The magnetic and crystal structures of the oxygen-deficient layered perovskite YBaCuFeO5+δ have been determined with powder neutron diffraction methods. It crystallizes in the noncentrosymmetric tetragonal space group P4mm with a = 3.8707(2) Å and c = 7.6711(4) Å (at room temperature). Structural motifs of corner-sharing severely elongated CuO5 (Cu-Oeq = 1.936(2) Å, Cu-Oax = 2.35(9) Å) and slightly squashed FeO5 (Fe-Oeq = 2.03(3) Å, Fe-Oax = 1.82(9) Å) square pyramids result in a novel lamellar structure with ordered oxygen defects. Local magnetic moments directed perpendicular to the unique axis (μ perpendicular to c, μ = 2.51 (15) μB at room temperature) appear at the Fe3+ sites at 442.4(5) K. The magnetic unit cell results by doubling the chemical one along all three crystallographic directions (magnetic propagation vector, k = (1/2,1/2,1/2))- Despite the long separation between successive FeO2 layers (~7.7 Å), the magnetic behavior can be successfully described by a three-dimensional Heisenberg spin Hamiltonian with a critical exponent β = 0.366(8). No evidence of ordering of the Cu sublattice is found on cooling to room temperature.
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