Suppression of intersite charge transfer in charge-disproportionated perovskite YCu₃Fe₄O₁₂

A novel iron perovskite YCu₃Fe₄O₁₂ was synthesized under high pressure and high temperature of 15 GPa and 1273 K. Synchrotron X-ray and electron diffraction measurements have demonstrated that this compound crystallizes in the cubic AA′₃B₄O ₁₂-type perovskite structure (space group Im3Ì..., No. 204) with a lattice constant of a = 7.30764(10) Å at room temperature. YCu ₃Fe₄O₁₂ exhibits a charge disproportionation of 8Fe^3.75+ → 3Fe⁵⁺ + 5Fe³⁺, a ferrimagnetic ordering, and a metal-semiconductor-like transition simultaneously at 250 K, unlike the known isoelectronic compound LaCu₃Fe₄O ₁₂ that currently shows an intersite charge transfer of 3Cu ²⁺ + 4Fe^3.75+ → 3Cu³⁺ + 4Fe³⁺, an antiferromagnetic ordering, and a metal-insulator transition at 393 K. This finding suggests that intersite charge transfer is not the only way of relieving the instability of the Fe^3.75+ state in the A³⁺Cu ²⁺₃Fe^3.75+₄O₁₂ perovskites. Crystal structure analysis reveals that bond strain, rather than the charge account of the A-site alone, which is enhanced by large A ³⁺ ions, play an important role in determining which of intersite charge transfer or charge disproportionation is practical.