Fast x-ray scattering reveals water’s two liquid phases

Frozen water can exist in at least 18 different crystalline forms, depending on the temperature, pressure, and preparation conditions. For decades, researchers have investigated—and at times, fiercely disputed—whether supercooled liquid water also possesses distinct high- and low-density phases separated by a phase boundary.

On the theoretical side, the two-liquid hypothesis got a boost in 2018 when the leading model that argued against the liquid–liquid phase transition was found to contain a coding error . But observing the phase transition in the lab has been a daunting experimental challenge. The boundary between the two phases, if it exists, occupies a region of water’s phase diagram ominously known as “no-man’s-land ,” so far below the familiar freezing point of 273 K that liquid water invariably turns to ice in a few microseconds, no matter how carefully the sample is rid of any crystal-nucleating impurities. Furthermore, the boundary appears to terminate at a critical point at some hundreds of atmospheres, with the phase transition present only at pressures even higher than that.

Now Stockholm University’s Anders Nilsson , Kyung Hwan Kim, Katrin Amann-Winkel, and their colleagues have devised an experiment that combines the requisite low temperature, high pressure, and fast measurement needed to reach the elusive phase transition. And they’ve observed a discontinuous structural change between two liquid-water states.

Rather than cooling water from room temperature, the experimenters approached the no-man’s-land regime from below, by heating amorphous ice. Made by chilling 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 a thin film of high-density amorphous ice with an IR laser pulse, the researchers created a small amount of liquid water that—for an instant, at least—was compressed to the equivalent of more than 2500 times atmospheric pressure.

The newly liquefied water rapidly decompressed before refreezing into crystalline ice. The dynamics of that expansion, which the researchers probed with an x-ray pulse precisely timed to follow the IR pulse, would reveal water’s phase behavior 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 x-ray scattering intensity (solid black lines) was dominated by a smooth distribution (gray) that corresponds to the high-density liquid. (Higher values of the 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 between—for delays of a few microseconds—the x-ray signal showed a second smooth hump (pink), the hallmark of a distinct liquid phase of lower density. By showing that the liquid–liquid transition exists and is experimentally accessible, the work paves the way for further study of water’s unusual behavior. (K. H. Kim et al., Science 370, 978, 2020 ; thumbnail image courtesy of Jerker Lokrantz and Anders Nilsson.)