Unique octahedral rotation pattern in the oxygen-deficient Ruddlesden–Popper compound Gd 3 Ba 2 Fe 4 O 12

A novel Ruddlesden–Popper-related compound, Gd 3 Ba 2 Fe 4 O 12 , 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. Gd 3 Ba 2 Fe 4 O 12 belongs to the tetragonal system P 4 2 / 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 3+ and Ba 2+ , 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 6 octahedra and FeO 5 pyramids represent valence states of +3 and +2.5, respectively, demonstrating a two-dimensional charge disproportionation. The corner-sharing FeO 6 octahedra and FeO 5 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 − b 0 c 0 / b 0 a − c 0 . In the perovskite blocks, the facing FeO 5 pyramids across the Gd layer rotate in the same sense, which is a unique rotation feature related to oxygen deficiency.