  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.
