Ice That Doesn't Melt: Unveiling the Mysteries of Room-Temperature Ice XXI

The Unseen World of Water: Unveiling the Mysteries of Room-Temperature Ice XXI

Water, the ubiquitous substance that covers over 70% of our planet's surface and constitutes up to 60% of the human body, is a molecule of profound and often deceptive simplicity. We know it as a liquid, a solid (ice), and a gas (steam). We see it fall from the sky, carve canyons through mountains, and fill the oceans. Yet, this familiar triad of states is merely the tip of the iceberg. In the hidden realms of extreme pressures and temperatures, water transforms into a bewildering array of solid forms, a veritable zoo of ices that defy our everyday intuition. The most recent and perhaps one of the most startling additions to this family is Ice XXI, a form of water that is solid at room temperature.

This discovery, which sounds like something from the pages of a science fiction novel, doesn't mean we will soon be building sculptures from ice that never melts in the summer sun. The reality is both more complex and more fascinating. Ice XXI is a child of immense pressure, a fleeting, metastable ghost in the machine of water's phase diagram. Its existence, confirmed by a global team of scientists using some of the most powerful experimental tools on the planet, forces us to reconsider the fundamental rules of how matter changes state. It opens new windows into the hearts of distant, icy worlds and even hints at novel applications in materials science and technology here on Earth. This is the story of Ice XXI, an exploration of the science that brought it to light, the bizarre gallery of ices it belongs to, and the profound implications of its fleeting existence.

Deconstructing the Familiar: What is "Ice"?

Before we can comprehend the strangeness of Ice XXI, we must first deconstruct our understanding of ordinary ice. The ice that floats in our drinks, known to scientists as Ice I_h (the 'h' stands for hexagonal), is just one of more than twenty known crystalline and amorphous solid phases of water. The defining characteristic of a particular ice phase is its crystal structure—the specific, repeating three-dimensional arrangement of its water molecules.

In Ice I_h, each oxygen atom is bonded to four hydrogen atoms—two covalently to form the H₂O molecule, and two via weaker hydrogen bonds to neighboring water molecules. This creates a tetrahedral arrangement that, when repeated, forms a hexagonal crystal lattice. A crucial and highly unusual property of this structure is that it is less dense than liquid water. The hydrogen bonds hold the molecules at a distance, creating a relatively open, porous lattice. This is why icebergs float and why pipes can burst in winter. As most other substances freeze, their molecules pack more tightly together, and the solid form sinks in its liquid. Water's anomaly is a cornerstone of life on Earth, allowing aquatic ecosystems to survive beneath a protective layer of surface ice.

However, this familiar, low-density structure is a product of our planet's relatively gentle atmospheric pressure. When water is subjected to pressures thousands or even millions of times greater than what we experience at sea level, the rules of the game change entirely. Under such duress, the open, hexagonal lattice of Ice I_h is no longer stable. The universe, in its relentless pursuit of the lowest energy state, forces the water molecules to rearrange into more compact, denser configurations, giving birth to a diverse and exotic menagerie of high-pressure ice polymorphs.

A Journey Through the Ice Zoo: From Bridgman to the 21st Century

Our journey into this high-pressure wonderland begins with the pioneering work of American physicist Percy W. Bridgman. In the early 20th century, Bridgman developed revolutionary new apparatus, most notably a self-sealing pressure container, that allowed him to achieve pressures far beyond what was previously possible, eventually reaching hundreds of thousands of atmospheres. With this new power, he began to systematically map the behavior of substances under extreme compression.

In 1912, Bridgman published a landmark paper detailing his discovery of five new phases of water ice, which he labeled Ice II, Ice III, Ice V, and Ice VI (Ice IV would be discovered later and found to be a metastable phase). He demonstrated that by squeezing water, one could force it to freeze at temperatures well above 0°C. He even created "hot ice," a form stable at temperatures higher than the boiling point of water, provided the pressure was maintained at tens of thousands of atmospheres. For his groundbreaking work in high-pressure physics, Bridgman was awarded the Nobel Prize in Physics in 1946.

Bridgman's work opened the floodgates. Over the subsequent decades, as technology advanced, scientists continued to explore the phase diagram of water, discovering an ever-growing list of new ice polymorphs.

