[Reading Science] Discovery of a New Ice Phase Under Ultra-High Pressure

KRISS Achieves World's First Identification of 'Ice XXI' at Room Temperature
A New Chapter for Space Life Exploration and Advanced Materials Research

The Korea Research Institute of Standards and Science (KRISS) has captured the moment when water transforms into ice under ultra-high pressure conditions exceeding 20,000 atmospheres at room temperature.

On October 27, KRISS researchers announced that they had observed the crystallization process of water in microseconds (μs, one-millionth of a second) and, for the first time in the world, discovered a previously unknown crystalline phase of ice, named "Ice XXI."

Phase diagram of various states of water and ice existing over a wide range of temperatures and pressures. The area marked in blue indicates the location of the newly discovered room-temperature ultra-high-pressure ice, "Ice XXI." Provided by KRISS

'Room-Temperature Ice' Created by Pressure

While water typically freezes below 0°C, both temperature and pressure determine its state. If the pressure is sufficiently high, water can become solid ice even at room temperature.

At room temperature, water transforms into "Ice VI" at about 9,600 atmospheres (0.96 GPa) or higher. In this study, the researchers directly observed the process in which water remains liquid and then turns into ice under much higher pressure conditions of 20,000 atmospheres (2 GPa).

Under such extreme pressure, the hydrogen bond structure between water molecules becomes intricately twisted and rearranged, resulting in the formation of various types of ice.

Until now, scientists worldwide have discovered 20 types of crystalline ice under various temperature and pressure conditions. However, the intermediate pressure range between 0 and 20,000 atmospheres is a "complex region" where multiple phase transitions overlap, making it difficult to reveal the internal structure.

Dr. Minju Kim, Postdoctoral Researcher (left), and Eunhee Lee, Senior Researcher (right) at KRISS are observing the crystallization process of excess pressure water implemented through a dynamic diamond anvil cell device. Provided by KRISS

Technology That Creates 20,000 Atmospheres in a Space as Thin as a Strand of Hair

The background of this discovery lies in KRISS's independently developed ultra-precision compression device, the "dynamic diamond anvil cell (dDAC)" technology.

This device places water in a micro-sized metal gap thinner than a strand of hair and applies ultra-high pressure by pressing it between two diamonds. Conventional devices required manual pressure adjustment and took tens of seconds to compress, but KRISS designed its system using a piezo actuator to apply pressure within just 10 milliseconds (one-thousandth of a second).

Thanks to this, the researchers were able to realize the "supercompression" state of water just before it turns into ice. This is a metastable liquid state in which water withstands much higher pressure than the pressure at which it should have already frozen, a region that has rarely been observed experimentally until now.

The team exposed water in this state to the European XFEL, the world's largest X-ray free-electron laser facility, firing millions of X-ray pulses per second to capture changes in the molecular structure of water. As a result, they discovered more than five new crystallization pathways at room temperature, one of which was identified as a previously unreported crystalline phase, "Ice XXI."

Larger Crystal Structure Than Conventional Ice... Similar to Planetary Interiors

The newly discovered Ice XXI exhibited a unique structure distinct from conventional ice. The size of its "unit cell," the smallest repeating unit of the crystal structure, was very large and formed a flat rectangular cuboid shape.

The KRISS research team analyzed the precise positions of water molecules using X-ray diffraction data and fully elucidated the structure of the new Ice XXI.

KRISS Space Extreme Measurement Group Ultra-High Pressure Research Team. From left: Yunhee Lee, Senior Researcher; Minju Kim, Postdoctoral Researcher; Jinkyun Kim, Postdoctoral Researcher; Geunwoo Lee, Senior Researcher. Provided by KRISS

Yunhee Lee, Senior Researcher of the KRISS Space Extreme Measurement Group, explained, "The density of Ice XXI is similar to that of the ultra-high-pressure ice layers predicted to exist inside the icy satellites of Jupiter and Saturn. This study will provide important clues for exploring the internal structures of planetary bodies and the potential for life beyond Earth."

Geunwoo Lee, Senior Researcher of the same group, emphasized, "By combining our proprietary dDAC technology with the ultrafast imaging capabilities of the XFEL, we captured moments that were previously inaccessible with conventional equipment. Research on the behavior of materials under extreme conditions will open new chapters in the development of advanced materials and the science of extreme environments."

'Ice Research' Expanding Beyond Physics to Space Science

This achievement goes beyond the discovery of a new form of ice; it is significant in that it experimentally demonstrates how the structure of water fundamentally changes under extreme conditions. The study was supported by the "Development of Ultra-High-Temperature Materials and Property Measurement Technology for 4,000 K-Class Rocket Engines" project, promoted by the National Research Council of Science & Technology.

The results were published in the October issue of the international journal "Nature Materials" (impact factor 38.5). The paper, titled "Multiple Freezing-Melting Pathways of High-Density Ice through Ice XXI Phase at Room Temperature," lists Yunhee Lee as the first author and Geunwoo Lee as the corresponding author.

KRISS is now recognized for having established the foundational technology to precisely measure the behavior of materials under ultra-high-pressure and ultra-high-temperature conditions, such as those found in extreme environments in space.