High-pressure Raman study of CH4 in melanophlogite (type I clathrate)

The evolution with pressure of the CH4 vibrational modes related to C-H stretching has been followed by in situ Raman spectroscopy on a single crystal of cubic melanophlogite from Varano Marchesi (Parma, Italy) containing only CH4 in the cages. The sample was compressed hydrostatically in a diamond anvil cell up to 6.5 GPa, using a non-penetrating P-transmitting medium. Two modes at 2900 and 2910 cm-1 (v1 and v2) were followed in response to the applied pressure, corresponding to C-H stretching of CH4 enclathrated in the 51262 and 512 cages, respectively. A change in slope of the frequency vs. P linear trend of the sharper peak at 2900 cm-1, observed between 1 and 1.5 GPa, is interpreted as evidence of the P-induced cubic to tetragonal transition, previously observed by X-ray diffraction at P >1.2 GPa. At pressures below the transition, the shift with P of the CH4 modes is comparable to that observed in methane ice hydrate, which becomes hexagonal at P > 0.9 GPa however. The ratio of the integrated areas of the two C-H stretching Raman peaks does not change significantly with pressure across the transition. At P > 5.1 GPa, a shoulder appears, close to the v1 peak, along with a slight change in slope of peak shift and peak broadening. The shoulder is maintained in decompression down to P = 4.4 GPa, showing slight hysteresis. At the same pressures however, X-ray diffraction shows no evidence of a phase transition, suggesting that a rearrangement of CH4 configuration occurs, without any effect on the tetrahedral framework.