Physicists Create New Form of Water

After many years of experiments, physicists have observed that under extreme temperatures and pressures, water takes on a new form called superionic ice. The results appear in the journal Nature Physics.

Visualization of molecular dynamics simulations showing the fast diffusion of hydrogen ions (pink trajectories) within the solid lattice of oxygen in superionic ice. Image credit: S. Hamel / M. Millot / J.Wickboldt / LLNL / NIF.

One of the most intriguing properties of water is that it may become superionic when heated to several thousand degrees at high pressure, similar to the conditions inside icy giant planets like Uranus and Neptune. This exotic state of water is characterized by liquid-like hydrogen ions moving within a solid lattice of oxygen.

Since this was first predicted in 1988, many physicists have confirmed and refined numerical simulations, while others used static compression techniques to explore the phase diagram of water at high pressure.

While indirect signatures were observed, no research group has been able to identify experimental evidence for superionic water ice — until now.

Using shock compression, Lawrence Livermore National Laboratory physicist Marius Millot and co-authors identified thermodynamic signatures showing that ice melts near 5,000 Kelvin at 190 GPa (gigapascals) — 4,000 K higher than the melting point at 0.5 megabar (Mbar) and almost the surface temperature of the Sun.

“Our experiments have verified the two main predictions for superionic ice: very high protonic/ionic conductivity within the solid and high melting point,” Dr. Millot said.

“Our work provides experimental evidence for superionic ice and shows that these predictions were not due to artifacts in the simulations, but actually captured the extraordinary behavior of water at those conditions.”

“This provides an important validation of state-of-the-art quantum simulations using density-functional-theory-based molecular dynamics.”

Using diamond anvil cells, the team applied 2.5 GPa of pressure to pre-compress water into the room-temperature ice VII, a cubic crystalline form that is different from ‘ice-cube’ hexagonal ice, in addition to being 60% denser than water at ambient pressure and temperature.

They then performed laser-driven shock compression of the pre-compressed cells. They focused up to six intense beams of a powerful laser, delivering a 1 nanosecond pulse of UV light onto one of the diamonds. This launched strong shock waves of several hundred GPa into the sample, to compress and heat the water ice at the same time.

“Because we pre-compressed the water, there is less shock-heating than if we shock-compressed ambient liquid water, allowing us to access much colder states at high pressure than in previous shock compression studies, so that we could reach the predicted stability domain of superionic ice,” Dr. Millot explained.

The authors used interferometric ultrafast velocimetry and pyrometry to characterize the optical properties of the shocked compressed water and determine its thermodynamic properties during the brief 10-20 nanosecond duration of the experiment, before pressure release waves decompressed the sample and vaporized the diamonds and the water.

“These are very challenging experiments, so it was really exciting to see that we could learn so much from the data — especially since we spent about two years making the measurements and two more years developing the methods to analyze the data,” Dr. Millot said.

This work also has important implications for planetary science because Uranus and Neptune might contain vast amount of superionic water ice.

Planetary scientists believe these ice giants are made primarily of a carbon, hydrogen, oxygen and nitrogen mixture that corresponds to 65% water by mass, mixed with ammonia and methane. Many scientists envision these planets with fully fluid convecting interiors.

Now, the experimental discovery of superionic ice should give more strength to a new picture for these objects with a relatively thin layer of fluid and a large ‘mantle’ of superionic ice.

In fact, such a structure was proposed a decade ago — based on dynamo simulation — to explain the unusual magnetic fields of these planets.

_____

Marius Millot et al. Experimental evidence for superionic water ice using shock compression. Nature Physics, published online February 5, 2018; doi: 10.1038/s41567-017-0017-4