  Hydrogen sulfides have recently received a great deal of interest due to the
record high superconducting temperatures of up to 203 K observed on strong
compression of dihydrogen sulfide (H2S). A joint theoretical and experimental
study is presented in which decomposition products and structures of compressed
H2S are characterized, and their superconducting properties are calculated. In
addition to the experimentally known H2S and H3S phases, our first-principles
structure searches have identified several energetically competitive
stoichiometries that have not been reported previously; H2S3, H3S2, and H4S3.
In particular, H4S3 is predicted to be thermodynamically stable within a large
pressure range of 25-113 GPa. High-pressure room-temperature X-ray diffraction
measurements confirm the presence of H3S and H4S3 through decomposition of H2S
that emerge at 27 GPa and coexist with residual H2S, at least up to the highest
pressure studied in our experiments of 140 GPa. Electron-phonon coupling
calculations show that H4S3 has a small Tc of below 2 K, and that H2S is mainly
responsible for the observed superconductivity of samples prepared at low
temperature (<100K).
