Structure and Composition of the 200K-Superconducting Phase of H2S at Ultrahigh Pressure: The Perovskite (SH-)(H3S+)

At ultrahigh pressure (>110GPa), H2S is converted into a metallic phase that becomes superconducting with a record T-c of approximately 200K. It has been proposed that the superconducting phase is body-centered cubic H3S (Im m, a=3.089 angstrom) resulting from the decomposition reaction 3H(2)S2H(3)S+S. The analogy between H2S and H2O led us to a very different conclusion. The well-known dissociation of water into H3O+ and OH- increases by orders of magnitude under pressure. H2S is anticipated to behave similarly under pressure, with the dissociation process 2H(2)SH(3)S(+)+SH- leading to the perovskite structure (SH-)(H3S+). This phase consists of corner-sharing SH6 octahedra with SH- ions at each Asite (the centers of the S-8 cubes). DFT calculations show that the perovskite (SH-)(H3S+) is thermodynamically more stable than the Im m structure of H3S, and suggest that the Asite hydrogen atoms are most likely fluxional even at T-c.