Ammonia water mixtures at extreme pressures and temperatures

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

Two state model for critical points and the negative slope of the melting-curve

We present a thermodynamic model which explains the presence of a negative slope in the melt curve, as observed in systems as diverse as the alkali metals and molecular hydrogen at high pressure. We assume that components of the system can be in one of two well defined states - one associated with low energy, the other with low volume. The model exhibits a number of measurable features which are also observed in these systems and are therefore expected to be associated with all negative Clapeyron-slope systems: first order phase transitions, thermodynamic anomalies along Widom lines. The melt curve maximum is a feature of the model, but appears well below the pressures where the change in state occurs in the solid: the solid-solid transition is related to the melt line minimum. An example of the model fitted to the electride transition in potassium is discussed

Stabilization of ammonia-rich hydrate inside icy planets

Significance The mantles of icy planets comprise large amounts of water, ammonia, and methane ices. To understand their interior structure, it is crucial to study these ices at the extreme pressure conditions they likely experience. Hitherto, such studies have mostly been restricted to individual ices and not considered formation of stable mixtures. We survey here mixtures of water and ammonia and show that high pressures stabilize ammonia hemihydrate, through a transformation from a molecular crystal into a fully ionic solid that involves complete deprotonation of water. We suggest that ammonia-rich hydrates can precipitate out of any ammonia–water mixture at sufficient pressures and are an important component inside icy planets.</jats:p

Squeezing Oil into Water under Pressure: Inverting the Hydrophobic Effect

The molecular structure of dense homogeneous fluid water-methane mixtures has been determined for the first time using high-pressure neutron-scattering techniques at 1.7 and 2.2 GPa. A mixed state with a fully H-bonded water network is revealed. The hydration shell of the methane molecules is, however, revealed to be pressure-dependent with an increase in the water coordination between 1.7 and 2.2 GPa. In parallel, ab initio molecular dynamics simulations have been performed to provide insight into the microscopic mechanisms associated with the phenomenon of mixing. These calculations reproduce the observed phase change from phase separation to mixing with increasing pressure. The calculations also reproduce the experimentally observed structural properties. Unexpectedly, the simulations show mixing is accompanied by a subtle enhancement of the polarization of methane. Our results highlight the key role played by fine electronic effects on miscibility and the need to readjust our fundamental understanding of hydrophobicity to account for these