Revisiting the phase diagram of methane

Combination of optical spectroscopy and in situ high-temperature–high-pressure techniques, were employed to investigate the melting curve and map out the location of methane’s solid phases up to 45 GPa and 1100 K. The experiments yield two distinct diagrams, one that demonstrates the kinetic phase transformations and the other presenting the equilibrium states usually reached with time. Raman spectroscopy demonstrates that the appearance and transitions between the higher pressure phases (VII, VIII, and IX) are strongly dependent on the pressure-temperature-time path. Combined visual observations and Raman spectroscopy indicate that the melting curve of methane extends to significantly higher temperatures than previously reported, e.g., ∼1000  K at 15 GPa. The study also suggests that some inconsistencies in the earlier melting data could be attributed to photochemical dissociation and/or a reaction induced by high-intensity light sources.