Cascade Charge Transitions of Unusually High and Mixed Valence Fe3.5+ in the A-Site Layer-Ordered Double Perovskite SmBaFe2O6

The A-site layer-ordered double perovskite SmBaFe₂O₆ was obtained by topochemical of oxidizing A-site layer-ordered SmBaFe₂O₅ in ozone at a low temperature. The compound contained unusually high and mixed valence Fe^3.5+ and was found to show cascade charge transitions, described as SmBaFe^3.5+₂O₆ → SmBa(Fe³⁺Fe⁴⁺)O₆ → SmBa(Fe³⁺Fe^(4-δ)+_0.5Fe^(4+δ)+_0.5)O₆ → SmBa(Fe³⁺_1.5Fe⁵⁺_0.5)O₆, to relieve its electronic instability. The first Verwey-like charge-order transition occurred at 340 K and was accompanied by a significant structural change and a sudden increase in magnetic susceptibility. The following transition was the charge disproportionation of metastable Fe⁴⁺ to Fe³⁺ and Fe⁵⁺, and each of the spins resulted in the antiferromagnetic ground state. The most plausible charge-ordered patterns are proposed based on the electrostatic lattice energy calculations.