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 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.