What superconducts in sulfur hydrides under pressure and why

The recent discovery of superconductivity at 190 K in highly compressed H₂S is spectacular not only because it sets a record high critical temperature, but because it does so in a material that appears to be, and we argue here that it is, a conventional strong-coupling BCS superconductor. Intriguingly, superconductivity in the observed pressure and temperature range was predicted theoretically in a similar compound, H₃S. Several important questions about this remarkable result, however, are left unanswered: (1) Does the stoichiometry of the superconducting compound differ from the nominal composition, and could it be the predicted H₃S compound? (2) Is the physical origin of the anomalously high critical temperature related only to the high H phonon frequencies, or does strong electron-ion coupling play a role? We show that at experimentally relevant pressures H₂S is unstable, decomposing into H₃S and S, and that H₃S has a record high Tc due to its covalent bonds driven metallic, which make this compound rather similar to MgB₂, but unlike most other good conventional superconductors.