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 (H 2 ) with sulfur (S) at high pressures. We find theoretically predicted Cccm and Im3¯m H 3 S to be the reaction products at 50 and 140 GPa, respectively. Im3¯m H 3 S is a stable crystalline phase above 140 GPa and it transforms to R 3 m H 3 S on pressure release below 140 GPa. Here, the latter phase is (meta)stable down to at least 70 GPa where it transforms to Cccm H 3 S upon annealing (T < 1300 K) to overcome the kinetic hindrance. Cccm H 3 S has an extended structure with symmetric hydrogen bonds at 50 GPa, and upon decompression it experiences a transformation to a molecular mixed H 2 S – H 2 structure below 40 GPa without any apparent change in the crystal symmetry.