We usually think of ice as just cooled water, simple, hard, and cold. But water is actually a "master of disguise". Although it is composed of only two atoms, hydrogen and oxygen, it can form more than 20 different types of ice, each of which has a unique internal structure. Some ice forms are smooth and familiar, such as ordinary ice cubes in household refrigerators; while in high-pressure environments such as deep in the Earth or on distant satellites, some strange "ice phases" may appear.

Scientists have been exploring these mysterious ice types for more than a century, not just out of curiosity but to understand how water behaves in extreme conditions, including where alien life might exist.

Previous research has discovered new types of ice such as Ice XIX, which has oxygen atoms arranged in the same way as Ice XV but hydrogen atoms arranged differently. There is also a new ice phase called Ice VIIIt, which occurs mainly deep in the Earth's mantle or on water-rich exoplanets, and was later classified as known as Ice X.

Recently, scientists from the Korea Institute of Standards and Science (KRISS) discovered a brand-new type of ice: officially named Ice XXI, its structure is completely different from any previously known type.

They used a powerful experimental device built with a diamond anvil and an X-ray laser to observe the behavior of extremely compressed water at room temperature. Unexpectedly, water did not freeze directly into ice in this pressure range, but experienced multiple freeze-thaw cycles, and finally ice XXI was born in the typical pressure range of ice VI.

What’s so special about Ice XXI? It has a unique atomic structure that is significantly different from the more than 20 currently known ice types. More importantly, it exhibits a "metastable state", that is, it can still exist briefly in an originally unstable environment, providing researchers with the opportunity to observe the formation process of ice under high pressure. This discovery is expected to help scientists understand the material environment of the icy planet and the deep Earth.

"Rapidly compressing the water allows it to remain liquid at higher pressures where it should have turned into ice VI," explained KRISS scientist Geun Woo Lee.

The research team used diamond anvils to create high pressure, conducted experiments on extremely pure water samples, and used high-speed cameras, laser sensors and real-time monitoring equipment to observe the freezing and melting processes of ice at normal temperatures. By adjusting the pressure, capturing structural transitions, and using Raman spectroscopy to analyze changes within the water molecules, every moment of water freezing was recorded. The fluorescence of microscopic ruby ​​crystals is used to accurately measure pressure.

Scientists also use powerful X-ray beams of synchrotron radiation, combined with high-precision detectors and analysis programs, to observe the exact moment when water forms "abnormal ice". In order to better match the freezing rhythm of the ice, the pressure is applied in an irregular triangular rhythm. Two types of detectors simultaneously captured experimental data at 560,000 times per second and 10 times per second respectively, allowing them to depict the "hidden dance" of water transforming into ice.

Molecular dynamics simulations used two models, SPCfw and TIP4P/Ice. TIP4P/Ice represents rigid water molecules, with basically unchanged angles and bond lengths, and is very suitable for the study of high-pressure ice; the SPCfw model is more flexible and can simulate the bending and stretching of hydrogen bonds between molecules under extreme high pressure. The simulation trends of the two are basically consistent and consistent with the experimental results.

In extremely compressed room-temperature water, scientists discovered that water does not freeze in a single step, but undergoes multiple ice-water cycles, eventually transforming into what is known as ice VI. In this pressure range (about 1.6GPa), they discovered ice XXI, whose crystal structure is body-centered tetragonal.

Ice XXI is very special: although its energy is higher than MS-Ice VII at room temperature and its stability is lower, the difference is not big. What's more interesting is that only Ice XXI can be converted into MS-Ice VII, but ordinary water cannot directly complete this transformation. However, if water is mixed with MS-Ice VII, both can be transformed into Ice VI under high pressure.

Using Europe's XFEL's powerful X-ray laser, scientists have discovered that water develops in at least five different ways when it freezes, even at room temperature.

Geun Woo Lee pointed out, "Using the unique X-ray pulses of the European XFEL, we revealed that H2O can undergo multiple crystallization pathways during rapid compression and decompression more than a thousand times."

Rachel Husband, a member of the research team, added, "These findings indicate that the high-temperature metastable ice phase and its transformation path may be far beyond previous knowledge, and are expected to bring new revelations to our understanding of the material structure of icy satellites."

Relevant research has been published in "Nature Materials".