Ice generally forms when water cools below 0 C, but can also form at room temperature or higher if the pressure is sufficient. When water is pressurized above 0.96 GPa at room temperature, it transitions into Ice VI. The process by which water freezes involves complex distortion and rearrangement of the hydrogen-bonded network, depending on both temperature and pressure, resulting in a variety of ice phases.

Over the last century, scientists have reported 20 crystalline ice phases by varying temperature and pressure. These include forms that exist across over 2,000 K and more than 100 GPa. The region between ambient pressure and 2 GPa is particularly complex, with over ten phases densely grouped.

KRISS's Space Metrology Group created a supercompressed liquid state - liquid water at room temperature under more than twice the known crystallization pressure - using an in-house developed dynamic diamond anvil cell (dDAC). The dDAC employs a pair of diamonds with piezoelectric actuators to dynamically control and observe pressure changes in microscopic water samples.

Unlike conventional diamond anvil cells, which use mechanical bolts causing pressure gradients and easy nucleation, the dDAC developed at KRISS minimizes mechanical shock and reduces the compression time from tens of seconds to just 10 milliseconds. This allowed the team to compress water to the required pressures for Ice VI and beyond.

The international research team tracked the crystallization of supercompressed water with a combination of the dDAC and the European XFEL X-ray free-electron laser, achieving microsecond time resolution. They discovered multiple complicated crystallization pathways at room temperature, which included the formation of the new Ice XXI - now recognized as the 21st crystalline phase of ice.

The research team not only uncovered multiple crystallization pathways but also determined the detailed structure of Ice XXI. Formed at room temperature, this new phase has a uniquely large and complex unit cell compared to those previously identified, showing a flattened rectangular structure with two equally long base edges.

This was achieved through collaboration among 33 scientists from Korea, Germany, Japan, the USA, England, the European XFEL, and DESY. The project was led by KRISS Principal Investigator Dr. Lee Geun Woo.

KRISS co-first author 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 commented, "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."

The study was supported by the National Research Council of Science and Technology's ultra-high temperature materials program. Results were published in Nature Materials in October.

Research Report:Multiple freezing-melting pathways of high-density ice through ice XXI phase at room temperature

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