Observation of Robust Compressed CuO₆ Octahedra and Exotic Spin Structure in CaCuFe₂O₅

CuO₆ octahedra usually show elongated distortion, leading to active d_x²-y² orbitals and planar exchange interactions, while compressed CuO₆ octahedra with active d_3z²-r² orbitals and unidirectional exchange interactions are exceptionally rare. Here, we design and synthesize a new frustrated antiferromagnet CaCuFe₂O₅ through a high-pressure and high-temperature approach, in which robust compressed CuO₆ octahedra are realized, separating the Fe₂O₅ sheets that comprise zigzag spin ladders. Magnetic susceptibility and specific heat measurements exhibit a long-range antiferromagnetic order below the Néel temperature of 165 K, which is further confirmed by neutron diffraction. Detailed magnetic refinement reveals a C-type spin structure, with its spin arrangement and orientation distinct from that of the isostructural CaFe₃O₅. By constructing a Heisenberg model, we find that this is due to the exchange redistribution between the CuO₆ octahedra and Fe₂O₅ sheets, during which some of the exchange interactions are selectively annihilated due to the specific orientation of the Cu d_3z²-r² orbitals in the compressed CuO₆ octahedra. Our results provide a unique example of robust compressed CuO₆ octahedra and show that it can selectively annihilate some of the exchange interactions and completely modify the spin structure and magnetic frustration accordingly.