Properties and phase diagram of (H2S)2H2

By combining hydrogen and sulfur within diamond-anvil cells we synthesize ${({\mathrm{H}}₂\mathrm{S})}₂{\mathrm{H}}₂$ 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 ${({\mathrm{H}}₂\mathrm{S})}₂{\mathrm{H}}₂$ within a wide $P\text{\ensuremath{-}}T$ range. On compression we observe the phase transition sequence of ${\text{I-II-II}}^{\ensuremath{'}}\text{-III}$, where ${\mathrm{II}}^{\ensuremath{'}}$ 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 $(<80\phantom{\rule{0.16em}{0ex}}\mathrm{K})$. Above these pressures we propose that phase III of ${({\mathrm{H}}₂\mathrm{S})}₂{\mathrm{H}}₂$ transforms to a nonmolecular ${\mathrm{H}}₃\mathrm{S}$ network. Our Raman and infrared measurements indicate that the transition from ${({\mathrm{H}}₂\mathrm{S})}₂{\mathrm{H}}₂$ to ${\mathrm{H}}₃\mathrm{S}$ is reversible at room temperature. X-ray diffraction reveals that the symmetry of the underlying S lattice of ${({\mathrm{H}}₂\mathrm{S})}₂{\mathrm{H}}₂$ and ${\mathrm{H}}₃\mathrm{S}$ is retained along this compression path up to at least 135 GPa.