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

Abstract

AbstractThe 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 (ν1 and ν2) 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 n1 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.