Superionic ice observed at extreme pressure and temperature

A 19th distinct form of solid water has been created and characterized by Federica Coppari and Marius Millot of Lawrence Livermore National Laboratory and their collaborators. Ice XVIII exists at pressures higher than 100 GPa and temperatures higher than 2000 K. To reach such extreme conditions, the researchers pummeled ultrathin containers of liquid water with nanosecond UV pulses generated by six of the lasers at the University of Rochester’s Omega Laser Facility. The pulses of 100–200 joules were so powerful that they explosively vaporized the diamond window that faced the beams and served as one side of the containers. Before each window was destroyed, it was driven into the back side of the container, which was also made of diamond. As a sample’s pressure and temperature soared, another fusillade of laser pulses struck a piece of iron foil to generate 6.7 keV x rays. The diffraction pattern formed by the x rays as they passed through the sample embodied the telltale structural information.

Firing a single, powerful shot at a sample would have pushed it beyond the expected melting point of ice XVIII. To avoid that outcome, Millot, Coppari, and company used several less powerful shots in quick succession. The pulse sequence was timed to bring the sample to its peak compression just before the pulse of x rays. Determining pressure and temperature was a two-step process. First, an interferometric technique known as ultrafast Doppler velocimetry recorded the arrival of the successive shock waves at the back window. Second, numerical simulations were run iteratively until they reproduced the window’s measured velocity. The physics built into the simulations yielded the pressure and temperature.

Water ice has so many solid forms in part because hydrogen atoms, being light, have large quantum zero-point motions and because water molecules, being bent, are linked by hydrogen bonds that can suffer geometric frustration. The possible existence of ice XVIII emerged from molecular dynamics calculations made in 1988 by Pierfranco Demontis, Richard LeSar, and Michael Klein. High pressure transforms ice from a body-centered cubic phase into a more compact face-centered cubic phase, which is what Coppari, Millot, and coworkers observed in their diffraction patterns. The high temperature needed to sustain ice XVIII frees hydrogen ions to move about the lattice in such high, “superionic” concentrations that the electrical conductivity of the phase, 10⁵ S/m, is comparable to that of arsenic, graphite, and other semimetals.

In their experiments, Millot, Coppari, and company established the conditions needed for ice XVIII for just a few nanoseconds. Those same conditions, along with abundant water, prevail inside the mantles of Neptune and Uranus. Exotic superionic ice XVIII could therefore exist in large quantities in the ice giants and host large-scale convective flows akin to the slow churning of Earth’s solid mantle. (M. Millot et al., Nature 569, 251, 2019 ; thumbnail credit: Marius Millot, Federica Coppari (LLNL), picture by Eugene Kowaluk.)