Scientists Discover Ice XXI: A New Metastable Room-Temperature Phase of Ice Using X-Ray Lasers

X-ray laser experiment reveals Ice XXI structure

Ice may seem simple at first glance, but its structure hides remarkable complexity. Over the past century, scientists have discovered more than twenty different solid phases of ice, each with distinct molecular arrangements. Now, an international team led by researchers from the Korea Research Institute of Standards and Science (KRISS), working at the European XFEL and DESY's PETRA III facilities, has identified a new metastable phase of ice — named Ice XXI — that forms at room temperature under extreme pressure.

This discovery sheds new light on how water behaves under high-pressure conditions and provides valuable insights into the complex crystallization pathways of H2O. The results were published in Nature Materials and mark a milestone in both fundamental physics and planetary science.

The Hidden Diversity of Ice

While everyday ice (known as Ice I) forms hexagonal crystals at ambient conditions, water exhibits a rich variety of structural arrangements when subjected to different combinations of pressure and temperature. Researchers have identified phases such as Ice II, Ice VI, Ice VII, and more, many of which occur only under extreme environments — like the interiors of icy moons and planets.

Understanding these phases is not only of academic interest; it has direct implications for planetary science. For example, Ice VI and Ice VII are believed to exist deep inside moons like Titan and Ganymede, influencing their internal dynamics and potential habitability.

Creating Ice XXI: High-Speed Compression Meets X-Ray Lasers

The new Ice XXI phase was discovered during experiments at the European XFEL, the world’s most powerful X-ray laser, and DESY’s PETRA III photon source in Germany. Using a dynamic diamond anvil cell setup, researchers compressed liquid water to pressures up to 2 gigapascals (20,000 times atmospheric pressure) within just 10 milliseconds. At such pressures, water typically crystallizes into Ice VI, but under rapid compression it remained liquid longer than expected — allowing a metastable intermediate phase to form.

By combining this rapid compression with ultra-fast X-ray imaging, the team could observe the ice formation process with microsecond precision. The X-ray flashes acted like a high-speed camera, capturing multiple crystallization pathways in real time. The result was the identification of a tetragonal crystal structure with unusually large unit cells, which the team classified as Ice XXI.

A Metastable Room-Temperature Phase

Unlike most other exotic ice phases that require low temperatures, Ice XXI forms and persists at room temperature. Although it is not the most stable structure under those conditions, it can exist for a significant amount of time before transforming into another phase. This kind of metastability is scientifically important because it reveals hidden intermediate states in the complex landscape of ice formation.

KRISS researcher Geun Woo Lee explained: “Rapid compression allows water to remain liquid up to higher pressures, where it should have already crystallized to Ice VI. This unique pathway enabled us to observe a previously unknown ice phase.”

Why This Discovery Matters

The identification of Ice XXI goes beyond adding another entry to the list of known ice phases. It demonstrates the power of combining dynamic compression techniques with cutting-edge X-ray laser imaging to explore states of matter that are normally inaccessible. It also offers new clues about the behavior of water in extraterrestrial environments.

As Rachel Husband from DESY’s HIBEF team noted, “Our findings suggest that a greater number of high-temperature metastable ice phases and their associated transition pathways may exist, potentially offering new insights into the composition of icy moons.”

This work is part of a broader “Water Call” initiative at European XFEL, which invites innovative studies on the physics of water. According to Scientific Director Sakura Pascarelli, “It is fantastic to see another great outcome from this initiative. We are looking forward to many more exciting discoveries ahead.”

Implications for Planetary Science and Beyond

The discovery of Ice XXI may help explain the internal structures of water-rich celestial bodies, inform models of ice layer dynamics under extreme pressure, and enhance our understanding of how water behaves in astrophysical settings. It also provides experimental evidence that can refine theoretical models of hydrogen bonding and phase transitions in H2O.

This is a powerful example of how advanced experimental tools like X-ray lasers can uncover hidden states of matter — revealing that even something as familiar as water still has secrets to tell.

Source: Phys.org – Ice XXI: Scientists use X-ray laser to identify new room-temperature phase

This article was prepared with the help of AI technologies.

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