Hydrogen sulfide at high pressure: change in stoichiometry

Hydrogen sulfide $({\mathrm{H}}_{2}\mathrm{S})$ 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 ${\mathrm{H}}_{2}\mathrm{S}$ becomes unstable with respect to formation of compounds with different structure and composition, including Cccm and a body-centered cubic like ($R3m$ or $Im\ensuremath{-}3m$) ${\mathrm{H}}_{3}\mathrm{S}$, 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 ${\mathrm{H}}_{3}\mathrm{S}, {\mathrm{H}}_{4}{\mathrm{S}}_{3}, {\mathrm{H}}_{5}{\mathrm{S}}_{8}, {\mathrm{H}}_{3}{\mathrm{S}}_{5}$, and $\mathrm{H}{\mathrm{S}}_{2}$ compounds that have not been reported previously at elevated pressures.