Scientists Discover New Ice Phase, ‘Ice XXI,’ Under Extreme Pressure
A groundbreaking study published in Nature Materials reveals the discovery of a new ice phase, dubbed Ice XXI, formed under pressures 10,000 times greater than atmospheric pressure. This finding reshapes our understanding of water’s complex behavior and opens new avenues for research in materials science, planetary science, and the search for extraterrestrial life.
Water, a substance seemingly simple, consistently defies expectations. Unlike most materials, water expands when it freezes, causing ice to float – a crucial phenomenon for life on Earth. This unusual property stems from the unique hydrogen bonds between water molecules, allowing for a multitude of arrangements depending on temperature and pressure.
Researchers at Lawrence Livermore National Laboratory (LLNL), in collaboration with scientists from the Korea Research Institute of Standards and Science (KRISS), embarked on a study to explore water’s behavior under extreme compression at room temperature. Their work revealed at least five distinct freezing-melting pathways, culminating in the identification of Ice XXI.
“Most people think of ice as just the solid that forms in a freezer, but water is far more complex,” explained a lead researcher. “Because water molecules are connected by hydrogen bonds, they can arrange themselves in many different ways depending on temperature and pressure. This has led to the discovery of more than 20 distinct crystalline phases of ice, from Ice I to Ice XXI, each with unique symmetry, density and bonding patterns.”
The team achieved these extreme pressures using a dynamic diamond anvil cell, compressing water to 10,000 times the atmospheric level. An X-ray free electron laser then captured snapshots of the water’s molecular structure during these rapid phase transitions. According to one scientist involved in the project, “The high X-ray flux and time resolution of the European X-ray free electron laser were ideal for probing the structural evolution of ice under dynamic compression in the diamond anvil cell.”
The observations revealed that Ice XXI forms as an intermediate step in the transformation to a stable ice phase. It possesses a tetragonal structure and is the largest and most complex unit of any ice phase discovered to date. Notably, once Ice XXI transitions to another phase, it does not reappear.
This discovery provides strong support for Ostwald’s step rule, which posits that crystallization often proceeds through less stable intermediate states before reaching the most stable form. The initial observation of this new phase occurred in 2018 at KRISS, but challenges with data interpretation and potential contamination initially prevented definitive confirmation. The researcher who first observed the phase revisited the data in 2023, after consistent signatures were observed by the KRISS team, ultimately leading to the identification of Ice XXI.
“This discovery not only reshapes our understanding of water’s phase diagram but also suggests that many more hidden metastable states may exist, not only in water but in salty solutions, biological systems and engineered materials,” the lead researcher stated. “The landscape of water under pressure is richer and more complex than anyone had imagined.”
The implications of this research extend beyond fundamental science. Understanding the diverse phases of water ice is critical for modeling the interiors of icy planets and moons, and assessing their potential to harbor extraterrestrial life. Further exploration of these high-pressure phases promises to unlock even more secrets hidden within this essential, yet remarkably complex, molecule.
