Ultrahigh-pressure induced decomposition of silicon disulfide into silicon-sulfur compounds with high coordination numbers

Silicon disulfide, ${\mathrm{SiS}}₂$, is thought to occur in interstellar dust and is of fundamental interest more generally among the silicon chalcogenides as a comparator to ${\mathrm{SiO}}₂$, an important component of terrestrial planets. However, the high-pressure behaviors of silicon sulfides are unclear. Here, using an efficient structure search method, we systematically explore the structural evolution of different Si-S stoichiometries up to 250 GPa. ${\mathrm{SiS}}₂$ is found to be stable below 155 GPa, above which it decomposes into two compounds, SiS and ${\mathrm{SiS}}₃$. SiS adopts a high-symmetry cubic structure consisting of eightfold-coordinated silicon in face-sharing ${\mathrm{SiS}}₈$ polyhedra, while ${\mathrm{SiS}}₃$ crystallizes in a rhombohedral structure containing ninefold-coordinated ${\mathrm{SiS}}₉$ polyhedra. Analyses suggest that the Si eightfold-coordination environment could be a common feature for group IV--VI compounds under high pressure. Our findings provide insights on the nature of Si-S compounds under ultrahigh pressure.