Water is weird. When ice cubes float at the top of a drink, they're defying the norm. Solids are generally denser than liquids, so they sink. But because of its hydrogen bonds, water produces unusual and complex behaviors.
Studying water ice and its various phases is crucial for understanding its strange properties. The knowledge is also critical for materials science, chemistry, physics and planetary science, where icy planets and moons may host extraterrestrial life.
In a recent study, published in Nature Materials, a team of researchers, including Yong-Jae Kim and Earl O'Bannon at Lawrence Livermore National Laboratory (LLNL), explored how water freezes under extreme compression at room temperature. In doing so, they discovered at least five different freezing-melting pathways, including an entirely new phase of ice that they named Ice XXI.
"Most people think of ice as just the solid that forms in a freezer, but water is far more complex. Because water molecules are connected by hydrogen bonds, they can arrange themselves in many different ways depending on temperature and pressure," said first author Kim. "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."
To uncover the newest phase of ice, the authors didn't cool water. Instead, at room temperature, they compressed it to 10,000 times the pressure of the atmosphere with a dynamic diamond anvil cell. During that compression, an X-ray free electron laser captured snapshots of the water's molecular structure as it underwent rapid phase transitions.
"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," said O'Bannon.
"Together, these tools make it possible to observe short-lived metastable states that would otherwise vanish before they could be studied," said Kim.
With this unparalleled, high-speed view, the scientists saw water evolve into a well-known, stable ice phase by passing through two metastable intermediate structures. The first of these was the newly discovered Ice XXI, which has a tetragonal structure.
Ice XXI only forms from water under strong supercompression. Once it morphs into another phase, it never reappears. And it is the largest and most complex unit of any ice phase discovered so far.
This work also provides strong evidence for Ostwald's step rule, which states that crystallization often proceeds through semi-stable intermediates before reaching the most stable phase.
Kim first observed the formation of this new ice phase using high-speed imaging and obtained its X-ray diffraction pattern at synchrotron beamlines while working at the Korea Research Institute of Standards and Science (KRISS) in 2018. But potential contamination in the water and the complexity of the diffraction pattern meant the findings weren't enough to stand on their own. Kim left KRISS and joined LLNL in 2019 with this new ice phase unresolved. By 2023, the KRISS team was continuously seeing the signatures, which drove Kim to revisit the old data and identify Ice XXI at last.
"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," said Kim. "The landscape of water under pressure is richer and more complex than anyone had imagined."
