Novel phases in ammonia-water mixtures under pressure

Abstract

While ammonia and water readily form hydrogen-bonded molecular mixtures at ambient conditions, their miscibility under pressure is not well understood, yet crucial e.g. to model the interior of icy planets. We report here on the behaviour of ammonia-water mixtures under extreme pressure conditions, based on first-principles calculations of 15 stoichiometries in the pressure range of 1 atm to 10 Mbar. We show that compression facilitates proton transfer from water to ammonia in all relevant mixtures. This favors ammonia-rich hydrates above 1 Mbar, stabilized by complete de-protonation of water and the formation of the unusual structural motifs O 2− •(NH + 4) 2 and O 2− •(N 2 H + 7) 2. The hydronitrogen cations persist to the highest pressures studied. We predict a new ammonia-rich 4:1-hydrate at intermediate pressures and find that by 5.5 Mbar, close to the core-mantle boundary of Neptune, all cold ammonia-water mixtures are unstable against decomposition into their constituents.

A perspective on conventional high-temperature superconductors at high pressure: Methods and materials

“…Two examples of high-pressure studies in hydrogen-storage materials are discussed by Kokail et al [479], who studied lithium borohydrides, or the studies on H-O-N at high pressure [480,481].…”

Plastic and superionic phases in ammonia–water mixtures at high pressures and temperatures

“…The P − T phase space for each mixture to be covered by AIMD was based on their low temperature stability established in previous works [24,25] and up to at least T = 5000 K. For example, the most stable high pressure mixture, AHH, was considered up to 500 GPa. The resulting phase diagrams are shown in Figure 1.…”

“…In general, high-pressure conditions up to hundreds of GPa inside ice giants can favor unexpected chemical motifs, and stabilize unusual compounds and stoichiometries. This has been shown for prototypical mineral compounds [10][11][12][13][14], individual planetary ices [15][16][17][18][19][20][21], and lately also for their mixtures [22][23][24][25][26].…”

“…A series of recent computational studies has explored the ground states of ammonia-water mixtures to higher pressures using crystal structure prediction methods [24,25,36,37]. Some studies are restricted to specific compounds, AMH [36] and ADH A c c e p t e d M a n u s c r i p t [37], and also explored the high-temperature regime using ab initio molecular dynamics (AIMD) simulations.…”

High Pressure Hydrocarbons Revisited: From van der Waals Compounds to Diamond

“…The current understanding of the interiors of icy planets such as Neptune and Uranus involves a mantle region formed by methane, ammonia and water ice, present roughly in solar abundances, which is exposed to pressures up to several 100s GPa and temperatures of thousands of Kelvin [1]. Ammonia and water can form hydrogen-bonded compounds, by themselves and as mixtures, and under extreme conditions exhibit structural transitions characterised by hydrogen bond rearrangements and symmetrisation, auto-ionisation, and superionicity [2][3][4][5][6][7][8][9]. Methane and heavier saturated hydrocarbons, on the other hand, contain no lone pairs, can not form hydrogen bonds and therefore will have different high pressure responses, perhaps favouring decomposition reactions instead.…”