Unveiling the Secrets of Ice's Many Faces: A New Phase Discovered
The Ice We Know is Just the Tip of the Iceberg (Literally)
Water ice, a familiar entity on Earth, has revealed a surprising new phase, dubbed Ice XXI. This discovery, made by researchers at the European XFEL and PETRA III facilities, challenges our understanding of ice's behavior under extreme conditions.
Earth's Ice: A Diverse Family
On our planet, ice takes on various forms, each with unique properties. The most common type, hexagonal ice (Ih), is named for its hexagonal lattice structure, which gives snowflakes their six-fold symmetry. However, when conditions get extreme, ice can transform into other structures. So far, scientists have identified 20 distinct ice phases, each with its own roman numeral designation (I, II, III, and so on up to XX).
Unveiling Ice XXI: A High-Pressure Mystery
Researchers from the Korea Research Institute of Standards and Science (KRISS) have now added a 21st member to this family by applying pressures of up to two gigapascals (GPa). This is roughly 20,000 times higher than the air pressure at sea level! Such extreme conditions allow ice to form even at room temperature, but only within a specialized device called a dynamic diamond anvil cell (dDAC).
Cornelius Strohm, part of the DESY HIBEF team that set up the experiment, explains, "In this pressure cell, samples are squeezed between diamond anvils, allowing us to compress them along a specific pathway."
A Tightly Packed Mystery: The Structure of Ice XXI
The structure of Ice XXI sets it apart from all other known ice phases. Its molecules are packed much more tightly, giving it the largest unit cell volume of any ice type discovered so far. KRISS scientist Geun Woo Lee describes it as "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 is intriguing; it's thought to exist inside icy moons like Titan and Ganymede. Its distorted structure might lead to complex pathways that result in metastable ice phases."
Unraveling the Secrets of Ice XXI's Structure
To understand how Ice XXI forms, the researchers subjected it to rapid compression and decompression over 1000 times within the diamond anvil cell. They captured images every microsecond using the European XFEL's high-frequency X-ray pulses. This revealed that liquid water crystallizes into different structures depending on how supercompressed it is.
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 far from simple, Lee recalls. Ice XXI grows upwards upon crystallization, making it challenging to analyze its crystal structure precisely. "The difficulty lies in keeping it stable long enough to make accurate structural measurements," Lee 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 mechanisms behind these phases' formation. "This could enhance our understanding of the formation and evolution of such phases on icy moons and planets," he tells Physics World.
And this is just the beginning! As we explore the extremes of pressure and temperature, who knows what other ice phases we might uncover?
