Dense and metallic nitric sulfur hydrides*

Hydrogen sulfide (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. Such mixtures dominate the interiors of icy planets and open up new routes towards hydrogen-rich superconductors. Here we report on stable phases in the H₂S-NH₃ system under extreme pressure conditions to 4 Mbar from first-principles crystal structure prediction. 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 300GPa is driven by a marked reversal of sulfur-hydrogen chemistry. Several stable phases exhibit metallic character, which enables superconductivity. Electron-phonon coupling calculations predict superconducting temperatures up to 50K, in the Cmma phase of (H₂S)(NH₃) at 150GPa. The present findings suggest a reservoir for hydrogen sulfide in the upper mantle regions of icy planets in a potentially metallic mixture, which has implications for their magnetic field formation. They also shed light on potential routes towards superconducting H₂S-containing molecular mixtures.