Hydrogen sulfide at high pressure: Change in stoichiometry

Hydrogen sulfide (H2S) was studied by x-ray synchrotron diffraction and Raman spectroscopy up to 150 GPa at 180-295 K and by quantum-mechanical variable-composition evolutionary simulations. The experiments show that H2S becomes unstable with respect to formation of compounds with different structure and composition, including Cccm and a body-centered cubic like (R3m or Im-3m) H3S, the latter one predicted previously to show a record-high superconducting transition temperature, a T-c of 203 K. These experiments provide experimental ground for understanding of this record-high T-c. The experimental results are supported by theoretical structure searches that suggest the stability of H3S, H4S3, H5S8, H3S5, and HS2 compounds that have not been reported previously at elevated pressures.