Ice II and Ice III: These are among the first high-pressure phases discovered by Bridgman. Ice II has a rhombohedral structure and is more ordered than Ice I_h, while Ice III has a tetragonal crystal structure.
Ice VI and Ice VII: These are denser phases found at even higher pressures. Ice VI, which is thought to exist in the interiors of icy moons like Ganymede and Titan, has a tetragonal structure. At pressures above roughly 2 GPa (about 20,000 times atmospheric pressure), Ice VI gives way to Ice VII. Ice VII has a simple cubic crystal structure and is significantly denser still.
Superionic Ice (Ice XVIII): Predicted in 1988 and confirmed experimentally in 2018, this is one of the most exotic forms of water. At incredibly high pressures (over 100 GPa) and temperatures (thousands of degrees Celsius), water molecules break apart. The oxygen atoms form a rigid, solid crystal lattice, but the hydrogen atoms (now ions, or protons) become delocalized and flow like a liquid through this oxygen framework. This unique state, which is both solid and liquid at the same time, is electrically conductive and is believed to constitute the mantles of ice giant planets like Uranus and Neptune, potentially explaining their bizarre, off-kilter magnetic fields.

By the 21st century, the number of confirmed ice phases had reached twenty, from Ice I to Ice XX, each with a unique crystal structure and a specific domain of stability on the pressure-temperature phase diagram. It was into this already crowded and complex landscape that a new and unexpected member was born: Ice XXI.

The Birth of an Impossible Ice: The Discovery of Ice XXI

The discovery of Ice XXI, announced in October 2025, was the result of a collaboration between researchers led by Dr. Geun Woo Lee of the Korea Research Institute of Standards and Science (KRISS) and scientists at two of Germany's premier research facilities: the Deutsches Elektronen-Synchrotron (DESY) and the European X-ray Free-Electron Laser (XFEL). Their experiment was not just about applying immense pressure, but about applying it with unprecedented speed and precision.

The team used a device called a dynamic diamond anvil cell (dDAC). A standard diamond anvil cell consists of two flawless, gem-quality diamonds with their tips (culets) polished to a flat surface. A tiny sample of water is placed in a small hole within a metal gasket, which is then squeezed between the two diamond tips. Because of diamond's incredible hardness, this simple mechanism can generate static pressures equivalent to those at the center of the Earth. The 'dynamic' aspect of the dDAC used in this experiment involves using powerful piezoelectric actuators to drive the diamonds together at incredibly high speeds.

The experiment proceeded as follows:

A minuscule water sample was placed in the dDAC at room temperature (around 25°C or 77°F).
The pressure was then ramped up with astonishing rapidity, reaching 2 gigapascals (GPa)—roughly 20,000 times the air pressure at sea level—in just a few milliseconds. This created a state known as "supercompressed water."
The pressure was then released more slowly, over the course of about one second.
This entire compression-decompression cycle was repeated over 1,000 times.

The key to observing what happened during these fleeting moments of extreme pressure was the European XFEL, the world's largest and most powerful X-ray laser. It produces incredibly brilliant and short pulses of X-ray light, acting like an ultra-high-speed camera. By synchronizing these X-ray flashes with the compression cycle, the team could capture images of the water's molecular structure every microsecond (a millionth of a second), effectively creating a stop-motion movie of the freezing and melting process.

The results were stunning. Instead of the water smoothly transitioning into the expected stable high-pressure phase for that region, Ice VI, the researchers observed multiple, complex freezing and melting pathways. And along one of these pathways, a completely new, previously unseen phase of ice appeared—a transient, or metastable, state they named Ice XXI. Further analysis at DESY's PETRA III light source facility helped to fully characterize the structure of this new ice. They determined that Ice XXI has a body-centered tetragonal crystal structure, but one that is surprisingly complex, with a very large repeating unit cell containing 152 water molecules. It has a density of approximately 1.413 grams per cubic centimeter at 1.6 GPa, making it significantly denser than ordinary ice and causing it to sink in water.

