Researchers have shown that hypercompressed water can undergo various freeze-thaw processes to convert into ice VI at room temperature, facilitated by a previously unidentified metastable ice form known as ice XXI.
Small ice crystals XXI formed through a nucleation and slow melting process. Image credit: Lee et al., doi: 10.1038/s41563-025-02364-x.
Water consists of only two elements, yet it has numerous polymorphs ranging from Ice Ih to Ice XX, along with four amorphous phases.
The exploration of formation and migration pathways for various water phases has captivated scientists for a century, especially in high-pressure physics and the quest for extraterrestrial life on icy moons.
“Water exhibits remarkable complexity in its solid forms,” stated Dr. Lee Geun-woo from the Korea Institute of Standards and Science and the National University of Science and Technology.
“Most phases are typically found under conditions of high pressure and low temperature.”
“When water is compressed rapidly, it can remain in a liquid state even under high pressures where it would normally crystallize into ice VI.”
“Ice VI is particularly fascinating, as it is believed to exist within icy moons like Titan and Ganymede.”
“Its highly distorted structure may facilitate intricate transition pathways that lead to metastable ice forms.”
“Most ice variants can only be observed under extreme conditions; hence, we utilized a diamond anvil cell to create these high-pressure conditions.”
“In this setup, the water sample is positioned between two diamonds, which can withstand and apply immense pressure.”
“The water was subjected to pressures as high as 2 gigapascals, roughly 20,000 times greater than typical atmospheric pressure.”
“This allows for ice formation even at room temperature, with molecules packed more tightly than in conventional ice.”
To examine ice formation under varied pressure conditions, researchers initially produced high pressures of 2 gigapascals in just 10 milliseconds.
They then took 1 second to release the anvil cell and repeated this procedure.
During these cycles, the scientists utilized Europe’s XFEL’s X-ray flashes to capture images of the sample every microsecond.
With extremely high X-ray pulse rates, they could even create videos illustrating the formation of ice structures.
Using PETRA III’s P02.2 beamline, the researchers confirmed that ice XXI possesses a tetragonal crystal structure composed of unusually large repeating units known as a unit cell.
“The unique X-ray pulses from Europe’s XFEL revealed multiple crystallization pathways in rapidly compressed and depressurized water over 1,000 times using a dynamic diamond anvil cell,” Dr. Lee explained.
“In this specialized pressure cell, the sample is squeezed between the tips of two opposing diamond anvils, following a defined pressure trajectory,” said Dr. Cornelius Strom from the Deutsche Electron Synchrotron.
“The crystallization structure of liquid water is influenced by the degree of supercompression,” added Dr. Lee.
“Our results hint at the potential existence of more high-temperature metastable ice phases and the related transition pathways, offering new insights into the compositions of icy satellites,” remarked Dr. Rachel Husband, also from the Deutsche Electron Synchrotron.
These findings were published in a magazine on October 10th in Nature Materials.
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YH. Lee et al. Multiple freeze-thaw paths of dense ice through the ice XXI phase at room temperature. Nat. Mater. published online on October 10, 2025. doi: 10.1038/s41563-025-02364-x
Source: www.sci.news
