of the ν3 peak position is observed at approximately 75 GPa, corresponding to the phase change previously observed using X-ray diffraction. The Gruneisen parameters for the principal Raman-active modes of methane in the simple cubic and high-pressure cubic phases are calculated. The predicted dissociation of methane at ultrahigh pressure to form C2H6 and H2 is not observed, but an additional discontinuous change in the pressure-induced shift of the Raman peaks is observed at 110 GPa. We suggest that this may be due to some reorientation or reordering of the methane molecules within the framework of the known cubic lattice.