Cu Site Preference and Structural Evolution in Cu‐Doped BiMn 7 O 12 Quadruple Perovskite

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Lightly Cu‐doped BiMn 7‐ x Cu x O 12 ( x = 0.05, 0.10, and 0.15) was investigated using high‐pressure synthesis, single‐crystal X‐ray diffraction, pair distribution function (PDF) analysis, Scanning Transmission Electron Microscope (STEM), magnetic measurements, and first‐principles calculations. All compositions retain an average monoclinic I 2/ m structure, while Cu substitution progressively suppresses the monoclinic distortion and drives the lattice toward a pseudo‐cubic metric symmetry. PDF analysis reveals increasing local structural disorder and reduced medium‐range coherence with increasing Cu concentration, despite preservation of the overall quadruple‐perovskite framework. Comparative single‐crystal refinements qualitatively favor Cu association with the octahedral Mn B sublattice over the square‐planar Mn A’ sites. However, due to the low Cu concentration and substantial systematic residual density, the Cu content was determined by EDS, and the local Cu distribution cannot be quantified independently of the X‐ray data. Magnetic measurements reveal an intrinsic transition near T M ∼ 50–60 K, accompanied by magnetic irreversibility, field‐dependent hysteresis, and unsaturated magnetization. An additional broad feature near T L ∼ 30K becomes weaker and less distinct with increasing Cu concentration. First‐principles calculations favor Cu occupation at the square‐planar sites, in contrast to the qualitative preference of the average B‐site refinement model.