Hydrogen sulfide at high pressure: change in stoichiometry
  Hydrogen-sulfide (H2S) was studied by x-ray synchrotron diffraction (XRD) 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 new compounds with different structure and composition, including Cccm and a body-centered-cubic (bcc) like (R3m or Im-3m) H3S, the latter one predicted previously to show a record-high superconducting transition temperature, 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 new H3S, H4S3, H5S8, H3S5, and HS2 compounds at elevated pressures. 
Alexander F. Goncharov, Sergey Lobanov, Ivan Kruglov, Xiao-Miao Zhao,   Xiao-Jia Chen, Artem R. Oganov, Zuzana Kon\^opkov\'a...
cond-mat.mtrl-sci