Symmetrization of Dense D$_{\mathrm{2}}$S under Pressure

Solid H$_{\mathrm{2}}$S, like H$_{\mathrm{2}}$O, is a typical hydrogen-bonded molecular crystal; yet, unlike H$_{\mathrm{2}}$O, the stability and chemistry of dense H$_{\mathrm{2}}$S is substantially more complex and less understood. We have investigated the phase diagram of D$_{\mathrm{2}}$S to 70 GPa in diamond anvil cells using confocal micro-Raman spectroscopy. The results show the formation of ``polymeric'' D$_{\mathrm{2}}$S (phase V) at 30 GPa at 300 K, analogous to "symmetric" ice-X. The formation of proton symmetrized D$_{\mathrm{2}}$S is evident by the characteristic single Raman peak at 460 cm$^{\mathrm{-1}}$ for symmetric bending/stretching vibrational mode, analogous to that of ice X at 730 cm$^{\mathrm{-1\thinspace }}$at 76 GPa. At low temperatures, we also found the proton-ordering transitions to phase IV' and VI, both of which transform to phase V at 40 GPa at 100 K. The present results indicate higher chemical stability of D$_{\mathrm{2}}$S in contrast to the previously suggested decomposition of H$_{\mathrm{2}}$S above 30 GPa.