Structural Diversity in Oxoiridates with 1D Ir_nO_3(n+1) Chain Fragments and Flat Bands

A previously unreported series of hexagonal-perovskite-based Rb-oxoiridates, Rb₅Ir₂O₉, Rb₇Ir₃O₁₂, and Rb₁₂Ir₇O₂₄, have been synthesized and structurally analyzed via N₂-protected single-crystal X-ray diffraction (SC-XRD). These materials exhibit different 1D Ir_nO_3(n+1)chain fragments along their c axes. IrO₆octahedra and RbO_x(x = 6, 8, and 10) polyhedra are their basic building blocks. The IrO₆octahedra are linked via face-sharing, forming Ir₂O₉dimers, Ir₃O₁₂trimers, and Ir₇O₂₄heptamers. The nonmagnetic RbO_x(x = 6, 8, and 10) polyhedra serve as both bridging units and spacers. Temperature-dependent SC-XRD shows all three to display positive thermal expansion and rules out structural transitions from their triangular symmetries down to 100 K. Density functional theory results suggest semiconducting-like behavior for the title compounds. The flatness of the electronic bands and our structural analysis are of potential interest for understanding and designing 1D quantum materials.