EBS flux reveals fate of over-compressed water

Water is so familiar to us that the ancients considered it one of the four basic elements. To modern physicists, however, it is a marvel – a liquid that, unlike almost all others, becomes not easier but harder to solidify at high pressures and, when it does solidify, expands rather than contracts. The behaviour results from the way the constituent hydrogen atoms bond with one another, and is vital for life. Without it, lakes and seas would freeze from the bottom up, killing everything inside.

In fact, the freezing of water is even more complicated than this. Under various pressures and temperatures, water is known to form at least 19 distinct phases of ice. The one we know well on Earth has its oxygen and hydrogen atoms in hexagonal rings. On the other hand, the most common phase in the Universe is likely to be a type of low-density amorphous ice, without any long-range crystal structure at all. Another very common phase with big scientific interest is the cubic-bonded ice VII, which is stable over a vast pressure range from 2 to 80 gigapascals, equivalent to those present on icy planets and moons.

The gateway to ice VII may be higher pressures, but the speed of compression is critical. Take it slowly, and normal water freezes at about one gigapascal into ice VI, a tetrahedral phase, before forming ice VII at about 2 gigapascals. Go faster, though, and the freezing is waylaid, occurring at higher and higher pressures.

Until now, no-one has been sure what water ultimately freezes into when it is compressed very quickly. The answer is important, because the freezing of water on other planets and moons could have taken place when it was over-compressed during planetary impact.

Charles Pépin, Paul Loubeyre  and colleagues at the CEA Laboratory for Materials at Extreme Conditions at the Université Paris-Saclay in France, together with scientists at the ESRF in France and the Paul Scherrer Institute (PSI) in Switzerland, have finally solved the mystery using a range of cutting-edge instrumentation for time-resolved X-ray diffraction.

One part of the toolkit was a special “dynamic-piezo” diamond anvil cell (d-DAC), designed by the CEA team to compress water in a well-controlled manner. Another was the latest Jungfrau detector – the result of a joint PSI–ESRF development – which can record an X-ray image every few microseconds. Most importantly, however, was the extremely high flux of X-rays streaming through the ID09 beamline, provided by the EBS.

“It was the coupling of our d-DAC with the ESRF instrumentation at ID09, plus the use of the Jungfrau detector, that made this experiment possible and successful,” says Loubeyre.

The results were clear: over-compressed water freezes into ice VII – not amorphous ice or even a novel solid phase, as other theorists had speculated. Loubeyre and colleagues could confirm this outcome for compression rates spanning over six orders of magnitude, by including their own data with the results of previous nanosecond dynamic-compression studies. They could also verify a theoretical model for the onset of freezing, which will help scientists to understand the composition of icy planets and moons.

Matteo Levantino, the scientist in charge of ID09 and a co-author of the study, says his beamline is ideally suited to this kind of research. “We succeeded to expose the evolution of the structure of water after rapid compression by collecting several snapshots within a single polychromatic X-ray pulse. Using the new Jungfrau detector available at the beamline in its so-called ‘burst mode’, it was possible to obtain up to 16 different X-ray diffraction patterns in less than 100 microseconds.”

“Our next steps are to investigate the mechanisms of solid phase transitions, and to further explore microsecond chemistry under high  pressures, which could be a path to synthesise novel materials,” says Loubeyre. “We want to perform a similar freezing measurement on  the liquid metal gallium.”

Reference:

Pépin, et al. Nat Commun 15, 8239 (2024). https://doi.org/10.1038/s41467-024-52576-z

Written by Jon Cartwright

Top image: The set-up of the experiment.