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

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.

High-pressure infrared study of solid methane: Phase diagram up to 30 GPa

“…7,8,12,23 This model was adopted by Fabre and co-workers 6,13,15 in the interpretation of their Raman results without indication of site symmetry. As to the factor-group symmetry, the D 4h was preferred to D 2d symmetry due to the lack of coincidence among infrared and Raman frequencies of the lattice modes.…”

“…This was confirmed by the evolution with pressure of the Raman phonon frequencies. 13 The transition from the disordered phase I to phase IV was also studied below 90 K and 0.9 GPa by NMR spectroscopy. 16 From our infrared spectrum the transition is observed between 50 and 70 K at pressure values ranging from 0.6 to 0.8 GPa, in good agreement with the NMR study.…”

“…According to these results a direct identification of the phase boundaries can be sketched, taking advantage also of the spectroscopic data obtained at low [6][7][8][13][14][15][16] and room temperature. 17,18 In Fig.…”

“…Despite the large amount of vibrational data as a function of temperature and/or pressure, only the low-pressure portion ( Pр0.5 GPa) of the diagram is well characterized. [5][6][7][8][9][10][11][12][13][14][15][16] Besides the fcc structure of plastic phase I, 5 to which liquid methane crystallizes between 100 and 300 K, only that of phase II, stable below 25 K, is known. The space group is Fm3c (fcc) with eight molecules per unit cell, six orientationally ordered on D 2d sites while the other two, on O sites, behave as weakly hindered rotators.…”

“…Raman experiments at 4 K up to 1.1 GPa suggest the occurrence of other three phases, in addition to III. 6,13,15 Evidence of two of them ͑IV and V͒ comes from slope changes in the frequency vs pressure plot of the lattice phonons and 3 side bands, while most other spectral features are similar to those of phase III. Phase VI is observed for pressures larger than 1 GPa, considering the spectral changes in the 1 band.…”