Synthesis of calcium polysulfides at high pressures

Chalcogenides have attracted wide attention on the structural characterizations and physicochemical properties due to their potential practicability and scientific interests, such as the intriguing stoichiometries and involved fascinating morphologies of sulfur under compression. In this work, our ab initio structure prediction simulations for the Ca-S system predicted unprecedented tetragonal $I4/mcm\text{\ensuremath{-}}\mathrm{Ca}{\mathrm{S}}₂$ and $P\text{\ensuremath{-}}{42}₁m\text{\ensuremath{-}}\mathrm{Ca}{\mathrm{S}}₃$ containing the isolated dimeric ${{\mathrm{S}}₂}^2--$ and V-shape trimeric ${{\mathrm{S}}₃}^2--$ units, respectively, under high pressure. Motived by these theoretical results, remarkably, we have successfully synthesized $\mathrm{Ca}{\mathrm{S}}₂$ and $\mathrm{Ca}{\mathrm{S}}₃$ as unambiguously identified through in situ x-ray diffraction and Raman spectra measurements. The present results have unraveled the high-pressure structure behavior of Ca-S system and further provided key insights for exploring other interesting polysulfides at extreme conditions.