Octahedral Rotation Change in the Ruddlesden-Popper Compound Gd3Ba2Fe4O12.5 by Topotactically Introduced Oxide Ions.

The A-site cation-ordered Ruddlesden-Popper compound Gd3Ba2Fe4O12.5 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. Gd3Ba2Fe4O12.5 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 Gd3Ba2Fe4O12.5 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.