Korea Research Institute of Standards and Science (KRISS) has discovered a new crystal structure of water that had not been known until now, "ice XXI (Ice 21)," for the first time in the world. It is the result of precisely observing the process of water turning into ice under ultrahigh pressure.
KRISS's Extreme Space Measurement Group said on the 27th that it observed in microseconds (one millionth of a second) the process in which water under ultrahigh pressure exceeding 20,000 atmospheres (2 GPa) at room temperature turns into ice, and noted that it identified ice XXI, the 21st new crystalline phase, in the process.
The ice we know forms below 0 degrees, but in fact ice is made not only by temperature but also by pressure. If water is subjected to pressure above 9,600 atmospheres (0.96 GPa) even at room temperature, it transforms into a crystalline form called "ice VI."
When water turns into ice like this, the hydrogen-bond network in which molecules are entangled changes in complex ways depending on temperature and pressure, and various shapes of the "phase of ice" emerge. So far, researchers around the world have found 20 types of ice by changing temperature and pressure.
The KRISS team applied more than 20,000 atmospheres at room temperature using a self-developed ultra-precise experimental device, the dynamic diamond anvil cell (dDAC). The device places water in a metal gap as thin as a strand of hair and applies pressure by squeezing it with two diamonds. With an attachment that finely controls pressure, it can raise the pressure much faster and more precisely than the previous hand-turned method.
With conventional equipment, even a slight increase in pressure would cause it to freeze quickly, but this device reduced the compression time to 10 milliseconds (one-thousandth of a second), keeping the water in a "supercompression" state above 20,000 atmospheres without freezing.
The team captured, in microseconds, the moment when water in a supercompression state turned into ice by using the world's largest X-ray laser facility in Germany, the European XFEL.
As a result, more than five previously unknown crystallization pathways were observed, and among them, a completely new form of ice, ice XXI, appeared.
Ice XXI had a much larger and flatter rectangular parallelepiped molecular arrangement than conventional ice. Lee Yun-hee, a principal researcher at KRISS, said the density of ice XXI is similar to the ultrahigh-pressure ice layers estimated to exist inside the icy moons of Jupiter or Saturn, and explained that the study could provide clues to understanding the origin of extraterrestrial life.
The study was carried out with support from the National Research Council of Science & Technology (NST) and was published in the international journal Nature Materials on the 10th.
References
Nat. Mater. (2025), DOI: https://doi.org/10.1038/s41563-025-02364-x