Not all water ice is the same, because water can crystallize in very different structural forms. Now researchers have discovered a new variant – the 21st crystal form of water. This ice XXI is formed when water is compressed with a pressure of two gigapascals within a few milliseconds. Under these conditions, the water molecules form a solid lattice with a tetragonal crystal structure even at room temperature, which is made up of surprisingly large unit cells, as the team discovered during analyzes in the X-ray laser. This metastable ice form and its formation conditions could indicate that water can also form other crystalline high-pressure phases.
Water has several chemical and physical properties, including density anomalies, self-dissociation and the ability to form hydrogen bonds between molecules. Also unusual is the fact that water can crystallize into an unusually large number of different structural variants depending on temperature and pressure. The range extends from the normal hexagonal ice of snow crystals to square shapes and cage-like structures. So far, researchers have known about 20 different ice phases – Ice I to Ice XX – and four amorphous ice variants.
Novel grid shape at 1.6 gigapascal pressure
Now a 21st crystal form has been added. This new ice phase was identified by a team led by Yun-Hee Lee from the Korea Research Institute of Standards and Science (KRISS) in Daejeon during experiments in the X-ray laser at the European XFEL. For their experiment, the researchers placed a small water sample in a diamond anvil cell – a small sample chamber between two diamond presses that can generate extreme pressures. They then compressed the water within ten milliseconds to the enormous pressure of two gigapascals – which corresponds to around 200,000 times atmospheric pressure – and relaxed the sample again immediately afterwards. During the cycles, Lee and his team used the European XFEL’s ultra-short X-ray flashes to take a snapshot of the state of the water every microsecond. The diffraction of the X-ray light reveals the arrangement in which the water molecules are present at that moment. The actual goal of the experiments was to bring water into the ice VI phase. This highly distorted ice structure is thought to occur, among other things, in the interior of icy moons such as Titan and Ganymede.
But the experiment showed that before ice VI forms, other, only temporarily stable structural variants of water arise. “Using the unique X-ray pulses from the European XFEL, we discovered several crystallization pathways in water,” reports Lee. As he and his colleagues discovered, one of these pathways creates a previously unknown form of water crystallization for a short period of time. This exists at 1.6 gigapascals and room temperature. “The X-ray diffraction pattern of this ice phase does not match any of the known ice phases I to XXX,” write the researchers. They have thus discovered another form of ice called Ice XXI. Closer analysis revealed that this newly discovered structural variant of water has a tetragonal crystal structure made up of surprisingly large unit cells: a single one of these basic units in the lattice contains 152 water molecules. In addition, this ice phase is metastable. This means that it exists for a short time, even though another form of ice would actually be more stable under these conditions.
There could be other metastable forms of ice
The results of this experiment and the newly discovered water ice variant provide new insights into how different ice phases form and change with pressure. “Thanks to rapid compression, water remains liquid even at higher pressures at which it should actually crystallize into ice VI,” explains senior author Geun Woo Lee from KRISS. “The structure in which liquid water then crystallizes depends on the degree of overcompression of the liquid. Crystallization goes through several steps and crystallization paths.” The new results also suggest that there could be other short-lived, previously undiscovered forms of ice in water at room temperature and high pressure. Research into these could also provide valuable insights into the forms of ice that occur inside the icy moons of the solar system and other celestial bodies. “Our results suggest that at high temperatures there may be a larger number of metastable ice phases and associated transition pathways that could provide new insights into the composition of icy moons,” says co-author Rachel Husband from the German Electron Synchrotron DESY in Hamburg.
Source: Yun-Hee Lee (Korea Research Institute of Standards and Science (KRISS), Daejeon) et al., Nature Materials, doi: 10.1038/s41563-025-02364-x