Phase behaviour of the 1:1 Ammonia-Hydrogen sulphide mixtures at high pressures and temperatures*

Although the major constituents of the gaseous atmosphere of the ‘ice giants’ are H and He, their mantle regions are postulated to be comprised of hot dense ices of CH₄, H₂O, NH₃ and H₂S. While the individual ices and their mixtures e.g., CH₄-H₂O¹, NH₃-H₂O^2,3  are studied under extreme conditions, the complete phase diagram of NH₃-H₂S mixtures remains unexplored, yet will contribute to accurate models of the interiors of ice giant planets. Using ab initio molecular dynamics, we construct the phase diagram of ammonia monosulphide (NH₃:H₂S = 1:1) up to 165 GPa and 3500K, focusing on the emerging plastic and superionic states, and the melting curve (MC).  Our calculated MC demonstrates a maximum in 40-70 GPa range followed by a dip at high-pressure regime as the mixture gives rise to short-lived di-sulphur molecules. Tracing the MC with the isentropes of Uranus and Neptune indicates that in the shallow mantle of these planets, H₂S can stabilize by forming NH₃-H₂S molecular fluid that could phase-separate from superionic NH₃-H₂O mixtures. Our study reveals that increasing temperature lowers the band-gap which is most prominent in the Cc crystalline phase at 20 GPa, indicating that this phase can switch from semi-conductor to conductor in the high-temperature liquid state beyond 3000K.

 ¹Pruteanu et al., Sci. Adv. 3, e1700240 (2017).

²Robinson et al., PNAS, 114, 9003 (2017).

³Robinson and Hermann, JPCM 32, 184004 (2020).