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Frozen water can exist in at least 18 different crystalline forms, depending on temperature, pressure and preparation conditions. For decades, researchers have investigated – and at times, fiercely contested – whether supercooled liquid water also possessed distinct high- and low-density phases separated by a phase boundary.
Theoretically, the two-liquid hypothesis got a boost in 2018 when it was discovered that the main model opposing the liquid-liquid phase transition contained a coding error. But observing the phase transition in the lab was a daunting experimental challenge. The boundary between the two phases, if any, occupies a region of the water phase diagram menacingly known as “no man’s land”, so far below the familiar freezing point of 273 K that liquid water invariably transforms in ice in a few microseconds, no matter how carefully the sample was cleared of nuclear impurities. Furthermore, the boundary appears to terminate at a critical point in a few hundred atmospheres, with the phase transition only present at pressures even higher than that.
Now Anders Nilsson of the University of Stockholm, Kyung Hwan Kim, Katrin Amann-Winkel and their colleagues have devised an experiment that combines the required low temperature, high pressure and velocity measurements required to achieve the elusive phase transition. And they observed a discontinuous structural change between two states of liquid water.
Instead of cooling the water from room temperature, the experimenters approached the no man’s land regime from below by heating amorphous ice. Produced by cooling water so quickly that it solidifies before it can crystallize, amorphous ice exists in high and low density forms that are thought to correspond to high and low density liquid phases. By zapping on a thin film of high-density amorphous ice with an IR laser pulse, researchers have created a small amount of liquid water that, at least for an instant, has been compressed to the equivalent of over 2500 times the atmospheric pressure.
The freshly liquefied water decompressed rapidly before refreezing into crystalline ice. The dynamics of that expansion, which the researchers probed with an X-ray pulse precisely programmed to follow the IR pulse, would reveal the phase behavior of water in no man’s land.
The results are shown in the figure. When the X-ray pulse followed the IR pulse by tens of nanoseconds or less, the intensity of the X-ray scattering (solid black lines) was dominated by a uniform (gray) distribution that corresponds to the high-density liquid. (Higher values of momentum transfer q correspond to shorter particle separations.) For delays of tens of microseconds or more, the series of discrete peaks (purple) signaled the formation of crystalline solid ice.
But in the middle, for delays of a few microseconds, the X-ray signal showed a second smooth (pink) hump, the hallmark of a distinct liquid phase of lower density. By demonstrating that the liquid-liquid transition exists and is experimentally accessible, the work opens the way for further studies on the unusual behavior of water. (KH Kim et al., Science 370, 978, 2020; miniature image courtesy of Jerker Lokrantz and Anders Nilsson.)
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