Novel phases in ammonia-water mixtures under pressure

Victor Naden Robinson; Miriam Marqués; Yanchao Wang; Yanming Ma; Andreas Hermann

doi:10.1063/1.5063569

Novel phases in ammonia-water mixtures under pressure

J Chem Phys. 2018 Dec 21;149(23):234501. doi: 10.1063/1.5063569.

Authors

Victor Naden Robinson¹, Miriam Marqués¹, Yanchao Wang², Yanming Ma², Andreas Hermann¹

Affiliations

¹ Centre for Science at Extreme Conditions and SUPA, School of Physics and Astronomy, The University of Edinburgh, Edinburgh EH9 3FD, United Kingdom.
² State Key Laboratory for Superhard Materials, College of Physics, Jilin University, Changchun 130012, China.

PMID: 30579307
DOI: 10.1063/1.5063569

Abstract

While ammonia and water readily form hydrogen-bonded molecular mixtures at ambient conditions, their miscibility under pressure is not well understood, yet crucial to model the interior of icy planets. We report here on the behavior of ammonia-water mixtures under extreme pressure conditions, based on first-principles calculations of 15 stoichiometries in the pressure range of 1 atm-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 $O^{2 -} \cdot {({N H}_{4}^{+})}_{2}$ and $O^{2 -} \cdot {(N_{2} H_{7}^{+})}_{2}$ . 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.