Superionic water, also called superionic ice or ice XVIII is a phase of water that exists at extremely high temperatures and pressures. In superionic water, water molecules break apart and the oxygen ions crystallize into an evenly spaced lattice while the hydrogen ions float around freely within the oxygen lattice. The freely mobile hydrogen ions make superionic water almost as conductive as typical metals, making it a superionic conductor. It is one of the 19 known crystalline phases of ice. Superionic water is distinct from ionic water, which is a hypothetical liquid state characterized by a disordered soup of hydrogen and oxygen ions.
While theorized for decades, it was not until the 1990s that the first experimental evidence emerged for superionic water. Initial evidence came from optical measurements of laser-heated water in a diamond anvil cell, and from optical measurements of water shocked by extremely powerful lasers. The first definitive evidence for the crystal structure of the oxygen lattice in superionic water came from x-ray measurements on laser-shocked water which were reported in 2019.
If it were present on the surface of the Earth, superionic ice would rapidly decompress. In May 2019, scientists at the Lawrence Livermore National Laboratory (LLNL) were able to synthesize superionic ice, confirming it to be almost four times as dense as normal ice. Superionic water is theorized to be present in the mantles of giant planets such as Uranus and Neptune.
As of 2013[update], it is theorized that superionic ice can possess two crystalline structures. At pressures in excess of 50 GPa (7,300,000 psi) it is predicted that superionic ice would take on a body-centered cubic structure. However, at pressures in excess of 100 GPa (15,000,000 psi) it is predicted that the structure would shift to a more stable face-centered cubic lattice.
History of theoretical and experimental evidence
Demontis et al. made the first prediction for superionic water using classical molecular dynamics simulations in 1988. In 1999, Cavazzoni et al. predicted that such a state would exist for ammonia and water in conditions such as those existing on Uranus and Neptune. In 2005 Laurence Fried led a team at Lawrence Livermore National Laboratory to recreate the formative conditions of superionic water. Using a technique involving smashing water molecules between diamonds and super heating it with lasers they observed frequency shifts which indicated that a phase transition had taken place. The team also created computer models which indicated that they had indeed created superionic water. In 2013 Hugh F. Wilson, Michael L. Wong, and Burkhard Militzer at the University of California, Berkeley published a paper predicting the face-centered cubic lattice structure that would emerge at higher pressures.
In 2018, researchers at LLNL squeezed water between two pieces of diamond with a pressure of 2,500 MPa (360,000 psi). The water was squeezed into type VII ice, which is 60 percent denser than normal water.
The compressed ice was then transported to the University of Rochester where it was blasted by a pulse of laser light. The reaction created conditions like those inside of ice giants such as Uranus and Neptune by heating up the ice thousands of degrees under a pressure a million times greater than the earth's atmosphere in only 10 to 20 billionths of a second. The experiment concluded that the current in the conductive water was indeed carried by ions rather than electrons and thus pointed to the water being superionic. More recent experiments from the same Lawrence Livermore National Laboratory team used x-ray crystallography on laser-shocked water droplets to determine that the oxygen ions enter a face-centered-cubic phase, which was dubbed ice XVIII and reported in the journal Nature in May 2019.
Existence in ice giants
It is theorized that the ice giant planets Uranus and Neptune hold a layer of superionic water. But there are also studies that suggest that other elements present inside the interiors of these planets, particularly carbon, may prevent the formation of superionic water.
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