Unique octahedral rotation pattern in the oxygen-deficient Ruddlesden–Popper compound Gd₃Ba₂Fe₄O₁₂

A novel Ruddlesden–Popper-related compound, Gd₃Ba₂Fe₄O₁₂, was discovered and its crystal structure was determined via single-crystal X-ray diffraction. The structure has an ordered structure of octahedra and pyramids along the c axis. Gd₃Ba₂Fe₄O₁₂ belongs to the tetragonal system P4₂/ncm, with a = 5.59040 (10) Å and c = 35.1899 (10) Å. The A-site ions in the Ruddlesden–Popper structure, i.e. Gd³⁺ and Ba²⁺, exhibit an ordering along the c axis. The perfect oxygen deficiency is accommodated at the GdO layers in the proper Ruddlesden–Popper structure. Using the bond-valence-sum method, the Fe ions in the FeO₆ octahedra and FeO₅ pyramids represent valence states of +3 and +2.5, respectively, demonstrating a two-dimensional charge disproportionation. The corner-sharing FeO₆ octahedra and FeO₅ pyramids are tilted in opposite directions, with the neighbours around one axis of the simple perovskite configuration, which, using Glazer's notation, can be represented as a⁻b⁰c⁰/b⁰a⁻c⁰. In the perovskite blocks, the facing FeO₅ pyramids across the Gd layer rotate in the same sense, which is a unique rotation feature related to oxygen deficiency.