Supercompressed water, under investigation by scientific researchers, has been observed to transition into ice VI at ambient temperatures by traversing a multitude of freezing and melting routes. This phenomenon proceeds through a hitherto undocumented form of unstable ice, now identified as ice XXI.
A nascent crystal of ice XXI, depicted as forming via nucleation and a gradual melting process. Image attributed to Lee et al., with DOI: 10.1038/s41563-025-02364-x.
Water, a compound comprising merely two constituent elements, exhibits the capacity to form an extensive array of polymorphic states, ranging from ice Ih through to ice XX, alongside four distinct amorphous configurations.
For a span of one hundred years, the intricate mechanisms and transitional trajectories governing these varied water phases have captivated the interest of scientists engaged in high-pressure physics and the broader quest for extraterrestrial life, particularly on celestial bodies like icy moons.
Dr. Geun Woo Lee, affiliated with the Korea Research Institute of Standards and Science and the University of Science and Technology, remarked, “Water demonstrates an astonishing level of complexity within its solid-state manifestations.”
“The preponderance of these phases typically manifests under conditions of elevated pressure and diminished temperature.”
“When water undergoes rapid compression, it retains its liquid state at pressures significantly higher than those at which it would ordinarily solidify into ice VI.”
“Ice VI is a particularly compelling phase, theorized to exist within the internal structures of celestial bodies such as Titan and Ganymede.”
“Its notably distorted atomic arrangement might facilitate complex transformation pathways that culminate in the generation of metastable ice forms.”
“Given that the majority of ice variants are observable solely under extreme environmental parameters, we engineered high-pressure environments utilizing diamond anvil cells.”
“Within this apparatus, the substance under examination—in this instance, water—is positioned between two diamond anvils, whose inherent hardness allows for the generation of substantial pressure.”
“The water was subjected to pressures reaching up to two gigapascals, a magnitude approximately 20,000 times greater than standard atmospheric pressure.”
“This extreme pressure induces ice formation even at ambient room temperatures, with the water molecules packed considerably more densely than in conventional ice.”
To meticulously observe the ice formation process under varying pressure regimes, the investigative team initially generated a pressure of two gigapascals within a remarkably short timeframe of 10 milliseconds.
Subsequently, the anvil cell was carefully decompressed over a period of one second, a cycle that was then systematically reiterated.
Throughout these iterative cycles, the researchers harnessed the brief, intense X-ray emissions from the European XFEL to capture high-resolution images of the sample at microsecond intervals.
The exceptionally high frequency of X-ray pulses enabled the creation of dynamic visual sequences illustrating the structural evolution of the ice.
Employing the P02.2 beamline at PETRA III, the study’s authors subsequently ascertained that ice XXI possesses a tetragonal crystalline structure characterized by surprisingly extensive repeating molecular arrangements, referred to as unit cells.
Dr. Lee elaborated, “Leveraging the unique capacity of the European XFEL’s X-ray pulses, we have identified multiple crystallization trajectories within water subjected to rapid compression and decompression, exceeding 1,000 cycles using a dynamic diamond anvil cell.”
Dr. Cornelius Strohm, a researcher at the Deutsches Elektronen-Synchrotron, explained, “In this specialized pressure cell, specimens are compressed between the opposing tips of two diamond anvils and can be modulated along a predetermined pressure gradient.”
“The specific crystalline architecture into which liquid water solidifies is contingent upon the extent of the liquid’s supercompression,” Dr. Lee further commented.
Dr. Rachel Husband, also from the Deutsches Elektronen-Synchrotron, posited, “Our discoveries suggest the potential existence of a greater number of high-temperature metastable ice phases and their related transition mechanisms, which could offer novel perspectives on the material composition of icy moons.”
The experimental results were officially documented and published on October 10 in the esteemed scientific journal Nature Materials.
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YH. Lee et al. Multiple freezing-melting pathways of high-density ice through ice XXI phase at room temperature. Nat. Mater, published online October 10, 2025; doi: 10.1038/s41563-025-02364-x
