More Than 50 Years after Its Discovery in SiO₂ Octahedral Coordination Has Also Been Established in SiS₂ at High Pressure

SiO₂ exhibits a high-pressure–high-temperature polymorphism, leading to an increase in silicon coordination number and density. However, for the related compound SiS₂ such pressure-induced behavior has not been observed with tetrahedral coordination yet. All four crystal structures of SiS₂ known so far contain silicon with tetrahedral coordination. In the orthorhombic, ambient-pressure phase these tetrahedra share edges and achieve only low space filling and density. Up to 4 GPa and 1473 K, three phases can be quenched as metastable phases from high-pressure high-temperature to ambient conditions. Space occupancy and density are increased first by edge and corner sharing and then by corner sharing alone. The structural situation of SiS₂ up to the current study resembles that of SiO₂ in 1960: Then, in its polymorphs only Si–O₄ tetrahedra were known. But in 1961, a polymorph with rutile structure was discovered: octahedral Si-O₆ coordination was established. Now, 50 years later, we report here on the transition from 4-fold to 6-fold coordination in SiS₂, the sulfur analogue of silica.