UNLV physicists find ice-X distorts into a less symmetric structure above 222 GPa in a diamond anvil cell

Physicists from the University of Nevada, Las Vegas, compressed water between diamond anvils and recorded a change in its atomic structure above 222(12) GPa, about 2.2 million times standard atmospheric pressure at sea level. The work was published in Communications Chemistry. According to the authors, ice-X transforms into a less symmetric structure at this pressure doi.orgCommunications Chemistrydoi.orgOpen source ↗.

Water governs the structure and dynamics of icy planets and, through the pressure-induced transition from hydrogen to ionic bonding, changes markedly across the pressure range of planetary interiors doi.orgCommunications Chemistrydoi.orgOpen source ↗. Researchers therefore study its state under conditions found in the interiors of bodies such as Uranus and Neptune.

Experiment and diffraction data

The structure was probed with synchrotron X-ray diffraction. Many materials are expected to favour simple, densely packed lattices under growing compression, and ice-X follows this pattern in the form of a cubic lattice with hydrogen atoms positioned symmetrically between oxygen atoms. In this experiment, however, the cubic lattice became distorted doi.orgCommunications Chemistrydoi.orgOpen source ↗.

The diffraction data are consistent with either Pbcm or P4₂/nnm symmetry. Ab initio calculations strongly favour the Pbcm variant, according to the report doi.orgCommunications Chemistrydoi.orgOpen source ↗. The diffraction data alone do not choose between the two candidates.

Upper limit of ice-X stability

The authors state that the transition marks the onset of a new regime of structural complexity in high-pressure H₂O and redefines the upper stability limit of ice-X doi.orgCommunications Chemistrydoi.orgOpen source ↗. Such behaviour has been seen in other elements: the complex hP4 phase of sodium is cited as an example of unexpected structural complexity at extreme pressures doi.orgCommunications Chemistrydoi.orgOpen source ↗.

The NV Techno account of the study adds that the lattice stretched and became asymmetric instead of forming a denser, more symmetric arrangement. The outlet links such a deformation to properties of planetary matter, including thermal conductivity and density. These links are the outlet's interpretation and are not part of the original abstract.

The result comes from a laboratory experiment on a small sample of water under static compression. The reported transition pressure is 222 GPa with an uncertainty of 12 GPa doi.orgCommunications Chemistrydoi.orgOpen source ↗.