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Neutron diffraction helps unlock the secrets of ice

New research undertaken by scientists using the UK’s ISIS Neutron and Muon Source is improving our understanding of the highly unusual properties of ice and this knowledge will be of importance to any future study where ice coexists with other materials in nature, for example on icy moons such as Jupiter’s Europa.

Although water is one of the most common elements, the complex properties of water and particularly ice are not well understood. There are many forms of ice, which are completely different to the ice you would find in a freezer.

As water freezes its’ molecules rearrange themselves, and high pressure causes the molecules to rearrange in different ways than they normally would. The many distinct phases of ice can be explained using a phase diagram, which shows the preferred physical states of matter at different temperatures and pressures.

Researchers from University College London (UCL) and STFC’s ISIS Neutron and Muon Source have used the PEARL high pressure neutron diffractometer at ISIS to investigate the impact of ammonium fluoride impurities on water’s phase diagram.

The scientists discovered that the addition of this impurity caused a particular phase of ice, known as ice II, to completely disappear from water’s phase diagram whereas the other phases were unaffected.

The many different phases of ice can be grouped into one of two types – hydrogen-ordered phases and hydrogen-disordered phases. In these different phases the orientation of water molecules is either firmly defined or disordered.

Ice II is a hydrogen-ordered phase of ice that forms under conditions of high pressure. Unlike other phases of ice, ice II remains thermodynamically stable and hydrogen-ordered up to very high temperatures and the origin of this anomalous result is not well understood.

Dr Christoph G. Salzmann, UCL said: “Without in-situ neutron diffraction we could not have performed this study. It was paramount to demonstrate that ice II has disappeared in the region of the phase diagram where it would normally exist.”

“Unlike the other phases the water molecules in ice II interact with each other over very long distances. In a sense, whatever happens to one water molecule in a crystal of ice II - the effect is “felt” by all other molecules. In our study, ice II experiences a disturbance by the ammonium fluoride which destabilizes all of the ice II and makes it disappear.”

This observation allowed researchers to infer important information on the highly unusual properties of ice II and the special properties of ice II provide a new explanation as to why the phase diagram of water displays so many anomalies, including liquid water.

The results have been published in Nature Physics.


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