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

H2S is converted under ultrahigh pressure (> 110 GPa) to a metallic phase that becomes superconducting with a record Tc of 200 K. It has been proposed that the superconducting phase is body-centered cubic H3S ( Im3m , a = 3.089 {AA}) resulting from a decomposition reaction 3H2S --> 2H3S + S. The analogy of H2S and H2O leads us to a very different conclusion. The well-known dissociation of water into H3O+ and OH- increases by orders of magnitude under pressure. An equivalent behavior of H2S is anticipated under pressure with the dissociation, 2H2S --> H3S+ + SH- forming a perovskite structure (SH-)(H3S+), which consists of corner-sharing SH6 octahedra with SH- at each A-site (i.e., the center of each S8 cube). Our DFT calculations show that the perovskite (SH-)(H3S+) is thermodynamically more stable than the Im3m structure of H3S, and suggest that the A-site H atoms are most likely fluxional even at Tc.