World’s first discovery of ice XXI: A new form of ice born under two gigapascals of pressure at room temperature

The Korea Research Institute of Standards and Science (KRISS, President Lee Ho Seong) has successfully observed, for the first time, the multiple freezing-melting process of water under ultrahigh pressure exceeding 2 gigapascals (2 GPa) at room temperature on a microsecond (μs, one-millionth of a second) timescale.

This breakthrough led to the world’s first discovery of a previously unknown crystallization pathway of water and a new 21st ice phase, named Ice XXI.

While ice generally forms when water cools below 0 °C, it can also form at room temperature or even at elevated temperatures above the boiling point. This is because crystallization—the process in which a liquid transforms into a solid—is influenced not only by temperature but also by pressure. At room temperature, pressurized water exceeding 0.96 GPa undergoes a phase transition into ice crystal, that is, Ice VI.

When water freezes, the hydrogen-bonded network among water molecules becomes intricately distorted and rearranged depending on temperature and pressure, resulting in a variety of ice phases during the crystallization process.

A deeper understanding of these complex phase transitions and structural formation mechanisms between water and ice—and the ability to control them under extreme pressure and temperature conditions—could lead to the creation of new materials never before found on Earth.

Over the past century, researchers around the world have identified 20 distinct crystalline ice phases* by varying temperature and pressure conditions. These phases have been discovered across an extremely wide range—over 2,000 K in temperature and more than 100 GPa in pressure. Among them, the region between ambient pressure (0 GPa) and 2 GPa is particularly important, as it represents the most complex zone of water’s phase transitions, where more than ten ice phases are densely clustered.

* 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.

The Space Metrology Group at KRISS successfully generated a supercompressed liquid state—where water remains liquid state under high pressure exceeding 2 GPa at room temperature, more than twice the known crystallization pressure—using an in-house developed dynamic diamond anvil cell (dDAC*).

* 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.

Unlike conventional diamond anvil cells (DACs), which increase pressure by tightening assembly bolts and often cause easy nucleation due to pressure gradients and mechanical perturbations, the dDAC developed by KRISS minimizes mechanical shock during compression and shortens the compression time from tens of seconds to just 10 milliseconds (ms). This innovation enabled water to be highly compressed into the pressure range of the Ice VI phase.

International collaboration research team led by the KRISS scientists captured the crystallization process of supercompressed water using the combination of dDAC and European XFEL (-the world’s largest X-ray free-electron laser facility) with microsecond time resolution. Through these observations, the team discovered very complicated multiple crystallization pathways that had previously been unknown at room temperature, and revealed that these pathways occurred via a new ice phase, named Ice XXI that is 21st crystalline ice phase for the first time in the world.

The KRISS research team identified the detailed structure of ice XXI as well as the multiple crystallization pathways. Surprisingly, the Ice XXI formed at room temperature possesses a remarkably large and complex unit cell—the smallest repeating unit of a crystal lattice—compared to previously known ice phases. Its crystal structure exhibits a flattened rectangular shape, in which the two base edges are equal in length.

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

The KRISS research team—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)—played a leading role in the experimental design, data acquisition, and structural analysis. Their leadership and sustained efforts were pivotal to the world’s first discovery of Ice XXI, marking a major step forward in high pressure and planetary science.

Dr. Lee Yun-Hee stated, “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 stated, “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 research 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 results were published in Nature Materials (Impact Factor: 38.5) in October.