Scientists have discovered a new form of water ice, dubbed Ice XXI, which exists at room temperature and has a unique structure. This groundbreaking finding, made by researchers at the European XFEL and PETRA III facilities, could shed light on the formation of different ice phases under high pressure, including those found on icy moons and planets. Ice XXI was produced by rapidly compressing water to pressures of up to 2 GPa, which is 20,000 times higher than normal air pressure at sea level. This extreme pressure allows ice to form even at room temperature within a specialized device called a dynamic diamond anvil cell (dDAC).
The structure of Ice XXI is distinct from all previously observed ice phases because its molecules are much more tightly packed, resulting in the largest unit cell volume of any known ice type. This metastable form of ice can exist even though another form, ice VI, would be more stable under the experimental conditions. The rapid compression of water enables it to remain liquid at higher pressures, where it should have already crystallized into ice VI, which is thought to be present in the interiors of icy moons like Titan and Ganymede. The highly distorted structure of ice VI may allow complex transition pathways that lead to metastable ice phases.
To study the formation of Ice XXI, researchers rapidly compressed and decompressed it over 1000 times in the diamond anvil cell while imaging it every microsecond using the European XFEL. They found that the liquid water crystallizes into different structures depending on its level of supercompression. The KRISS team then used the P02.2 beamline at PETRA III to determine that Ice XXI has a body-centered tetragonal crystal structure with a large unit cell at approximately 1.6 GPa. This unit cell contains 152 water molecules, resulting in a density of 1.413 g/cm³.
Despite the challenges, the researchers were able to analyze the crystal structure of Ice XXI. The difficulty lies in keeping the ice stable for long enough periods to make precise structural measurements. The multiple pathways of ice crystallization uncovered in this study suggest that many more ice phases may exist, and it is crucial to analyze the mechanism behind their formation. This could help scientists better understand the formation and evolution of these phases on icy moons and planets.
