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

SiS₂ is thought to occur in inter-stellar dust and is of interest more generally among the silicon chalcogenides as a comparator to SiO₂, an important component of terrestrial planets. However, the high-pressure behaviors of silicon sulfides are unclear. Here, using an efficient structure search method, we systematic explore the structural evolution of different Si-S stoichiometries up to 250 GPa. We find that SiS₂ is only stable below 155 GPa, which then decomposes into two previously-unreported compounds, SiS and SiS₃, at higher pressures. SiS adopts a high symmetry Pm-3m structure consisting of 8-fold coordinated silicon in face-sharing SiS₈ polyhedra, while SiS₃ crystallizes in R3m structure containing 9-fold coordinated SiS₉ polyhedra. Analysis suggests the new Si 8-fold coordination environment could be a common feature for group IV-VI compounds under high pressure. Our findings provide key insights on the nature of the Si-S compounds under ultrahigh pressure.