Ammonia water mixtures at extreme pressures and temperatures
Naden Robinson, Victor Laurance Alexander
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. Yet it is unknown how these compounds organize themselves under the extreme conditions of pressure and temperature in the planetary interiors - for instance, would they occur as a mixture, or instead as well-separated layers within the planets. While individual ices at high pressures and temperatures have been studied in great detail, the properties of their mixtures are much less explored. Experiments have previously investigated ammonia water mixtures to moderate pressures of 10-40 GPa finding rich phase diagrams. Here the binary phase diagram of ammonia-water mixtures is explored computationally as a function of composition, pressure and temperature close to planetary conditions. Crystal structure prediction methods utilizing the particle swarm optimization approach were employed to find stable solid phases at different densities reflecting the pressure ranges found in ice giants. Accurate energetics of different solid structures was ensured by utilizing electronic structure methods within the framework of density functional theory. Ammonia and water were investigated individually in the ground state to gauge the computational methodology and allow comparisons with the ground state mixtures. Benchmark crystal structure prediction results for the individual ices confirmed results of previous experimental and computational studies. For the ammonia hydrates at low pressures the canonical mixing ratios previously seen in experiments (1:2, 1:1, and 2:1) are found to be stable. These mixtures form molecular compounds and, with increasing pressure, ionic phases due to proton transfer from water to ammonia. For all hydrates, new high-pressure structures are presented that supersede existing literature results. The phase evolution of the different hydrates is discussed in terms of energetics, vibrational and electronic properties. An overarching study of all hydrates reveals that at pressures above 1 Mbar ammonia-rich hydrates dominate, stabilized by a remarkable structural evolution involving fully ionic phases with O2−(NH+ 4 )2 units in the 2:1 hydrate, and O2−(N2H+ 7 )2 in a newly predicted 4:1 hydrate. In those compounds, all water molecules are completely deprotonated, an unexpected bonding phenomenon not seen before. Beyond 500-550 GPa, close to the core-mantle boundary of Neptune, all mixtures are predicted to become unstable towards decomposition into the constituents ammonia and water. Ammonia-water mixtures that were found stable in the static ground state binary phase diagram were studied at elevated temperatures using ab initio molecular dynamics simulations. Heating these mixtures resulted in the emergence of plastic and superionic phases in all mixtures. The former is characterized by excited molecules and ionic species rotating and are also able to exhibit symmetry breaking due to temporary proton transfer depending on the mixture and the speciﬁc crystal structure. The latter exhibit fast diﬀusing protons in three dimensions that travel through the solid O-N sub-lattice. Further heating results in full melting, with melt lines established for all mixtures and found to be close to the Uranus and Neptune isentropes. The dynamical properties of these heated mixtures were then analyzed in terms of local structure, diﬀusivity, chemical abundances, and bond life-times. Covalent N-H bonds were found to be more persistent than O-H bonds, suggesting the high temperature convex hull of these mixtures may still favour ammonia-rich hydrates. Although ionicity stabilized the cold ammonia-rich hydrates, the relative abundance of ionic vs charge-neutral species decreased with temperature, leading to a more charge-balanced system. A pressure-temperature phase diagram of the ammonia-water system is presented for four diﬀerent mixing ratios and up to 600 GPa and 7000 K, indicating regions of molecular, ionic, plastic, superionic, and ﬂuid character.