Hydrogen sulfide at high pressure: Change in stoichiometry

Hydrogen sulfide (H$_2$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 H$_2$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−3m) H$_3$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 H$_3$S, H$_4$S$_3$, H$_5$S$_8$, H$_3$S$_5$, and HS$_2$ compounds that have not been reported previously at elevated pressures