Researchers at the European XFEL and DESY are investigating unusual forms of ice that can exist at room temperature when subjected to extreme pressure.
Ice comes in many forms, even when made of nothing but water molecules. Scientists have now identified more than 20 unique solid structures, or “phases,” of ice, each with its own molecular arrangement. These variations are labeled with Roman numerals, such as ice I, ice II, and ice III.
In a recent breakthrough, an international team of researchers led by scientists from the Korea Research Institute of Standards and Science (KRISS) has discovered a completely new phase known as ice XXI. Using advanced X-ray facilities at the European XFEL and PETRA III, the team captured and analyzed this previously unknown structure. Their findings have been published in Nature Materials.
Ice XXI is unlike any other form of ice observed so far. It develops when liquid water is subjected to rapid compression, creating what scientists call “supercompressed water” at room temperature. This phase is metastable, meaning it can persist for a time even though another type of ice would normally be more stable under the same conditions. The discovery provides valuable new insights into how ice behaves and transforms under extreme pressure.
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 learned 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.
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