Scientists have predicted a new phase of superionic ice, a special form of ice that could exist on Uranus and Neptune, thanks to a computer solution performed by a team of researchers at Princeton University.
Unlike water or regular ice, in superionic ice the water molecules dissociate into charged atoms called ions, with the oxygen ions locked in a solid lattice, while the hydrogen ions move like the molecules in a liquid. The model predicts an entirely new type of superionic ice that they call the P21/c-SI phase, which occurs at pressures even higher than in the interior of giant ice planets of our solar system. Two other phases of superionic ice thought to exist on the planets are body centered cubic superionic ice (BCC-SI) and, close-packed superionic ice (CP-SI).
Each phase has a unique arrangement of oxygen ions that gives rise to distinct properties. For example, each of the phases allows hydrogen ions to flow in a characteristic way. The effects of this ionic conductivity could be observed by planetary scientists in search of superionic ice.
Unlike Earth, which has two magnetic poles (north and south), ice giants can have many local magnetic poles, which leading theories suggest may be due to superionic ice and ionic water in the mantle of these planets. In ionic water both oxygen and hydrogen ions show liquid-like behavior. Scientists have proposed that heat emanating outward from the planet's core may pass through an inner layer of superionic ice, and through convection, create vortices on the outer layer of ionic water that give rise to local magnetic fields.
By using simulations, the researchers were able to model states of superionic ice that may not exist experimentally. They simulated pressures that were beyond the highest possible pressures attainable in the laboratory with instruments called diamond anvil cells. Extreme pressure can be achieved through shockwave experiments but these rely on creating an explosion and are difficult to interpret.
The researchers calculated the ionic conductivity of each phase of superionic ice and found unusual behavior at the transition where the low temperature crystal, in which both oxygen and hydrogen ions are locked together, transforms into superionic ice. In known superionic materials, generally the conductivity can change either abruptly (type I) or gradually (type II), but the type of change will be specific to the material. However, superionic ice breaks from convention, as the conductivity changes abruptly with temperature across the crystal to close-packed superionic transition and continuously at the crystal to P21/c-SI transition.
Citation: Sun, J.; Clark, B. K.; Torquato, S.; Car, R. "The phase diagram of high pressure superionic ice." Nature Communications, Published online Aug. 28, 2015.