Multiple superionic states in helium-water compounds

Superionic states are phases of matter that can simultaneously exhibit some of the properties of a liquid and of a solid. For example, in superionic ice, hydrogen atoms can move freely while oxygen atoms are fixed in their sublattice. 'Superionicity' has attracted much attention in both fundamental science and applications. Helium is the most inert element in nature and it is generally considered to be unreactive. Here, we use ab initio calculations to show that He and H2O can form stable compounds within a large pressure range that can exist even close to ambient pressure. Surprisingly, we find that they can form two previously unknown types of superionic state under high pressure and high temperature. In the first of these phases, the helium atoms exhibit liquid behaviour within a fixed ice-lattice framework. In the second phase, both helium and hydrogen atoms move in a liquid-like fashion within a fixed oxygen sublattice. As the He-O interaction is weaker than the H-O interaction, the helium atoms in these superionic states have larger diffusion coefficients and lower 'melting' temperatures than those of hydrogen, although helium is heavier than hydrogen. The insertion of helium atoms substantially decreases the pressure at which superionic states may be formed, compared to those in pure ice.