Ice is a very versatile material. In snowflakes or ice cubes, oxygen atoms are arranged hexagonal. This form of ice is called an ice (ice I). “Obviously, however, these are not exactly perfect crystals, but incorrect systems where water molecules are randomly oriented in different spatial directions,” explains Thomas Loerting of the Institute of Physical Chemicals at the University of Innsbruck, Austria.
Including ice I, 18 forms of crystalline ice are described to date, which differ in their atomic arrangement. The various types of ice, known as polymorphisms, depend on pressure and temperature and have very different properties. For example, the melting point varies by several hundred degrees Celsius. “It is similar to diamonds and graphite, both of which are made of pure carbon,” said the chemist.
When conventional ice I is very cold, the hydrogen atom can periodically regulate itself in addition to the oxygen atom if the experiment is done correctly. Below 200 degrees Celsius, this can lead to the formation of ice XI, where all water molecules are ordered according to a specific pattern. The form of ice that is ordered as such is different from the irregular shape of the parent, especially in its electrical properties.
Under current employment, Innsbruck chemists deal with the mother form of ice VI, which is formed under high pressure, for example in the Earth’s mantle. Like hexagonal ice, this form of high pressure ice is not a complete crystal. More than 10 years ago, researchers at the University of Innsbruck produced a hydrogen-derived variant of this ice, which entered the textbook as ice XV.
By changing the manufacturing process, three years ago the Thomas Loerting team was successful for the first time in creating a second form for ice VI. To do this, scientists significantly slow down the cooling process and increase the pressure to 20 kbar. This allows them to regulate hydrogen atoms in a second way in the oxygen grid and produce XIX ice. “We found clear evidence at the time that it was a new variant, but we could not explain the crystal structure.” Now his team has succeeded in only using the gold standard for structural determination – neutron diffraction.
The crystal structure is completed
For clarification of the crystal structure, important technical barriers must be overcome. In studies using neutron diffraction, it is necessary to replace light hydrogen in water with deuterium (“heavy hydrogen”).
“Unfortunately, this also changes the scales of time to order in the process of producing ice,” Loerting said. “But Ph.D. student Tobias Gasser then came up with the important idea of adding a few percent of normal water to heavy water – which turned out to be very quick to order.”
With ice obtained in this way, Innsbruck scientists were finally able to measure neutron data on high-resolution HRPD instruments at the Rutherford Appleton Laboratory in the UK and to decisively solve the XIX ice crystal structure. This required searching for the best crystal structure of several thousand candidates from the measured data – such as searching for needles in a haystack. The Japanese research group confirmed Innsbruck’s results in another experiment under different pressures. Both papers have now been published together in Natural Communication.
Six forms of ice were found in Innsbruck
Despite the abundance of conventional ice and snow on Earth, no other form is found on our planet’s surface – except in research laboratories. However, the high pressure forms of ice VI and ice VII were found to be covered in diamonds and therefore have been added to the list of minerals by the International Mineralogical Association (IMA). Many types of water ice are formed in a wide range of spaces under specific pressure and temperature conditions. They exist, for example, in such celestial bodies JupiterGanymede moon, which is covered with layers of different types of ice.
Ice XV and XIX ice represent the first pair of siblings in ice physics that have the same oxygen lattice, but the pattern of order of hydrogen atoms is different. “It also means that for the first time now we will be able to realize the transition between the two forms of ice ordered in the experiment,” Thomas Loerting likes to report. Since the 1980s, researchers at the University of Innsbruck, Austria, have been responsible for finding four crystals as well as two forms of amorphous ice.
References: 18 February 2021, Natural Communication.
DOI: 10.1038 / s41467-021-21161-z
Research work is currently underway within the framework of the Research Platform for Materials and Nanoscience at the University of Innsbruck and is financially supported by the Austrian Science Fund FWF.