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Revisiting the Phase Diagram of Methane

Mengnan Wang1,2, Miriam Peña-Alvarez1,*, Ross T. Howie1,3, and Eugene Gregoryanz3,4,1,5,†

  • *Contact author: miriam.pena.alvarz@ed.ac.uk
  • †Contact author: e.gregoryanz@sharps.ac.cn
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Phys. Rev. Lett. 136, 046101 – Published 26 January, 2026

DOI: https://doi.org/10.1103/7hxd-hhjf

Abstract

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.

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Viewpoint

Methane’s Elaborate Phases and Where to Find Them

Published 26 January, 2026

A systematic exploration of the phase diagram of methane resolves inconsistencies of earlier studies, with potential ramifications for our understanding of planetary interiors.

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Phys. Rev. Lett. 136, 046101– Published 26 January, 2026
  • Received 29 May 2025
  • Accepted 20 November 2025
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

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