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 Tc of 203 K. These experiments provide experimental ground for understanding of this record-high Tc. 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.