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Researchers explore unusual forms of ice at room temperature under extreme pressure at the European XFEL and DESY
Ice cream comes in many different flavors. But even pure ice, which consists only of water molecules, has been discovered to exist in more than 20 different solid forms or phases that differ in the arrangement of the molecules. The phases are named with Roman numerals, like ice I, ice II or ice III. Now, researchers led by scientists from the Korea Research Institute of Standards and Science (KRISS) have identified and described a new phase called ice XXI. The results are published in the journal Nature Materials.
The international team of researchers conducted their experiments at European XFEL, the world’s largest X-Ray laser, and DESY’s high energy photon source Petra III. Ice XXI is structurally distinct from all previously observed phases of ice. It forms when water is rapidly compressed to supercompressed water at room temperature and is metastable, meaning it can exist for some time even though another form of ice would be more stable at those conditions. The discovery offers important insights into how high-pressure ice forms.
Water or H2O, despite being composed of just two elements, exhibits remarkable complexity in its solid state. The majority of the phases are observed at high pressures and low temperatures. The team has learnt more about how the different ice phases form and change with pressure. “Rapid compression of water allows it to remain liquid up to higher pressures, where it should have already crystallized to ice VI,” KRISS scientist Geun Woo Lee explains. Ice VI is an especially intriguing phase, thought to be present in the interior of icy moons such as Titan and Ganymede. Its highly distorted structure may allow complex transition pathways that lead to metastable ice phases.
Because most ice variants exist only under extreme conditions, the researchers created high pressure conditions using diamond anvil cells. The sample – in this case water – is placed between two diamonds, which can be used to build up very high pressure due to their hardness. Water was examined under pressures of up to two gigapascals, which is about 20,000 times more than normal air pressure. This causes ice to form even at room temperature, but the molecules are much more tightly packed than in normal ice.
In order to observe ice formation under different pressure conditions, the researchers first generated a high pressure of two gigapascals within 10 milliseconds – a millisecond is one thousandth of a second – with a compression rate of 120 gigapascals per second. They then released the anvil cell over a period of 1 second. This was achieved with a piezo-electric drive, which exploits the ability of piezoelectric materials to expand or contract when an electric field is applied. During these cycles, the team used the X-ray flashes of the European XFEL to capture images of the sample every microsecond – one millionth of a second. With its extremely high rate of X-ray pulses – working like a high speed camera – they could make movies of how the ice structure formed. During a follow-up experiment at the P02.2 beamline at PETRA III, they determined that ice XXI has a tetragonal crystal structure built of surprisingly large repetitive units, called unit cells.
“With the unique X-ray pulses of the European XFEL, we have uncovered multiple crystallization pathways in H2O which was rapidly compressed and decompressed over 1000 times using a dynamic diamond anvil cell”, explains Lee. “In this special pressure cell, samples are squeezed between the tips of two opposing diamond anvils and can be compressed along a predefined pressure pathway,” states Cornelius Strohm from the DESY HIBEF team that implemented this set-up at the High Energy Density (HED) instrument of European XFEL.
“The structure in which liquid H2O crystallizes depends on the degree of supercompression of the liquid”, says Lee. “Our findings suggest that a greater number of high temperature metastable ice phases and their associated transition pathways may exist, potentially offering new insights into the composition of icy moons”, Rachel Husband from the DESY HIBEF team adds.
Sakura Pascarelli, Scientific Director at European XFEL notes: “It is fantastic to see another great outcome from our Water Call, an initiative inviting scientists to propose innovative studies on water. We are looking forward to many more exciting discoveries ahead.”
Lee et al. (2025) Multiple freezing-melting pathways of high-density ice through a new metastable ice phase at room temperature. Nature Materials, https://www.nature.com/articles/s41563-025-02364-x
DOI: https://doi.org/10.1038/s41563-025-02364-x
https://www.xfel.eu/news_and_events/news/index_eng.html?openDirectAnchor=2828&am... Images are available on the European XFEL website
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