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August, 2019

Superionic materials are simultaneously liquids and solids. In a compound the different components might melt independently, at different temperatures. In a collaboration between University of Cambridge and Nanjing University, it has been discovered computationally that helium-water compounds under pressure are expected to exhibit multiple superionic states, with the hydrogen and helium melting independently while the oxygen atoms remain fixed.

First principles structure prediction techniques were used to find that water and unreactive helium form compounds at surprisingly low pressures. Molecular dynamics simulations were then performed on these new structures, at increasing temperatures. It was found that the helium atoms started to wander freely through the water ice framework, the hallmark of superionicity. At higher temperatures still, the hydrogen sub-lattice also melted, giving rise to a multiply superionic phase.

Figure caption:  Molecular dynamics trajectories showing the superionic motions of the hydrogen and helium atoms while the oxygen sub-lattice remains fixed.

C. Liu, H. Gao, Y. Wang, R. J. Needs, C. J. Pickard, J. Sun, H.-T. Wang & D. Xing, "Multiple superionic states in helium–water compounds", Nature Physics July, 2019

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