The scientific community has documented a major breakthrough in understanding the fundamental characteristics of water: the identification of the twenty-first distinct form of ice, officially designated Ice XXI. This revelation, detailed in the journal Nature Materials, fundamentally challenges existing assumptions regarding how H2O molecules arrange themselves in a solid state, particularly when conventional temperature and pressure boundaries are breached. Ice XXI exists as a metastable phase, characterized by a tetragonal crystalline structure, which was successfully stabilized at ambient room temperature, though only under the influence of immense compressive forces.
The groundbreaking research was executed utilizing the powerful capabilities of the European X-ray Free-Electron Laser (XFEL) and the PETRA III photon source, situated within German research hubs, including experts from DESY. Scientists from the Korean Research Institute of Standards and Science (KRISS) played a pivotal role in the investigation. The core of the experiment involved manipulating a water sample with unprecedented speed: it was subjected to compression reaching 2 gigapascals—a pressure roughly equivalent to 20,000 atmospheres—in a mere 10 milliseconds. This rapid pressurization was achieved using a specialized diamond anvil cell.
To capture the fleeting molecular changes, the process was repeated thousands of times. The crystallization dynamics were monitored with exceptional precision, capturing data at a rate of one million images per second. This new structure stands apart from the twenty previously cataloged modifications of ice. Its tetragonal lattice is notable for possessing unusually large elementary cells. Analysis conducted at the PETRA III P02.2 beamline confirmed that these cells encapsulate 152 water molecules.
Researchers, including Gyeong Woo Lee from KRISS, highlighted that such extremely fast compression enabled the water to maintain its liquid state even at pressures where it would typically transition into Ice VI. Ice VI is a phase hypothesized to exist deep within the interiors of icy satellites, such as Titan and Ganymede. The ability to bypass this expected phase transition is key to understanding the formation mechanism of Ice XXI.
While the immediate practical application of this discovery in everyday life is limited due to the extreme conditions required for its formation, its significance for astrophysics is profound. Grasping the behavior of water under these high-pressure regimes opens up new avenues for modeling the internal architecture of icy planets and their moons. This knowledge acts as a catalyst for revising current planetary models, as every new phase, like Ice XXI, illuminates the hidden potential of matter. The very demonstration of such complex variability in water—a substance so familiar to us—serves as a powerful reminder of the boundless potential for discovery concealed within even the simplest systems, urging researchers to continue the search for undiscovered high-temperature metastable states.