Scientists have demonstrated that supercompressed water turns into ice VI at room temperature through multiple freeze-thaw pathways that occur through a previously unknown form of metastable ice called ice XXI.
The small ice crystal XXI formed through nucleation and slow melting. Image credit: Lee etc.., doi: 10.1038/s41563-025-02364-x.
Water, consisting of only two elements, forms numerous polymorphic phases from ice Ih to ice XX and four amorphous phases.
Understanding the formation and transition of different water phases has been of interest for a century in high-pressure physics and the search for life in space and on icy moons.
“Water exhibits amazing complexity in its solid state,” said Dr. Geun-Woo Lee, a researcher at the Korea Standards and Science Research Institute and the University of Science and Technology.
“Most phases are observed at high pressures and low temperatures.”
“The rapid compression of water allows it to remain liquid to higher pressures, where it would already have crystallized to ice VI“
“Ice VI is a particularly intriguing phase that is believed to be present inside icy moons such as Titan and Ganymede.”
“Its highly distorted structure may allow complex transition pathways that lead to metastable ice phases.”
“Since most varieties of ice only exist in extreme conditions, we created high-pressure conditions using diamond anvil cells.”
“The sample—in this case, water—is placed between two diamonds, which, because of their hardness, can be used to create very high pressure.”
“Water has been tested at pressures of up to two gigapascals, which is approximately 20,000 times normal air pressure.”
“This results in ice formation even at room temperature, but the molecules are packed much more densely than in regular ice.”
To observe ice formation under different pressure conditions, the researchers first created a high pressure of two gigapascals for 10 milliseconds.
They then released the anvil for 1 second and then repeated the process.
During these cycles, scientists used Europe's XFEL X-ray flares to image the sample every microsecond.
Thanks to the extremely high frequency of the X-ray pulses, they could make films of how the ice structure formed.
Then, using the P02.2 beam in PETRA III, the authors determined that ice XXI has a tetragonal crystal structure, composed of surprisingly large repeating units called unit cells.
“Using the unique X-ray pulses of the European XFEL, we discovered multiple crystallization pathways in water that was rapidly compressed and decompressed more than 1,000 times using a dynamic diamond anvil cell,” Dr Li said.
“In this special pressure chamber, samples are compressed between the tips of two opposing diamond anvils and can be compressed along a specified pressure path,” said Dr. Cornelius Strohm, a researcher at Deutsches Elektronen-Synchrotron.
“The structure in which liquid water crystallizes depends on the degree of supercompression of the liquid,” Dr. Lee said.
“Our results suggest that there may be a greater number of high-temperature metastable ice phases and associated transition pathways, potentially providing new insights into the composition of icy moons,” said Dr Rachel Husband, also from Deutsches Elektronen-Synchrotron.
conclusions were published on October 10 in the magazine Natural materials.
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Yu.H. Lee etc.. Multiple freeze-thaw paths for high-density ice through XXI phase ice at room temperature. Night. Materpublished online October 10, 2025; doi: 10.1038/s41563-025-02364-x
