Structure and Composition of the 200 K-Superconducting Phase of H2 S at Ultrahigh Pressure: The Perovskite (SH(-) )(H3 S(+) ).

At ultrahigh pressure (>110 GPa), H2 S is converted into a metallic phase that becomes superconducting with a record Tc of approximately 200 K. It has been proposed that the superconducting phase is body-centered cubic H3 S (Im3‾ m, a=3.089 Å) resulting from the decomposition reaction 3 H2 S→2 H3 S+S. The analogy between H2 S and H2 O led us to a very different conclusion. The well-known dissociation of water into H3 O(+) and OH(-) increases by orders of magnitude under pressure. H2 S is anticipated to behave similarly under pressure, with the dissociation process 2 H2 S→H3 S(+) +SH(-) leading to the perovskite structure (SH(-) )(H3 S(+) ). This phase consists of corner-sharing SH6 octahedra with SH(-) ions at each A site (the centers of the S8 cubes). DFT calculations show that the perovskite (SH(-) )(H3 S(+) ) is thermodynamically more stable than the Im3‾ m structure of H3 S, and suggest that the A site hydrogen atoms are most likely fluxional even at Tc  .