The starting point of the study is a principle that physicists have relied on for more than a century: the identity of a phase of matter is tied to its symmetries. The everyday example is water. Liquid water and steam look and behave very differently, yet at the level of fundamental symmetry they are the same. In both states the molecules have no preferred position and no preferred direction, so nothing in principle forbids one from being continuously deformed into the other. Indeed, if water is heated under high pressure, beyond a specific critical point the boundary between liquid and gas simply disappears, and the two states merge into a single thermodynamic phase. Ice is a different story. When water freezes, the molecules lock into a crystal lattice with fixed positions, breaking the translational and rotational symmetries that liquids and gases preserve. Because the symmetries differ, ice can never be smoothly transformed into liquid water; a genuine phase boundary must always separate them.

For decades, this symmetry-based picture has served as the organizing principle of condensed matter physics. But it leaves an uncomfortable question hanging over any pair of states that share the same symmetries yet look strikingly different. If symmetry cannot tell two states apart, is there any guarantee that they are actually connected by a continuous path, or could some subtler, non-symmetry-based mechanism keep them separated? Answering that question in a controlled and general way has proven remarkably difficult, and it is precisely the challenge that the HKUST-led team set out to confront.

Their testing ground was one of nature’s most counterintuitive substances: dense ice. Under extreme pressures, water freezes into a series of exotic crystalline forms with names like ice VII, ice VIII, and ice X. All of these share the same skeleton of oxygen atoms, arranged in an identical crystal framework. What distinguishes them is the placement of the hydrogen atoms that sit between the oxygens. In ice VII, the hydrogen atoms are distributed randomly among the available positions. In ice VIII, they settle into an ordered, periodic arrangement. In ice X, they drift to the midpoints of the bonds between neighboring oxygen atoms, becoming shared equally by two oxygens at once. The puzzle is that ice VII and ice X, when averaged over time, preserve exactly the same underlying crystal symmetries. By the standard symmetry criterion, there is no obvious reason they should be distinct phases, yet experiments and intuition have long suggested that something meaningful separates them.

The key insight of the new work is that this problem in high-pressure ice can be recast, exactly, as a problem in magnetism. The midpoints of the oxygen-oxygen bonds in these ice structures form a geometric pattern known as the pyrochlore lattice, a three-dimensional network of corner-sharing tetrahedra that is famous in condensed matter physics as the architecture underlying so-called spin ice materials. In those magnets, the magnetic moments behave as if they were hydrogen atoms in ice, obeying the same local rules. The correspondence runs deeper still: the three possible positions available to each hydrogen atom in ice map precisely onto the three allowed states of a quantum magnetic moment with spin equal to one. This mapping transforms a question about water under pressure into a question about magnets on a pyrochlore lattice, where a powerful theoretical toolkit becomes available.

In a companion study published in Physical Review B, the team first analyzed a simplified version of the problem in which the ordering of ice VIII is excluded. The result was striking. Ice VII and ice X are separated by a sharp, genuine phase transition only at absolute zero temperature. At any temperature above that point, thermally excited defects begin to roam the crystal, and these defects behave exactly like the celebrated magnetic monopoles of spin ice physics. The monopoles screen out the field that would otherwise hold the two states apart, dissolving the sharp transition into a smooth, gradual crossover. The conclusion is unambiguous: at any realistic temperature, ice VII and ice X are not distinct phases of matter but different manifestations of a single phase. Ice VIII, by contrast, remains genuinely separate, because its ordered hydrogen arrangement breaks a crystal symmetry that the other two forms preserve, placing it beyond the reach of any continuous connection.

The primary Physical Review X paper then generalizes this result into a comprehensive framework covering classical pyrochlore spin-ice models with arbitrary spin values, not just the spin-one case relevant to ice. The framework identifies what actually keeps two states apart in these systems: a network of string-like defects threading through the crystal. As long as those strings close on themselves, forming closed loops, the two states remain genuinely distinct phases. But the framework also exposes a sharp divide that earlier work could not see, one that depends on whether the spin of the magnetic moments is a whole number or a half-integer. Above absolute zero, thermally excited monopoles cut the strings open, and once the strings are severed, the topological distinction they carried is washed out. Almost every sharp phase boundary in the family then gives way to a smooth crossover, echoing the fate of the ice VII to ice X transition.

There is one dramatic exception to this rule. When the spin takes the value three-halves, the theory predicts that an abrupt transition between two distinct liquid-like magnetic states survives at finite temperature, ending at a critical point without any need for an external magnetic field to stabilize it. This makes spin three-half pyrochlore magnets an especially promising target for experimentalists searching for a phase transition that defies the conventional symmetry-based classification. Finding such a transition in the laboratory would demonstrate a form of order that persists at finite temperature through topology alone, something the new framework says should be impossible everywhere else in this family of models.

The implications reach well beyond dense ice and idealized magnets. Pyrochlore spin-ice physics appears in frustrated magnetic materials, in engineered nanomagnet arrays where artificial moments are arranged on the same lattice geometry, and in trapped cold-atom systems where researchers can simulate these models with exquisite control. For all of these platforms, the new theory provides a practical recipe: examine the symmetries, examine the string-like topological defects, and check whether the spin is integer or half-integer. The answers determine whether an observed change of behavior reflects a true phase boundary or merely a crossover, a distinction that governs what experiments can hope to observe and what theoretical surprises may remain hidden.

For Professor Watanabe, the deeper message of the work is a reaffirmation of a classical idea, sharpened by modern tools. As he reflected on the findings, our results ultimately support the conventional view that the phases of ordinary matter are classified by broken symmetry, and by nothing else. Wherever symmetry could not distinguish between two states, a continuous path connected them without crossing a phase boundary. Yet he is quick to point out that this conclusion is an invitation as much as an answer. It would be far more interesting, he notes, to discover a topological phase that genuinely survives at finite temperature beyond this classification, and that is exactly what the team is looking for now. In redrawing the boundary between phases of matter, the HKUST-led collaboration has also drawn a map of where the next discovery might lie.

Subject of Research: Classification of phases and phase transitions in pyrochlore spin-ice magnets and high-pressure ice

Article Title: HKUST researchers redraw the boundary between phases of matter

Article References: HKUST researchers redraw the boundary between phases of matter. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: phases of matter, phase transitions, pyrochlore lattice, spin ice, topological defects, magnetic monopoles, high-pressure ice, ice VII, ice X, symmetry breaking, condensed matter physics, HKUST

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