Scientists have identified a new phase of solid water, dubbed Ice XXI, that forms under rapid, extreme compression at room temperature. An international team of researchers utilized a powerful X-ray laser to observe this previously unknown structure, revealing a novel pathway for water to crystallize under intense pressure. The discovery pushes the boundaries of how scientists understand water’s behavior in extreme environments, from the deep interiors of distant moons to the frontiers of materials science.
The new phase, cataloged as the 21st known crystalline structure of ice, is considered metastable, meaning it can exist temporarily under conditions where another form would be more stable. Led by scientists from the Korea Research Institute of Standards and Science (KRISS), the research team found that by compressing liquid water to pressures 20,000 times greater than Earth’s atmosphere in mere milliseconds, they could bypass other expected ice formations to create Ice XXI. This finding, published in Nature Materials, provides critical insight into the complex phase transitions of water and suggests that even more forms of ice may be waiting to be discovered.
A Crowded Family of Ices
While most people are familiar with the hexagonal crystalline ice that floats in a beverage, known as Ice Ih, it is just one of more than 20 documented solid phases of water. These different forms, or polymorphs, are created under varying conditions of temperature and pressure, each with a unique arrangement of water molecules and distinct physical properties. The different phases are named with Roman numerals, such as Ice II, Ice III, and so on. The discovery of Ice XXI adds a new member to this diverse family, but one that forms through a unique kinetic process rather than settling into a state of thermal equilibrium.
This new form is distinct from previously identified high-pressure ices. For example, Ice VI is a phase believed to exist inside icy moons like Jupiter’s Ganymede and Saturn’s Titan. The experiments revealed that by compressing water rapidly, the liquid state can be maintained at pressures far higher than previously thought, delaying the transition into phases like Ice VI and allowing the metastable Ice XXI to form instead. This persistence in a supercompressed liquid state before crystallizing into a novel form is a key aspect of the discovery, highlighting the importance of the speed of compression in determining the final ice structure.
Advanced Experimental Techniques
The creation and verification of Ice XXI required state-of-the-art experimental facilities capable of exerting immense pressures and observing molecular structures on incredibly short timescales. The research was conducted at two major European research centers: the European X-ray Free-Electron Laser (XFEL) and the PETRA III high-energy photon source at the Deutsches Elektronen-Synchrotron (DESY).
Rapid Compression with Diamond Anvils
To generate the extreme conditions needed, the scientists used a device called a dynamic diamond anvil cell. This instrument uses the tips of two flawless diamonds to squeeze a small sample of liquid water trapped between them. The team was able to apply pressure up to 2 gigapascals—roughly 20,000 times standard atmospheric pressure—and, crucially, to do so very quickly. This rapid compression was performed over 1,000 times, allowing the researchers to study the various crystallization pathways water can take when pushed far from equilibrium. The ability to apply pressure so swiftly was essential for creating the supercompressed water state from which Ice XXI crystallizes.
Illumination by X-ray Lasers
Observing the atomic structure of the ice as it formed required the unique capabilities of the European XFEL. This facility produces intense and ultrashort flashes of X-rays, which allowed the team to perform diffraction experiments on the tiny sample inside the diamond anvil cell. By analyzing how the X-rays scattered off the water molecules, the scientists could determine their arrangement in real time, capturing the moment of crystallization into the new Ice XXI phase. The High-Energy Density (HED) experiment station at European XFEL was specifically used for this analysis.
The Unique Structure of Ice XXI
Following the initial discovery at the European XFEL, the team conducted further analysis to fully characterize the structure of the new ice phase. Using the P02.2 beamline at the PETRA III facility, they confirmed that Ice XXI possesses a tetragonal crystal structure. A key feature of this structure is its surprisingly large repetitive units, known as unit cells. The complexity and size of this repeating pattern set it apart from other known ice forms.
The researchers determined that the specific crystallization pathway of liquid water under pressure depends heavily on the degree of supercompression it experiences. This means the pressure history—how quickly and to what extent the water is squeezed—dictates which form of ice will ultimately emerge. The findings suggest a rich landscape of possible ice structures that can be accessed through dynamic compression, many of which may not be achievable through slower, more conventional methods of crystallization.
Implications for Planetary Science and Beyond
The discovery of new high-pressure ice phases has significant implications for our understanding of the interiors of icy celestial bodies. Planets and moons throughout the solar system and beyond are believed to contain vast quantities of water under pressures and temperatures far different from those on Earth’s surface. Understanding how water behaves under these conditions is crucial for accurately modeling the internal structure, geology, and potential habitability of these worlds.
The existence of a metastable phase like Ice XXI suggests that the interiors of icy moons could be more complex than previously thought. Models that assume water will always form the most stable ice phase for a given pressure and temperature may need revision. According to the research team, the findings open the door to the existence of numerous other high-temperature metastable ice phases and their corresponding transition pathways. This could introduce new layers of complexity into the composition and dynamics of these planetary bodies, influencing everything from internal heating to the behavior of subsurface oceans.
