Raman spectra of solid CH4 under high pressure. II. New phases below 9 kbar at 4.2 K

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Abstract
We have studied the Raman spectra of solid CH4 at 4.2 K below 9 kbar. The vibration-libration structures of the intramolecular ν1–ν4 spectra were essentially the same as the previous data of 1.5 kbar, so that the three-site (S1, S2, and W) model of phase III proposed in paper I was utilized to analyze the observed data. It is noteworthy that one of the three bands in the ν4 spectrum exhibits a decrease in frequency with pressure. This ’’softening’’ is interpreted as a result of distortion of the local potential fields of methanes with pressure. On the other hand, the lattice Raman spectrum exhibits a marked change in the II–III phase transition at 0.5 kbar, and furthermore shows changes in the splittings of certain bands at 1.9 and 4.9 kbar. In the ν-P plots of both intramolecular and lattice vibrational bands, prominent discontinuities are observed at 1.9 and 4.9 kbar, indicative of the existence of phase II (0–0.5 kbar), phase III (0.5–1.9 kbar), and new phases IV (1.9–4.9 kbar) and V (4.9 kbar), below 9 kbar. Mode Grüneisen parameters γ have been estimated. It is found that the γ’s of phase III are much larger than those of phases IV and V. Assignments of the lattice Raman bands are made using the observed ν3 Raman spectrum, the ν-P plots and the existing far-infrared spectrum. It is concluded that the lattice Raman spectrum mainly consists of two translational bands and three librational bands arising from S1, S2, and W sites. Finally, we have estimated from the observed data the magnitude of the A-T tunneling rotational splittings of methane as a function of pressure. Results imply that the splitting at the W (weak field) sites is as large as ∼3.6 cm−1, even at P = 8 kbar. On the basis of this result, we discuss the tunneling splittings in solid CD4 phase III at normal pressure.