Anion–Anion Bonding and Topology in Ternary Iridium Seleno–Stannides

The synthesis and physical properties of two new and one known Ir–Sn–Se compound are reported. Their crystal structures are elucidated with transmission electron microscopy and powder X-ray diffraction. IrSn_0.45Se_1.55 is a pyrite phase which consists of tilted corner-sharing IrX₆ octahedra with randomly distributed (Sn–Se)^4– and (Se–Se)^2– dimers. Ir₂Sn₃Se₃ is a known trigonally distorted skutterudite that consists of cooperatively tilted corner-sharing IrSn₃Se₃ octahedra with ordered (Sn–Se)₂^4– tetramers. Ir₂SnSe₅ is a layered, distorted β-MnO₂ (pyrolusite) structure consisting of a double IrSe₆ octrahedral row, corner sharing in the a direction and edge sharing in the b direction. This distorted pyrolusite contains (Se–Se)^2– dimers and Se^2– anions, and each double row is “capped” with a (Sn–Se)_n polymeric chain. Resistivity, specific heat, and magnetization measurements show that all three have insulating and diamagnetic behavior, indicative of low-spin 5d⁶ Ir³⁺. Electronic structure calculations on Ir₂Sn₃Se₃ show a single, spherical, nonspin–orbit split valence band and suggest that Ir₂Sn₃Se₃ is topologically nontrivial under tensile strain due to inversion of Ir-d and Se-p states.