Ice Reaches a Record Density as Ultrahigh Pressure Forges a New Phase

Water is arguably the most familiar substance on Earth, yet it remains one of the least settled problems in condensed matter physics. A new study published in Nature Materials reports that the crystal structure of ice X, the phase in which ordinary hydrogen bonds have been fully symmetrized, does not simply compress in a passive, uniform way when squeezed to ultrahigh pressures at room temperature. Instead, the lattice is observed to distort before undergoing a transformation into a previously unobserved polymorph, designated ice XXII, which now stands as the densest form of water ever realized in the laboratory. The finding adds a new entry to the ever-growing family of water’s solid phases and sharpens a long-standing question about how the simplest of molecular liquids behaves when pushed to the limits of compression.

The significance of the result lies in what it says about the intermediate regime between known phases. For decades, high-pressure physicists have mapped the phase diagram of water with increasing precision, identifying nearly twenty distinct crystalline forms of ice, from the hexagonal ice Ih that falls as snow to exotic proton-ordered varieties stabilized only at low temperature and high pressure. Ice X occupies a special place in that catalog. Above roughly sixty gigapascals, the oxygen atoms lock into a body-centered arrangement and the hydrogen atoms settle at the midpoints between them, so that each proton is shared equally by two neighboring oxygens. The material ceases to be describable as discrete water molecules bound by hydrogen bonds; it becomes a network of oxygen ions threaded by protons, a so-called symmetrized hydrogen-bonded crystal.

What happens beyond that regime has been the subject of intense theoretical debate. Computational studies have predicted that as pressure climbs further, the oxygen sublattice itself should reorganize, exploring structures such as the cubic Pn-3m framework and more densely packed arrangements in which the coordination of oxygen atoms increases beyond the eightfold geometry of ice X. Some predictions place such transitions at pressures approaching the terapascal range, conditions comparable to those found in the interiors of giant icy planets such as Uranus and Neptune, where water is believed to constitute a substantial fraction of the mantle. Confirming any of these predictions experimentally has been extraordinarily difficult, because the pressures involved push the capabilities of diamond anvil cell technology to their limits.

The new work observes that before any wholesale transformation occurs, the ice X lattice begins to distort. Rather than maintaining the ideal symmetric geometry assumed in textbook treatments, the crystal develops a structural deviation under extreme compression at room temperature. This distortion is not a trivial elastic response; it signals that the symmetrized hydrogen-bond network, long treated as a stable endpoint of compression, is itself unstable at sufficiently high pressure. The observation provides direct experimental evidence that the pathway from ice X to denser structures is mediated by a symmetry-lowering intermediate state, a detail that theoretical models will now need to reproduce.

Following this distortion, the material transforms into ice XXII, the newly identified phase. According to the study, ice XXII is the most dense H2O polymorph that has been experimentally observed to date. Density is the central currency of high-pressure physics: when a crystal adopts a denser packing, it lowers its Gibbs free energy under compression, and the sequence of phases a substance passes through as pressure increases is essentially a sequence of increasingly efficient ways of stacking its atoms. That water, a molecule of just three atoms, continues to find new packing solutions at pressures far above those at which most substances have long since surrendered their molecular identity, is a striking illustration of the richness hidden in its phase diagram.

Experiments of this kind rely on diamond anvil cells, instruments that compress a microscopic sample between two gem-quality diamond anvils. The sample chamber in such an experiment is measured in micrometers, and the pressures achieved can exceed those at the center of the Earth. Structural information is typically extracted by X-ray diffraction, in which a synchrotron beam is focused through the diamonds onto the tiny compressed sample, and the resulting diffraction pattern is used to reconstruct the arrangement of atoms. At the highest pressures, the diffraction signals become faint and the analysis demanding, since the diamond itself contributes background and the sample may be non-hydrostatically strained. Detecting a subtle distortion of the ice X lattice, and then the emergence of an entirely new diffraction signature corresponding to ice XXII, therefore represents a considerable experimental achievement.

The room-temperature character of the transition is also noteworthy. Many of water’s high-pressure phases were originally discovered or characterized at low temperatures, where kinetics slow down enough for metastable structures to be trapped and proton ordering to occur. A transition observed at ambient temperature demonstrates that the transformation is driven by thermodynamic stability under compression rather than by temperature-assisted rearrangement, and it means the new phase is accessible under conditions closer to those relevant to planetary interiors, where water-bearing layers are hot as well as compressed. This strengthens the connection between laboratory measurements and geophysical models of ice giant planets, where the behavior of water at megabar and higher pressures influences magnetic field generation, heat transport, and interior structure.

For theorists, the observation of a distorted ice X precursor poses a concrete benchmark. First-principles calculations based on density functional theory have long predicted a landscape of candidate high-pressure structures for water, but the calculated transition pressures and the ordering of phases have been sensitive to computational details, including the treatment of proton quantum effects such as zero-point motion and tunneling. Protons are light enough that their quantum behavior significantly modifies the energy landscape, and nuclear quantum effects are believed to influence the precise pressure at which hydrogen bonds symmetrize. Any successful model of the ice X to ice XXII transition must now account for both the symmetry-lowering distortion and the density of the final structure, constraints that should help discriminate among competing computational approaches.

The discovery also extends a numbering scheme that has become a running chronicle of water’s complexity. Ice phases were historically labeled in order of discovery, from ice II and ice III in the early twentieth century through the recent additions of ice XIX, ice XX, and ice XXI in the past few years, several of which were identified through careful calorimetry and dielectric measurements of low-temperature, high-pressure samples. Ice XXII now continues that sequence, and its identification as the densest experimentally observed polymorph underscores that the inventory of water’s solid forms is still incomplete. Each new phase refines the phase diagram that planetary scientists, chemists, and physicists share, and each raises the question of where the sequence ends, or whether it ends at all before water’s molecules give way to an ionic or superionic state at the highest compressions.

Beyond its fundamental interest, the result speaks to a broader theme in modern materials physics: that even substances as thoroughly studied as water conceal structural surprises when probed under extreme conditions. The techniques refined in this work, combining ultrahigh static compression with high-resolution structural probes, are applicable to other hydrogen-dominated systems, including ammonia and methane, which together with water shape the interiors of icy planets. As synchrotron and free-electron laser sources deliver ever brighter beams and anvil geometries improve, the frontier of static compression continues to advance, bringing laboratory conditions closer to the interiors of the solar system’s most enigmatic worlds. With ice XXII now on the books, the high-pressure chapter of water’s story has gained a new and unexpectedly dense installment, and the search for the next polymorph is already underway in laboratories around the world.

Subject of Research: Ultrahigh-pressure structural distortion of ice X and its transition to the dense ice XXII polymorph of water

Article Title: Ultrahigh pressure phase transition in ice

Article References: Osmond, I., Kuzovnikov, M. A., Lewis, D. A., Shuttleworth, H. A., Marqueño, T., Dixon, M. J., Robertson, C. E. A., Cherepnalkoski, V., Wang, B., Hermann, A., Peña-Alvarez, M., & Howie, R. T. (2026). Ultrahigh pressure phase transition in ice. Nature Materials. https://doi.org/10.1038/s41563-026-02732-1

Image Credits: AI Generated

DOI: 10.1038/s41563-026-02732-1

Keywords: ice X, ice XXII, high-pressure physics, water polymorphs, phase transition, diamond anvil cell, hydrogen bonds, crystal structure, ice giant planets, X-ray diffraction, condensed matter physics, dense ice