By combining hydrogen and sulfur within diamond-anvil cells we synthesize ${\left({\mathrm{H}}_2\mathrm{S}\right)}_2{\mathrm{H}}_2$ at 5 GPa and 373 K. Through a series of Raman spectroscopy, infrared spectroscopy, and synchrotron x-ray diffraction experiments we have constrained the phase diagram of ${\left({\mathrm{H}}_2\mathrm{S}\right)}_2{\mathrm{H}}_2$ within a wide $P\text{−}T$ range. On compression we observe the phase transition sequence of ${\text{I-II-II}}^{\prime }\text{-III}$, where ${\mathrm{II}}^{\prime }$ is a previously unreported phase; at room temperature this sequence spans from 5 to 47 GPa, while the application of low temperatures stabilizes this sequence to 127 GPa $(<80\mathrm{K})$. Above these pressures we propose that phase III of ${\left({\mathrm{H}}_2\mathrm{S}\right)}_2{\mathrm{H}}_2$ transforms to a nonmolecular ${\mathrm{H}}_3\mathrm{S}$ network. Our Raman and infrared measurements indicate that the transition from ${\left({\mathrm{H}}_2\mathrm{S}\right)}_2{\mathrm{H}}_2$ to ${\mathrm{H}}_3\mathrm{S}$ is reversible at room temperature. X-ray diffraction reveals that the symmetry of the underlying S lattice of ${\left({\mathrm{H}}_2\mathrm{S}\right)}_2{\mathrm{H}}_2$ and ${\mathrm{H}}_3\mathrm{S}$ is retained along this compression path up to at least 135 GPa.

• Received 7 October 2019
• Revised 31 March 2020
• Accepted 1 April 2020

DOI:https://doi.org/10.1103/PhysRevB.101.174511