Optical studies of methane under high pressure

Abstract

Physical properties of solid methane have been studied at room temperature under high pressure in a diamond-anvil cell. Internal modes have been followed up to 20 GPa by Raman scattering. The refractive index has been measured by a newly developed method up to 12 GPa and the elastic properties determined by Brillouin scattering up to 32 GPa. In the plastic phase I, the experimental equation of state and the elastic properties were analyzed with reference to rare-gas solids, i.e. , with use of self-consistent harmonic calculations with various pair potentials. The best agreement is obtained with Aziz s Hartree-Fock+dispersion -C krypton-krypton potential. From analogies with rare-gas and NH3 solids, a hcp structure is proposed for phase IV. The IV-VII phase transition is shown to be orientational in nature.

Generation of methane in the Earth's mantle: In situ high pressure–temperature measurements of carbonate reduction

“…1 A, hydrocarbon-rich regions were found after laser heating at Ϸ1,500°C at 5.7 GPa. The hydrocarbon was clearly identified as methane based on the sharp band at 2,972 cm Ϫ1 , matching the position of bulk methane at this pressure (20). The broad feature near 3,200 cm Ϫ1 is caused by O-H stretching in ice VII (21).…”

The crystal structure of methane B at 8 GPa—An α-Mn arrangement of molecules

“…Previous discussions of methane's structural transformation assumed that it would adopt the hcp structure at high pressure 4 and subsequent studies were interpreted on this basis. 3,6 This assumption was challenged by later work in which methane B was determined to have a cubic unit cell.…”

“…2 The individual molecules have therefore been treated as spheres in previous studies and, as a result, the higher-pressure structures of solid methane have been discussed in terms of a "bad rare gas" model. [3][4][5] This has led to assumptions that methane's structural progression would be towards hexagonal-close packing (hcp) at high pressures above 25 GPa. 6 The progression to a hcp structure in solid methane was questioned when the high-pressure phase methane B was proposed to have cubic unit cell, 7 but this has remained unconfirmed.…”

“…4 The phase above 9 GPa was originally named methane VII but has later been renamed methane B 6 to distinguish it from the low-temperature solid phases. To date, methane B has not been the subject of a full structural study and no atomic or molecular positions have been proposed.…”

“…1 In the ice series HF, H 2 O, NH 3 , and CH 4 , methane is the sole member that does not form hydrogen bonds in its solid phases. Its structures are therefore determined by the interplay of van der Waals interactions and steric effects.…”