Raman study of phase transitions in compressed methane using moissanite anvil cells

Abstract

Thorough investigations using Raman spectra of solid methane (CH4) were conducted using moissanite anvil cells in the range of 1−43 GPa at room temperature to identify high-pressure phases. Two novel phases that are distinguishable from previously reported phases were uncovered. One, with a narrow pressure range of 1 GPa centered at around 12.5 GPa and another, with a range of 3 GPa centered around 40.5 GPa. Our results do not support the existence of a phase transformation around 25 GPa. The high quality Raman spectra, obtained by using single crystal samples, narrow the boundaries of seven methane phases at pressures up to 43.2 GPa.

Carbon precipitation from heavy hydrocarbon fluid in deep planetary interiors

“…The similarity is due to the orientational disorder of CH 4 molecules in the crystal structure of phase I. However, at pressures of 412 GPa, n 1 and n 3 fundamentals split as was observed in the Raman spectra 36,37 . This splitting is believed to be due to the proton ordering in the crystal lattice 38 .…”

Orientational Ordering, Locking-in, and Distortion of CH4 Molecules in Methane Hydrate III under High Pressure

“…As far as data of refs 14,19,21 are concerned, the additional stretching band indeed matches the frequency of the CH stretching mode in pure methane. 46,47 We could imagine The orientation of the methane molecules enclosed within the water frame was investigated by calculating the orientational probability density function (OPDF) P CH (θ, φ) of the CH bonds for different pressures: the OPDF for one molecule is shown in Figure 4a. For the sake of simplicity, the high-pressure case (P = 25.3GPa) is considered first: four well defined peaks corresponding to the four CH bonds are observed.…”

The Search for Superconductivity in High Pressure Hydrides

“…Instead, we briefly describe the high pressure behavior of methane and methane/hydrogen mixtures, which have been intensely studied because of their relevance in planetary sciences. Methane has a rich phase diagram, 252 and various binary molecular compounds with the general formula (CH 4 ) n (H 2 ) m (n = 1, 2 and m = 1, 2, 4) have been characterized spectroscopically up to 30 GPa. 253 A wide variety of CSP techniques have been used to predict the phases methane adopts under pressure, but none of them found any stable metallic structures up to pressures as high as 550 GPa.…”