Abstract
Studies of molecular mixtures containing hydrogen sulfide (H2S) could open up new routes towards hydrogen-rich high-temperature superconductors under pressure. H2S and ammonia (NH3) form hydrogen-bonded molecular mixtures at ambient conditions, but their phase behavior and propensity towards mixing under pressure is not well understood. Here, we show stable phases in the H2S–NH3 system under extreme pressure conditions to 4 Mbar from first-principles crystal structure prediction methods. We identify four stable compositions, two of which, (H2S) (NH3) and (H2S) (NH3)4, are stable in a sequence of structures to the Mbar regime. A re-entrant stabilization of (H2S) (NH3)4 above 300 GPa is driven by a marked reversal of sulfur-hydrogen chemistry. Several stable phases exhibit metallic character. Electron–phonon coupling calculations predict superconducting temperatures up to 50 K, in the Cmma phase of (H2S) (NH3) at 150 GPa. The present findings shed light on how sulfur hydride bonding and superconductivity are affected in molecular mixtures. They also suggest a reservoir for hydrogen sulfide in the upper mantle regions of icy planets in a potentially metallic mixture, which could have implications for their magnetic field formation.
Highlights
Studies of molecular mixtures containing hydrogen sulfide (H2S) could open up new routes towards hydrogen-rich high-temperature superconductors under pressure
We study here the formation of nitric sulfur hydrides in the form of hydrogen sulfide-ammonia mixtures under high-pressure conditions up to 400 GPa using crystal structure prediction and ab-initio calculations, and present a theoretical overview of their phase diagrams, including stable metallic and superconducting phases with novel bonding configurations, driven by a reversal of sulfur-hydrogen chemistry that transforms sulfur from an electronegative isolated S2− anion to an electropositive S6+ polyhedra-former
While only two mixed compounds, NH4SH and (NH4)2S, are known to exist at or near ambient conditions we report a series of new phases across four different stoichiometries with very different high-pressure properties
Summary
Studies of molecular mixtures containing hydrogen sulfide (H2S) could open up new routes towards hydrogen-rich high-temperature superconductors under pressure. Electron–phonon coupling calculations predict superconducting temperatures up to 50 K, in the Cmma phase of (H2S) (NH3) at 150 GPa. The present findings shed light on how sulfur hydride bonding and superconductivity are affected in molecular mixtures. The present findings shed light on how sulfur hydride bonding and superconductivity are affected in molecular mixtures They suggest a reservoir for hydrogen sulfide in the upper mantle regions of icy planets in a potentially metallic mixture, which could have implications for their magnetic field formation. Electronic structure calculations have studied various mixtures of molecular ices and their interactions with the lighter atmosphere constituents hydrogen and helium and predict a plethora of stable compounds and exotic states of matter, such as plasticity and staged superionicity[20,21,22,23,24,25,26,27]. H2S, despite confirmed via atmospheric observations of Uranus and Neptune to exist in those planets[28,29,30], is considered a minor component of their overall composition and its mixtures are little studied
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