Ionic ammonia-rich hydrates at planetary conditions*

The “ice giants” Uranus and Neptune, and exoplanets like them, contain large amounts of water, ammonia, and methane ices, as well as hydrogen in various forms. It is unknown how these compounds organize themselves under the extreme conditions of pressure and temperature in the interior of the planets - as a mixture, or as separated layers. Individual ices have been studied in detial under pressure, but properties of their mixtures are less explored. We computationally explore the binary ammonia–water mixtures as a function of pressure and temperature, using crystal structure prediction methods to find stable solid phases and explore superionicity and melting. At lower pressures the canonical mixtures are found to be stable, forming molecular compounds and ionic phases with increasing pressure. At higher pressures ammonia-rich hydrates dominate, due to a structural evolution involving fully ionic phases with completely deprotonated O²⁻ units and a new 4:1 hydrate. Above 500 GPa, close to the core-mantle boundary of Neptune, all mixtures are predicted to be unstable towards decomposition into ammonia and water.