First principles study of dense and metallic nitric sulfur hydrides
Abstract Studies of molecular mixtures containing hydrogen sulfide (H 2 S) could open up new routes towards hydrogen-rich high-temperature superconductors under pressure. H 2 S and ammonia (NH 3 ) 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 H 2 S–NH 3 system under extreme pressure conditions to 4 Mbar from first-principles crystal structure prediction methods. We identify four stable compositions, two of which, (H 2 S) (NH 3 ) and (H 2 S) (NH 3 ) 4 , are stable in a sequence of structures to the Mbar regime. A re-entrant stabilization of (H 2 S) (NH 3 ) 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 (H 2 S) (NH 3 ) 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 more »
Authors:
; ; ; ;
Award ID(s):
Publication Date:
NSF-PAR ID:
10312146
Journal Name:
Communications Chemistry
Volume:
4
Issue:
1
ISSN:
2399-3669
5. Sub-Neptunes are common among the discovered exoplanets. However, lack of knowledge on the state of matter in$H2$O-rich setting at high pressures and temperatures ($P−T$) places important limitations on our understanding of this planet type. We have conducted experiments for reactions between$SiO2$and$H2$O as archetypal materials for rock and ice, respectively, at high$P−T$. We found anomalously expanded volumes of dense silica (up to 4%) recovered from hydrothermal synthesis above ∼24 GPa where the$CaCl2$-type (Ct) structure appears at lower pressures than in the anhydrous system. Infrared spectroscopy identified strong OH modes from the dense silica samples. Both previous experiments and our density functional theory calculations support up to 0.48 hydrogen atoms per formula unit of ($Si1−xH4x$)$O2 (x=0.12)$. At pressures above 60 GPa,$H2$O further changes the structural behavior of silica, stabilizing a niccolite-type structure, which is unquenchable. From unit-cell volume and phase equilibrium considerations, we infer that the niccolite-type phase may contain H with an amount at least comparable with or higher than that of the Ct phase. Our results suggest that the phases containing both hydrogen and lithophile elements could bemore »