Unique octahedral rotation pattern in the oxygen-deficient Ruddlesden-Popper compound GdBaFeO.

A novel Ruddlesden-Popper-related compound, GdBaFeO, 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. GdBaFeO belongs to the tetragonal system P4/ncm, with a = 5.59040 (10) Å and c = 35.1899 (10) Å. The A-site ions in the Ruddlesden-Popper structure, i.e. Gd and Ba, 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 FeO octahedra and FeO pyramids represent valence states of +3 and +2.5, respectively, demonstrating a two-dimensional charge disproportionation. The corner-sharing FeO octahedra and FeO 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 abc/bac. In the perovskite blocks, the facing FeO pyramids across the Gd layer rotate in the same sense, which is a unique rotation feature related to oxygen deficiency.