New Ice XXI Emerges When Water Is Crushed to Extreme Pressures

Inside the dynamic diamond anvil cell used by the KRISS research team to generate the supercompressed state (A pair of diamond anvils can be seen.) Credit: Korea Research Institute of Standards and Science (KRISS)

A microsecond freeze under crushing pressure has revealed Ice XXI, an entirely new form of ice.

The Korea Research Institute of Standards and Science (KRISS, President Lee Ho Seong) has recorded water repeatedly freezing and melting at pressures above 2 gigapascals (2 GPa) while still at room temperature. These rapid cycles occurred within microseconds (μs, one-millionth of a second), making it the first time scientists have directly observed this process.

This achievement led to the identification of a previously unknown crystallization pathway in water and resulted in the discovery of a new ice phase. The newly recognized structure has been named Ice XXI, making it the 21st crystalline form of ice.

Phase diagram of water and ice showing the newly discovered room-temperature high-pressure phase, Ice XXI (blue region). Credit: Korea Research Institute of Standards and Science (KRISS)

How Pressure Creates New Ice Forms

Although ice usually forms below 0 °C, pressure can also trigger crystallization. Under high enough pressure, water can freeze at room temperature and even at temperatures above its normal boiling point. When water is pressurized beyond 0.96 GPa at room temperature, it changes into Ice VI.

As water freezes, hydrogen bonds reorganize in complex ways that depend on both pressure and temperature. These molecular adjustments produce a wide range of ice structures during crystallization.

Gaining insight into how these transitions occur, and learning how to control them in extreme environments, could ultimately help scientists develop new materials that do not naturally occur on Earth.

Historical timeline of ice phase discoveries from Ice I to Ice XX spanning more than a century. Credit: Korea Research Institute of Standards and Science (KRISS)

A Century of Mapping Ice Phases

Over the past hundred years, researchers have identified 20 crystalline ice phases[1] by altering pressure and temperature. These structures form across an enormous range that spans more than 2,000 K and over 100 GPa. The section of the phase diagram between 0 GPa and 2 GPa is one of the most intricate areas, containing more than ten closely packed ice phases.

Creating a Supercompressed Liquid State

The Space Metrology Group at KRISS produced a liquid state that remained stable at pressures greater than 2 GPa and room temperature, even though this is more than twice the pressure normally required for crystallization. They achieved this using their in-house dynamic diamond anvil cell (dDAC)[2], a tool designed for precise high-pressure experiments.

Standard diamond anvil cells (DACs) increase pressure by tightening bolts, a method that can introduce mechanical disturbances and uneven pressure that cause early nucleation. The KRISS dDAC reduces mechanical shock and decreases the compression time from tens of seconds to only 10 milliseconds (ms). This allowed researchers to compress water into the Ice VI pressure region while keeping it in liquid form.

KRISS researchers (from left: Dr. Lee Yun-Hee, Dr. Kim Minju, Dr. Lee Geun Woo, and Dr. Kim Jin Kyun) with Dr. Cornelius Strohm, beamline operator at the European XFEL, in front of the IC2 high-vacuum X-ray chamber used in the study. Credit: European XFEL

Capturing the Formation of Ice XXI

In partnership with scientists from around the world, the KRISS team used the dDAC together with the European XFEL (the world’s largest X-ray free-electron laser facility) to observe the crystallization of supercompressed water with microsecond accuracy. These measurements revealed complex crystallization routes that had never been documented at room temperature. All of these transitions occurred through Ice XXI, establishing it as the 21st crystalline ice phase.

KRISS researchers also determined the internal structure of Ice XXI and documented the pathways that produce it. Ice XXI has a remarkably large and intricate unit cell, the smallest repeating pattern in a crystal lattice. The structure forms a flattened rectangular shape in which the two base edges are identical in length.

Unit cell structure of ice XXI. Credit: Korea Research Institute of Standards and Science (KRISS)

A Global Scientific Effort

This breakthrough was achieved through a collaboration of 33 scientists from South Korea, Germany, Japan, the USA, and England, working together with researchers at the European XFEL and DESY. The project was proposed and led by KRISS under the direction of principal investigator (PI) Dr. Lee Geun Woo.

The KRISS research group—including Dr. Kim Jin Kyun (co-first author, postdoctoral researcher at KRISS), Dr. Kim Yong-Jae (co-first author, formerly postdoctoral researcher at KRISS and now at Lawrence Livermore National Laboratory), Dr. Lee Yun-Hee (co-first author, Principal Research Scientist), Dr.

Kim Minju (co-author, Postdoctoral Researcher), Dr. Cho Yong Chan (co-author, Principal Research Scientist), and Dr. Lee Geun Woo (corresponding author, Principal Research Scientist)—led the experiment design, data collection, and structure analysis. Their combined work made the discovery of Ice XXI possible and represents a significant advancement in high-pressure and planetary science.

Implications for Planetary Science

Dr. Lee Yun-Hee said, “The density of Ice XXI is comparable to the high-pressure ice layers inside the icy moons of Jupiter and Saturn. This discovery may provide new clues for exploring the origins of life under extreme conditions in space.”

Dr. Lee Geun Woo added, “By combining our in-house developed dDAC technology with the XFEL, we were able to capture fleeting moments that had been inaccessible with conventional instruments. Continued research into ultrahigh-pressure and other extreme environments will open new frontiers in science.”

This work was supported by the 4000 K-class Rocket Engine Ultra-High Temperature Materials and Measurement Technologies Development Project of the National Research Council of Science & Technology (NST). The findings were published in Nature Materials.

Explore Further: Scientists Discover a New Form of Ice That Shouldn’t Exist

Notes

1. Previously, ice phases from Ice I to Ice XX had been reported. Ice I exists in two structural forms: the hexagonal Ice Ih and the cubic Ice Ic.

2. The dDAC is a high-pressure device that uses a pair of diamonds and piezoelectric actuators to dynamically control and observe pressure changes in a microscopic water sample.

Reference: “Multiple freezing–melting pathways of high-density ice through ice XXI phase at room temperature” by Yun-Hee Lee, Jin Kyun Kim, Yong-Jae Kim, Minju Kim, Yong Chan Cho, Rachel J. Husband, Cornelius Strohm, Emma Ehrenreich-Petersen, Konstantin Glazyrin, Torsten Laurus, Heinz Graafsma, Robert P. Bauer, Felix Lehmkühler, Karen Appel, Zuzana Konôpková, Minxue Tang, Anand Prashant Dwivedi, Jolanta Sztuck-Dambietz, Lisa Randolph, Khachiwan Buakor, Oliver Humphries, Carsten Baehtz, Tobias Eklund, Lisa Katharina Mohrbach, Anshuman Mondal, Hauke Marquardt, Earl Francis O’Bannon, Katrin Amann-Winkel, Choong-Shik Yoo, Ulf Zastrau, Hanns-Peter Liermann, Hiroki Nada and Geun Woo Lee, 10 October 2025, Nature Materials.

DOI: 10.1038/s41563-025-02364-x

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