Tag: ice

EBS flux reveals fate of over-compressed water

EBS flux reveals fate of over-compressed water

ESRF users have exploited the high X-ray flux of the EBS to confirm that water freezes into a particular ‘cubic’ form of ice when it is compressed very quickly. Published in Nature Communications, the results clear up a long-standing mystery in high-pressure physics, and will provide insights into the composition of the Solar System’s icy moons.

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.

Read more at ESRF beamline

Frozen noble gas in the accelerator

Frozen noble gas in the accelerator

Researchers at European XFEL in Schenefeld near Hamburg have taken a closer look at the formation of the first crystallisation of nuclei in supercooled liquids. They found: The formation starts much later than previously assumed. The findings could help to better understand the creation of ice in clouds in the future and to describe some processes inside the Earth more precisely.

Every child knows that water freezes into ice when it gets icy cold. For water, this normally happens below zero degrees Celsius, the melting temperature of water. This is a fixed point on the Celsius temperature scale that we use.

However, the transition from the liquid to the solid phase is a very complex process and is difficult to study experimentally at the atomic level. One reason for this is that crystals are formed randomly: You don’t know exactly when and where it will happen. Furthermore, a liquid can remain in a metastable state for a long time: It remains liquid even though it should actually freeze and become solid. This makes it extraordinarily difficult to pinpoint the right moment for a crystal to form and watch its growth.

However, these effects are highly relevant in nature. For example, they play a decisive role in the formation of ice in clouds or in processes inside Earth.

Using the intense X-ray flashes of the European XFEL’s X-ray free-electron laser, an international team of researchers at the European XFEL in Schenefeld near Hamburg has now succeeded in precisely measuring the nucleation of supercooled liquids. The experiments took place in a vacuum so that the X-ray light does not interact with the molecules in the air, which would interfere with the experiments. Because of its complexity, however, water is one of the most difficult liquids to model. For that reason, the researchers used instead argon and krypton in liquid form in their experiments. In fact, supercooled noble-gas liquids are the only systems for which reliable theoretical predictions can be presently made.

The researchers explicitly investigated the so-called crystal nucleation rate J(T). This is a measure of the probability that a crystal will form in a certain volume within a certain time. The rate at which this happens is an important parameter, for example in order to be able to mathematically describe real processes in models – in weather forecasting, for example, or in climate models.

Read more on XFEL website

Image: X-ray of a crystal. The diffraction pattern results from 34,000 single-pulse x-ray exposures of a krypton jet shortly after the onset of crystal nucleation. The rings indicate x-ray scattering from specific molecular planes within the small crystals.

Credit: European XFEL