The Physics of Fleeting Forms: Supercompression and Kinetic Trapping

The existence of Ice XXI is a masterclass in the difference between thermodynamics and kinetics. Thermodynamics tells us what the most stable state of a system should be—its lowest energy configuration. For water at room temperature and pressures between roughly 1 and 2 GPa, this stable state is Ice VI. However, thermodynamics doesn't tell us how or how fast the system will get there. That is the domain of kinetics, the study of reaction rates and pathways.

The key to forming Ice XXI is the incredible speed of compression. When pressure is applied slowly, water molecules have enough time to jiggle around, find their lowest-energy positions, and neatly organize themselves into the stable Ice VI crystal lattice. But by compressing the water in mere milliseconds, the researchers don't give the system time to find this optimal arrangement. The rapid increase in pressure effectively "jams" the molecules together, forcing the liquid into a state of "supercompression" where it remains liquid at pressures where it should have already frozen.

From this highly stressed, supercompressed liquid state, the system is desperate to solidify and release energy. But because it has been pushed so far from equilibrium so quickly, it can get lost on its way to the most stable structure. Instead of following the direct path to Ice VI, it can fall into a nearby, temporary energy well—a state that is stable enough to exist for a short time but is not the absolute lowest energy state. This phenomenon is known as kinetic trapping.

Imagine a ball rolling down a bumpy hill. The bottom of the valley is the most stable, lowest-energy state (like Ice VI). If the ball rolls slowly, it will navigate the bumps and settle at the very bottom. But if you give it a very hard, fast push, it might get bounced into a small divot or pothole partway down the hill. It's stable in that pothole (like Ice XXI), but a little jiggle (like a change in pressure or waiting long enough) will knock it out, and it will continue its journey down to the true valley floor.

The discovery showed that the transition from liquid water to high-pressure ice is not a single, simple step. Instead, it's a complex energy landscape with multiple possible pathways, some of which lead through these transient, metastable states. The existence of Ice XXI proves that by controlling the rate of change, not just the temperature and pressure, scientists can trap water in these unusual configurations and unveil "hidden" phases that would otherwise be impossible to observe.

Clarifying the Concept: What "Ice That Doesn't Melt" Really Means

The headline "Ice That Doesn't Melt" is provocative and captures the imagination, but it requires careful clarification. Ice XXI is indeed solid at room temperature, a condition under which ordinary Ice I_h would instantly turn to liquid. However, it can only maintain its solid form because it is being held together by immense pressure, roughly 20,000 times that of our atmosphere. If you could somehow remove a cube of Ice XXI from the diamond anvil cell, it would not sit on your countertop as a stable, room-temperature ice cube. The moment the immense pressure was released, it would instantly and explosively revert to liquid water.

This is fundamentally different from the "non-melting ice cubes" you can buy for your home bar. These products, often marketed as whiskey stones or reusable ice cubes, are typically small cubes made of stainless steel, granite, or soapstone. Some stainless steel versions contain a non-toxic gel. Their function is based on simple heat capacity and thermal conductivity. You store them in the freezer, and when placed in a drink, they absorb heat from the liquid, making it cooler. They "don't melt" because they are already stable solids at room temperature and are not undergoing a phase change. They simply transfer thermal energy. While they prevent the dilution of a drink that comes from melting water ice, they are also less effective at cooling, as they lack the powerful cooling effect (latent heat of fusion) that a substance provides as it melts.

So, while Ice XXI is technically an "ice that doesn't melt" at room temperature, this is only true within the extreme, artificial environment of a high-pressure laboratory.

Echoes in the Cosmos: The Importance for Planetary Science

While creating a permanent, non-melting ice cube for our drinks is not on the horizon, the discovery of Ice XXI and the principle of metastable ice formation have profound implications for our understanding of the universe, particularly the icy moons of our outer solar system.

Worlds like Jupiter's moon Europa and Saturn's moons Ganymede and Titan are believed to harbor vast liquid water oceans beneath their thick, icy shells. These shells are not static. They are subjected to immense pressures from their own weight and powerful tidal forces from their parent gas giants.

The interiors of these moons are not simple blocks of Ice I_h. The pressure and temperature profiles change dramatically with depth, creating conditions ripe for the formation of high-pressure ice phases like Ice III, Ice V, and, at greater depths, Ice VI. Current models of these moons' interiors are based on the assumption that water transitions to its most thermodynamically stable ice phase at any given depth.

