Unveiling the Secrets of Ice's Many Faces
In a groundbreaking discovery, researchers have unveiled a new phase of water ice, creatively named Ice XXI. This revelation, made at the European XFEL and PETRA III facilities, has sparked curiosity and opened doors to understanding the diverse forms ice can take.
The Many Faces of Ice on Earth
Ice, a familiar substance, can transform into various forms, each with unique properties. The most common naturally occurring ice is hexagonal ice (Ih), named for its hexagonal lattice structure, which gives snowflakes their six-fold symmetry. However, under extreme conditions, ice can adopt different structures, and so far, we've identified 20 distinct ice phases, each with its own roman numeral designation (from Ice I to Ice XX).
Unveiling Ice XXI: A New Phase at Room Temperature
Researchers from the Korea Research Institute of Standards and Science (KRISS) have pushed the boundaries further, creating a 21st ice phase, Ice XXI, by applying pressures of up to two gigapascals. This pressure is approximately 20,000 times higher than normal air pressure at sea level, and it's incredible that ice can form at room temperature under such conditions, albeit within a specialized device called a dynamic diamond anvil cell (dDAC).
Cornelius Strohm, a member of the DESY HIBEF team, explains, "In this pressure cell, samples are squeezed between diamond anvil tips, allowing us to compress them along a specific pressure pathway."
The Unique Structure of Ice XXI
What sets Ice XXI apart is its tightly packed molecular structure, giving it the largest unit cell volume of all known ice types, according to KRISS scientist Geun Woo Lee. It's also metastable, meaning it can exist even though another ice form, Ice VI, would be more stable under those conditions. Lee explains, "Rapid compression keeps water liquid at higher pressures, where it should have crystallized into Ice VI."
Ice VI, found in the interiors of icy moons like Titan and Ganymede, has a highly distorted structure, which may lead to complex transition pathways and metastable ice phases.
Unraveling the Crystal Structure of Ice XXI
To study Ice XXI's formation, researchers rapidly compressed and decompressed it over 1000 times within the diamond anvil cell, imaging it every microsecond using the European XFEL's high-frequency X-ray pulses. They found that liquid water crystallizes into different structures depending on its level of supercooling.
The KRISS team then used the P02.2 beamline at PETRA III to determine that Ice XXI has a body-centred tetragonal crystal structure with a large unit cell (a = b = 20.197 Å and c = 7.891 Å) at approximately 1.6 GPa. This unit cell contains 152 water molecules, resulting in a density of 1.413 g cm−3.
The Challenge of Studying Ice XXI
The experiments were challenging, as Lee recalls. Ice XXI grows upwards upon crystallization, making it difficult to analyze its crystal structure precisely. "The difficulty lies in keeping it stable long enough to make precise structural measurements in single-crystal diffraction studies," he says.
Implications and Future Directions
The multiple pathways of ice crystallization revealed in this work, detailed in Nature Materials, suggest that many more ice phases may exist. Lee emphasizes the importance of analyzing the mechanism behind the formation of these phases, which could enhance our understanding of their formation and evolution on icy moons and planets.
A Thought-Provoking Conclusion
And here's where it gets controversial: Could the discovery of Ice XXI lead us to question our understanding of ice formation in extreme environments? What implications might this have for our exploration of icy celestial bodies? Feel free to share your thoughts and opinions in the comments below!
