Ice XIX is the second hydrogen-partially-ordered phase of ice VI, a previously known form of ice that exists at pressures around 1 GPa and temperatures between 130 and 355 K.
Illustration showing the relationship between ice VI and XIX unit cells viewed down their c-axes, and the differences in their diffraction patterns, with color coding red for ice XIX and blue for ice VI. Image credit: University of Innsbruck.
“The different types of ice form depending on pressure and temperature and have very different properties,” said Dr. Thomas Loerting from the Institute of Physical Chemistry at the University of Innsbruck and his colleagues.
“Including conventional ice I, 18 crystalline forms of ice were known so far, which differ in the arrangement of their atoms.”
“When ice I is cooled strongly, the hydrogen atoms can arrange themselves periodically in addition to the oxygen atoms if the experiment is conducted correctly.”
“Below 73 K, this can lead to the formation of so-called ice XI, in which all water molecules are ordered according to a specific pattern. Such ordered ice forms differ from the disordered parental forms, especially in their electrical properties.”
In 2009, a team of researchers in Europe produced a hydrogen-ordered variant of ice VI, which found its way into textbooks as ice XV.
By changing the manufacturing process, Dr. Loerting and colleagues recently succeeded in creating a second ordered form of ice VI.
To do this, they significantly slowed down the cooling process and increased the pressure to around 2 GPa.
This enabled them to arrange the hydrogen atoms in a second way in the oxygen lattice and produce the new form of ice, named ice XIX.
“We found clear evidence at that time that it is a new ordered variant, but we were not able to elucidate the crystal structure,” Dr. Loerting said.
Using ex situ calorimetry and high-resolution neutron powder diffraction, the scientists investigated the crystal structure of ice XIX.
“Based on common practice, the availability of the crystal structure allows us to assign a Roman numeral to this ice phase,” they said.
“Following the latest discovery of the superionic phase ice XVIII appearing at ultrahigh-pressure, we add ice XIX to the ice phase diagram in the versatile intermediate pressure range between 0.5 and 2.0 GPa, in which 13 ice polymorphs are now known.”
“Ice XIX represents an ordered phase sharing the same oxygen atom topology with its parent, disordered ice VI, and its sibling, partially antiferroelectrically ordered ice XV.”
“Upon heating ice XIX transforms in two steps first to ice XV above 107 K, and then to ice VI at 128 K.”
A separate team of physicists, led by Dr. Ryo Yamane from the University of Tokyo, confirmed the results in an experiment under different pressure conditions.
The findings appear in two papers in the journal Nature Communications.
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T.M. Gasser et al. 2021. Structural characterization of ice XIX as the second polymorph related to ice VI. Nat Commun 12, 1128; doi: 10.1038/s41467-021-21161-z
R. Yamane et al. 2021. Experimental evidence for the existence of a second partially-ordered phase of ice VI. Nat Commun 12, 1129; doi: 10.1038/s41467-021-21351-9
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