New crystalline ice form EurekAlert! Science News

The ice of the material is very flexible. In avalanches or ice cubes, the oxygen atoms are arranged hexagonally. This form of ice is called ice one (ice I). “Firmly speaking, however, these are not perfect crystals, but disordered systems in which the water molecules are randomly directed in different spatial directions,” explained Thomas Loerting of the University’s Institute of Physical Chemistry Innsbruck, Austria. Introducing ice I, 18 crystalline ice forms have been known to date, which differ in the arrangement of the atoms. The different types of ice, called polymorphs, form according to pressure and temperature and have very different properties. For example, their melting points vary by several hundred degrees Celsius. “It compares to diamond and graphite, and they are both made of real carbon,” the chemist explains.

When normal ice has cooled vigorously, the hydrogen atoms can occasionally reorganize themselves as well as the oxygen atoms if the test is performed correctly. Below less than 200 degrees Celsius, this can lead to the formation of so-called XI ice, in which each water molecule is ordered according to a specific pattern. Prescribed ice forms are thus different from the disordered parent forms, especially in their electrical properties. In the current work, Innsbruck chemists treat the parent form of ice VI, which is formed by high pressure, for example in the Earth’s crust. Like hexagonal ice, this form of high-pressure ice is not like a full-order crystal. More than 10 years ago, researchers at the University of Innsbruck made this hydrogen-ordered ice variety, which found its way into textbooks as XV ice. By changing the manufacturing process, three years ago the Thomas Loerting team succeeded for the first time in creating a second ordered form for ice VI. To do this, the scientists significantly delayed the cooling process and increased the weight to about 20 kbar. This allowed them to form the hydrogen atoms in a second way in the oxygen and ice field of XIX. “We received clear evidence at the time that it was a new ordered version, but we were unable to clarify the crystal structure.” Now his team has succeeded in doing so using the gold standard for structural determination – neutron differentiation.

To clarify the crystal structure, a necessary technical hurdle must be overcome. In a study using neutron differentiation, the light hydrogen in water needs to be replaced by deuterium (“heavy hydrogen”). “Unfortunately, this also changes the time rates for ordering in the ice manufacturing process,” Loerting says. “But then Ph.D. student Tobias Gasser had the crucial idea of ​​adding a few percent of normal water to the heavy water – which turned out to speed up the order significantly.” With the ice obtained in this way, Innsbruck scientists were finally able to quantify neutron data on the high-resolution HRPD instrument at the Rutherford Appleton Laboratory in England and accurately solve the XIX ice crystal structure. This required finding the optimal crystal structure out of several thousand candidates from the measured data – similar to finding a needle in a haystack. A Japanese research group confirmed the result of Innsbruck in another experiment under different pressure conditions. The two papers are now published together Nature Communication.

Although conventional ice and snow are plentiful on Earth, other forms are not found on the surface of our planet – except in research laboratories. However, the high-pressure forms of ice VI and ice VII are available as inclusions in diamonds and have therefore been added to the list of minerals by the International Mining Association (IMA). Many types of water ice are formed in large expanses under certain conditions and temperatures. They are found, for example, on celestial bodies such as Jupiter Ganymede’s moon, which is covered with layers of different types of ice.

Ice XV and ice XIX represent the first pair of sibling in ice physics in which the oxygen laser is the same, but the pattern of how hydrogen atoms are ordered is different. “This also means that for the first time it will now be possible to achieve the transition between two ordered ice forms in experiments,” Thomas Loerting is pleased to report. The University of Innsbruck, Austria, is now responsible for the discovery of four crystals as well as two amorphous ice forms.

The routine research work was carried out within the framework of the Research Platform for Materials and Nanoscience at the University of Innsbruck and was financially supported by the Austrian Science Fund FWF.

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