Stable high-pressure phases in the H-S system determined by chemically reacting hydrogen and sulfur

Synchrotron x-ray diffraction and Raman spectroscopy have been used to study the chemical reactions of molecular hydrogen (H₂) with sulfur (S) at high pressures. We find theoretically predicted Cccm and Im3̄m H₃S to be the reaction products at 50 and 140 GPa, respectively. Im3̄m H₃S is a stable crystalline phase above 140 GPa and it transforms to R3m0.16em0exH₃S 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.