Scientists Unveil Unique Ice XXI, A New Form of Frozen Water

Researchers from the Korea Research Institute of Standards and Science (KRISS) have discovered a new form of ice, officially designated as ice XXI, which exhibits unique properties and formation processes. This remarkable finding, reported in the journal Nature Materials on March 15, 2024, challenges conventional understandings of how ice behaves under varying pressures and temperatures.

Ice is typically perceived merely as frozen water, but it consists of over 20 distinct forms, each with unique atomic structures. While some types of ice, such as those found in household freezers, are familiar, others require extreme conditions to manifest. Previous studies have documented forms like ice XV and ice VIIt, which exist under high pressure, but ice XXI stands out due to its formation characteristics at ambient temperatures.

Using advanced technology, including diamond anvil cells and powerful X-ray lasers, scientists observed how super-compressed water behaves at room temperature. Instead of freezing in a straightforward manner, the water underwent multiple freeze–melt cycles within the pressure range where ice VI typically forms. The result was the emergence of ice XXI, which possesses a body-centered tetragonal crystal structure that differentiates it from all previously identified ice types.

Geun Woo Lee, a scientist at KRISS, explained the significance of this discovery: “Rapid compression of water allows it to remain liquid up to higher pressures, where it should have already crystallized to ice VI.” This unique behavior of ice XXI indicates a metastable state, meaning it can persist for a duration despite its inherent instability under those conditions.

The research team meticulously created high-pressure environments by loading ultra-pure water into a metal chamber no thicker than a hair. They utilized high-speed cameras and laser-based sensors to monitor the water’s transition from liquid to solid in real time. By varying the pressure rhythmically, they captured detailed snapshots of the freezing process, revealing how the water molecules interact and rearrange themselves.

In their experiments, scientists discovered that water does not freeze in a singular step but follows at least five distinct pathways before settling into ice VI. This finding suggests that the process of ice formation is more complex than previously understood, with ice XXI representing a previously unrecognized phase of frozen water.

The pressure required for ice XXI’s formation is approximately 1.6 gigapascals, and it holds more energy than MS-ice VII at room temperature, indicating a lower stability. Notably, ice XXI can transform into MS-ice VII, a transition that regular water cannot achieve under similar conditions. In experiments utilizing the European XFEL, researchers highlighted the intricate behavior of water during the crystallization process.

Rachel Husband, another member of the research team, remarked that these findings imply the existence of additional high-temperature metastable ice phases. This could provide new insights into the characteristics of icy moons and the potential for life beyond Earth.

The implications of discovering ice XXI extend beyond scientific curiosity. Understanding the properties of various ice forms can enhance our knowledge of planetary bodies, including icy exoplanets and Earth’s mantle. This research not only adds to the growing body of knowledge about water but also opens new avenues for exploration in astrobiology and geology.

As scientists continue to investigate the complexities of ice formation, ice XXI stands as a testament to the remarkable versatility of water and its ability to adapt to extreme conditions.