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  • Open Access

Distinction between ice phases VII, VIII, and X

Graeme J. Ackland* and Andreas Hermann

Kazuki Komatsu

Keishiro Yamashita and J. S. Loveday

  • Centre for Science at Extreme Conditions, School of Physics and Astronomy, University of Edinburgh, Edinburgh EH9 3FD, United Kingdom

  • Centre for Science at Extreme Conditions, School of Physics and Astronomy, University of Edinburgh, Edinburgh EH9 3FD, United Kingdom

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Phys. Rev. B 112, 094116 – Published 25 September, 2025

DOI: https://doi.org/10.1103/nmlm-rtxz

Abstract

Ice phases VII, VIII, and X are all based on a body-centered cubic arrangement of molecules, the differences coming from how molecular orientation breaks the symmetry. It is debatable as to whether these should even be considered distinct phases given that the standard definition of a transition between distinct phases involves a discontinuity in any derivative of the free energy. This can be hard to prove experimentally, and most previous theoretical works have been based on models which either have continuously differentiable free energies or offer no straightforward way to determine the free energy. Here we build a free-energy model based on the common definitions of the phases: ordered ice-VIII, orientationally disordered ice VII, and centered-proton ice X. All transitions in this model might or might not be associated with a discontinuity in the specific heat, depending on parameterization. By comparing with data, we find that a VII-X transition line exists, but it ends in a critical point hidden within the stability field of phase VIII. If the model is correct, there is a discontinuity between VII and X, so they are separate phases. We propose that the hidden phase boundary might be demonstrated experimentally by compression of supercooled ice VII.

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Phys. Rev. B 112, 094116– Published 25 September, 2025
  • Received 9 April 2025
  • Accepted 29 August 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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