A team of researchers at the University of Innsbruck have used neutron diffraction to confirm that the new form of ice they had observed in their laboratory takes a novel structure, different to any of the 18 other forms of ice previously observed.
Different ices
Ice is a very versatile material. In snowflakes or ice cubes, the form of ice present is called 'ice one', or ice I. Different types of ice have different arrangements of the hydrogen and oxygen atoms. Which one is formed depends on the pressure and temperature.
These forms also have very different properties depending on the structural arrangement and packing of the molecules. "It's comparable to diamonds and graphite, both of which are made of pure carbon," explains Thomas Lörting from the Department of Physical Chemistry at the University of Innsbruck, Austria.
While conventional ice and snow are abundant on Earth, no other forms are found on the surface of our planet - except in research laboratories. However, many varieties of water ice are formed in space under special pressure and temperature conditions. They are found, for example, on celestial bodies such as Jupiter's moon Ganymede, which is covered by layers of different ice varieties.
Parents and siblings
In conventional ice I, the oxygen atoms are arranged hexagonally and the hydrogen atoms are randomly spread. This is known as a 'parent' structure. When it is cooled quickly under certain conditions to below -200°C, the hydrogen atoms can also arrange themselves periodically, leading to the formation of ice XI. Ordered ice forms such as these have different characteristics to their disordered parental forms, especially in their electrical properties.
In the current work, the Innsbruck chemists used ice VI as the 'parent'. Ice VI is formed at high ambient pressure, such as is present in the Earth's mantle. Like ice I, this form of ice is not a completely ordered crystal.
Over 20 years ago, researchers at the University of Innsbruck produced a hydrogen-ordered variant of this ice, known as ice XV. Three years ago, by changing the manufacturing process, Thomas Lörting's team succeeded for the first time in creating a second ordered form with ice VI as the parent. To do this, they significantly slowed down the cooling process and increased the pressure to around 20 kbar. This enabled them to arrange the hydrogen atoms in a second way within the oxygen lattice and produce ice XIX.
"We found clear evidence at that time that it was a new ordered variant, but we were not yet able to elucidate the crystal structure," explains Thomas. “Now my team has succeeded in doing just that, using the gold standard for structure resolution - neutron diffraction."
Neutron experiments
However, the neutron diffraction experiments were not straightforward. To use the technique, the hydrogen atoms have to be replaced with heavy hydrogen, deuterium. "Unfortunately, this also changes the behavior in the ice manufacturing process," says Thomas. "But doctoral student Tobias Gasser had the crucial idea of adding a few percent of normal water to the heavy water."
With the ice obtained in this way, the Innsbruck scientists were finally able to measure neutron data on the HRPD instrument at ISIS. After the measurement of ice XIX, the group had to go through the painstaking process of solving the crystal structure. “This required finding the best crystal structure out of several thousand candidates," explains Thomas; “much like searching for a needle in a haystack."
Alongside the work of the Innsbruck researchers, a Japanese research group has confirmed their result in another experiment under different pressure conditions. Both papers have now been published jointly in Nature Communications.
Ice XV and Ice XIX represents the first sibling pair in ice physics in which the oxygen lattice is the same, but where the hydrogen atoms are arranged differently. “This also means that, for the first time, it will be possible to realize the transition between two fully ordered ice forms in experiments," adds Thomas.
Further information
The full paper can be found at DOI: 10.1038/s41467-021-21161-z
Since the 1980s, researchers at the University of Innsbruck have been responsible for the discovery of four crystalline as well as two amorphous ice forms.
The current 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.
