New Form of Ice Discovered: Ice VIIt

by johnsmith

Physicists have observed the transition from a previously known cubic phase, ice VII, to the newly-discovered intermediate, and tetragonal, phase, ice VIIt, before settling into another known phase, ice X.

Phase diagram of ice: dark blue-, green-, and red-shaded regions denote ice VII, VIIt and X, respectively, and projected phase boundaries separating high-pressure ice phases from our work are shown as solid black lines; ice X phase boundaries connect the measured transition at 30.9 GPa and 300 K to the inflection point in the melt curve, which have been associated with the transition from molecular to ionic fluid; dashed lines show measured melting curves; superionic boundary is highlighted. Image credit: Grande et al., doi: 10.1103/PhysRevB.105.104109.

Water ice is like many other materials in that it can form different solid materials based on variable temperature and pressure conditions, like carbon forming diamond or graphite.

However, water is exceptional in this aspect as there are at least 20 solid forms of ice known to scientists.

Zach Grande from the Department of Physics and Astronomy at the University of Nevada Las Vegas and colleagues pioneered a new method for measuring the properties of water under high pressure.

The water sample was first squeezed between the tips of two opposite-facing diamonds — freezing into several jumbled ice crystals.

The ice was then subjected to a laser-heating technique that temporarily melted it before it quickly re-formed into a powder-like collection of tiny crystals.

By incrementally raising the pressure, and periodically blasting it with the laser beam, the physicists observed the transition from cubic ice VII to a structure of tetragonal symmetry, ice VIIt.

They also demonstrated that the transition to ice X, when water stiffens aggressively, occurs at much lower pressures than previously thought.

While it’s unlikely we’ll find the new phase of ice anywhere on the surface of Earth, it is likely a common ingredient within the mantle of Earth as well as in large moons and water-rich planets outside of our Solar System.

“This work has demonstrated that the transformation to an ionic state occurs at much, much lower pressures than ever thought before,” said Dr. Ashkan Salamat, also from the Department of Physics and Astronomy at the University of Nevada Las Vegas.

“It’s the missing piece, and the most precise measurements ever on water at these conditions.”

“The work also recalibrates our understanding of the composition of exoplanets.”

“Ice VIIt could exist in abundance in the crust and upper mantle of expected water-rich planets outside of our Solar System, meaning they could have conditions habitable for life.”

The study was published in the journal Physical Review B.


Zachary M. Grande et al. 2022. Pressure-driven symmetry transitions in dense H2O ice. Phys. Rev. B 105 (10): 104109; doi: 10.1103/PhysRevB.105.104109

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