Octahedral Rotation Change in the Ruddlesden-Popper Compound Gd₃Ba₂Fe₄O_12.5 by Topotactically Introduced Oxide Ions

The A-site cation-ordered Ruddlesden-Popper compound Gd₃Ba₂Fe₄O_12.5 was synthesized using the topotactic chemical reaction of Gd₃Ba₂Fe₄O₁₂. The crystal structure was determined through single-crystal X-ray diffraction and scanning transmission electron microscopy. Gd₃Ba₂Fe₄O_12.5 crystallizes in a tetragonal unit cell with P4₂/mnm symmetry, where a = 5.57240(10) Å and c = 35.3289(8) Å, differing from the P4₂/ncm space group of Gd₃Ba₂Fe₄O₁₂. The topotactically introduced oxide ions randomly occupy half of the atomic sites on the central layer (Gd layer) of the perovskite block, causing the iron ions on both sides of the layer to form six-coordinate octahedra and five-coordinate pyramids. Consequently, these polyhedra are randomly distributed within the perovskite blocks. Mössbauer spectroscopy revealed the presence of three independent trivalent Fe sites, corresponding to two FeO₆ octahedra and one FeO₅ pyramid. The introduction of oxide ions into Gd₃Ba₂Fe₄O₁₂ to form Gd₃Ba₂Fe₄O_12.5 altered the polyhedral rotation pattern from the same direction to the opposite direction relative to the ⟨110⟩ direction at the center of each perovskite block, although both compounds equally exhibited the a^-b⁰c⁰/b⁰a^-c⁰ manner. The occupation of O ions in the Gd layers of Gd₃Ba₂Fe₄O₁₂ induces a change in the crystal symmetry, which is associated with the octahedral rotation pattern and coordination environment.