Scientists unveil Ice XXI, an ice so extreme it forms at room temp

An international team led by the Korea Research Institute of Standards and Science (KRISS) has discovered a new crystalline form of ice, named Ice XXI, which forms at room temperature under extremely high pressures exceeding 2 gigapascals (GPa). This represents the 21st known crystalline ice phase and was identified through a novel crystallization pathway involving rapid pressure changes applied via a dynamic diamond anvil cell (dDAC) combined with observations from the world’s largest X-ray free-electron laser facility. The researchers captured multiple, previously unknown freezing and melting events at microsecond resolution, revealing that water undergoes complex phase transitions rather than a single-step freeze under these conditions. Ice XXI is structurally distinct from all previously known ice phases, featuring a large, flattened rectangular unit cell. Its density is comparable to the high-pressure ice layers believed to exist inside icy moons of Jupiter and Saturn, suggesting that these celestial bodies may have more complex internal structures than previously thought. This discovery not only advances understanding of