  Synchrotron X-ray diffraction and Raman spectroscopy have been used to study
chemical reactions of molecular hydrogen (H2) with sulfur (S) at high
pressures. We find theoretically predicted Cccm and Im-3m H3S to be the
reaction products at 50 and 140 GPa, respectively. Im-3m H3S is a stable
crystalline phase above 140 GPa and it transforms to R3m H3S on pressure
release below 140 GPa. The latter phase is (meta)stable down to at least 70 GPa
where it transforms to Cccm H3S upon annealing (T<1300 K) to overcome the
kinetic hindrance. Cccm H3S has an extended structure with symmetric hydrogen
bonds at 50 GPa and upon decompression it experiences a transformation to a
molecular mixed H2S-H2 structure below 40 GPa without any apparent change in
the crystal symmetry.