The discovery of Ice XXI challenges this assumption. It demonstrates that rapid changes in pressure, which could be caused by meteorite impacts, internal convection, or cryovolcanism, could lead to the formation of metastable ice phases within these ice shells. These metastable ices have different properties—such as density, crystal structure, and thermal conductivity—than their stable counterparts.

The implications are significant:

Geological Activity: The presence of less dense or more dense pockets of metastable ice could affect the buoyancy and convection within the ice shell, potentially influencing geological activity on the surface, such as the formation of the chaotic terrains seen on Europa.
Heat Transfer: The thermal conductivity of the ice shell dictates how heat from the moon's core and from tidal flexing escapes. If layers of metastable ice with different thermal properties exist, it would change our models of the ocean's temperature and the overall thermal evolution of the moon.
Habitability: The conditions at the interface between the subsurface ocean and the overlying ice shell (or the underlying rocky seafloor) are crucial for assessing the potential habitability of these ocean worlds. The existence of different ice phases and transition dynamics could affect the chemical exchanges between the ocean and the ice, and potentially the delivery of nutrients necessary for life.

The researchers, including Rachel Husband from DESY, have emphasized this point, stating that their findings suggest a greater number of high-temperature metastable ice phases and transition pathways may exist, offering new insights into the composition of these distant worlds.

The Frontier of Creation: Speculative Applications on Earth

Beyond helping us understand alien worlds, the ability to create and control novel, metastable materials opens up intriguing, if speculative, possibilities here on Earth. Metastable materials are all around us; diamond itself is a metastable phase of carbon that would, over geological timescales, prefer to be graphite. Many of the most advanced materials used in modern technology are prized precisely for their unique, metastable properties.

The principles learned from creating Ice XXI could be applied to other substances, using rapid compression and kinetic trapping to synthesize novel materials with desirable characteristics.

Energy Storage: High-pressure synthesis is a known route to creating high-energy-density materials. Metastable phases, by their very nature, store energy within their structure which is released when they transition to a more stable form. While highly speculative, understanding these processes could one day lead to new forms of energy storage or advanced propellants.
Pharmaceuticals and Manufacturing: The crystal structure of a drug (its polymorph) can have a dramatic effect on its solubility, stability, and bioavailability. Some metastable polymorphs of drugs are more effective than their stable counterparts. High-pressure synthesis and kinetic trapping could offer new ways to produce and stabilize desirable but elusive drug polymorphs.
Advanced Electronics and Superconductors: High-pressure synthesis is a key tool in the search for new superconducting materials, such as the record-breaking superhydrides. The ability to create novel crystal structures and electronic configurations through kinetic trapping could accelerate the discovery of materials with exotic electronic or magnetic properties.

While the direct use of Ice XXI itself outside the lab is unlikely due to the extreme pressures required, the knowledge gained from its creation—the mastery over the kinetics of phase transitions—is a powerful new tool in the materials scientist's toolkit. It represents a step towards a new paradigm of "materials by design," where we can create substances with tailored properties by precisely guiding them along specific formation pathways.

The Uncharted Future of a Familiar Molecule

The discovery of Ice XXI is a testament to the enduring mystery of water. It is a molecule that we encounter every second of our lives, yet it continues to surprise us with its complexity. This single discovery has pulled back the curtain on a hidden world of kinetic pathways and fleeting structures, suggesting that the already sprawling family of ice polymorphs may be even larger than we imagined.

The future of this research is as dynamic as the compression process that created Ice XXI. Scientists will undoubtedly push to explore the pressure-temperature-rate landscape even further. Are there other metastable ices waiting to be found? What happens at even faster compression rates or different temperature regimes? Next-generation diamond anvil cells are constantly being developed, pushing the achievable pressures and control ever higher, promising access to even more extreme states of matter.

The study of Ice XXI and its brethren is not merely an academic exercise. It is a fundamental exploration into the rules that govern the assembly of matter. It helps us model the interiors of planets and moons hundreds of millions of miles away, informs our search for extraterrestrial life, and provides a new conceptual framework for creating the advanced materials that will shape our future. It reminds us that even in the most familiar of substances, there are unseen worlds and profound mysteries waiting to be unveiled, one gigapascal at a time.