Cascade Charge Transitions of Unusually High and Mixed Valence Fe 3.5+ in the A-Site Layer-Ordered Double Perovskite SmBaFe 2 O 6 .

The A-site layer-ordered double perovskite SmBaFe 2 O 6 was obtained by topochemical of oxidizing A-site layer-ordered SmBaFe 2 O 5 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+ 2 O 6 → SmBa(Fe 3+ Fe 4+ )O 6 → SmBa(Fe 3+ Fe (4-δ)+ 0.5 Fe (4+δ)+ 0.5 )O 6 → SmBa(Fe 3+ 1.5 Fe 5+ 0.5 )O 6 , 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 Fe 4+ to Fe 3+ and Fe 5+ , 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.