Novel Doubly Superionic Behavior in Icy Compounds at Planetary Interior Conditions*

Superionic phases, in which protons diffuse like a liquid through stable lattices of heavier nuclei, have been observed in water and ammonia at high pressures. Here, we describe a novel state of matter: double superionicity. With density functional molecular dynamics and machine learning molecular dynamics simulations we show that hydrogen and one heavier species become mobilized at elevated temperature while the heaviest nuclei provide a stable sublattice until the entire material melts at yet higher temperature. We further demonstrate that proton superionicity is ubiquitous in H-C-N-O materials at sufficiently high pressure and temperature. Superionic and doubly superionic phases may exist in the interiors of Uranus and Neptune and thus may influence their magnetic dynamo because of their high ionic conductivities.