A new study by a team at the University of Chicago, published in the October 14, 2021 issue of the journal Nature Physics, reports on the creation of a new phase of ice called "superionic ice". Turns out, the ice that tinkles in our glasses of Coke, known as Ih, is actually just one of at least 19 different phases of ice.

 

Structure of regular ice. Source: Psiĥedelisto/Wikimedia Commons

Formed from water, ice is comprised of only hydrogen and oxygen atoms in the famous H2O configuration of two hydrogen atoms attached to one oxygen atom.

One intriguing idea is that ice may become superionic when heated at very high temperatures and pressures. This exotic state would contain liquid-like hydrogen ions moving within a solid lattice of oxygen.

Superionic ice was first predicted in 1988, and since then a number of research groups have used simulation and static compression techniques to try and study this phase of ice.

The first experimental evidence for superionic water ice came from a 2018 study by scientists from the Lawrence Livermore National Laboratory (LLNL), UC Berkeley, and the University of Rochester. They first sandwiched a droplet of water between two diamonds that functioned like miniature anvils, squeezing the droplet with 2.5 GPa of pressure (25 thousand atmospheres). This "pre-compressed" water into the room-temperature ice VII, a cubic crystalline form of ice.

The team then shifted to the University of Rochester's Laboratory for Laser Energetics where they bombarded one of the diamonds with up to six intense beams of UV light. This launched strong shock waves of several hundred GPa into the sample, to compress and heat the ice at the same time. The result verified the existence of super-ionic ice but was only able to create it for a few nanoseconds before it melted away - not long enough to measure its properties.

In a more recent study, conducted in 2019, the team was able to create a more stable form of the ice by squeezing a water droplet with a 0.2-carat diamond anvil and blasting it with a laser, pressurizing the droplet to 3.5 million times Earth's atmospheric pressure at temperatures hotter than the surface of the sun. The ice was the eighteenth form to be discovered, and so was named Ice XVIII ("Ice 18").

In Ice XVIII, oxygen atoms in the droplet took up stationary positions, while the hydrogen atoms, which were stripped of their electrons thus turning them into positively charged ions, were free to flow throughout the ice, acting like a fluid. The free-flowing ions blocked all light from passing through the ice, making the ice black in color.

The work published in 2021 by the team in Chicago used similar methods to elucidate what may be another phase of superionic ice. They squeezed water droplets in a diamond anvil to pressures of 20 GPa, and shoot lasers through the diamonds to heat the sample up. Finally, they sent a beam of X-rays through the sample, and pieced together the arrangement of the atoms inside the superionic ice by observing how the X-rays scattered off the sample. 

The positively-charged, free-flowing hydrogen ions in superionic ice also create a magnetic field, and this is highly interesting to scientists because many icy bodies in our solar system, such as Neptune, Uranus, and Jupiter's moons, Europa, Io, and Ganymede, have magnetic fields. Scientists are now wondering if those magnetic fields are caused by the presence of superionic ice in the cores of those bodies.

This question is vital since a planet's magnetic field, or magnetosphere, is what prevents dangerous cosmic rays and UV radiation from reaching a planet's surface and obliterating all life. If superionic ice is common in the cores of planets outside our solar system, that would make the possibility of life on other planets that much more likely.

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