Cascade Charge Transitions of Unusually High and Mixed Valence Fe in the A-Site Layer-Ordered Double Perovskite SmBaFeO.

The A-site layer-ordered double perovskite SmBaFeO was obtained by topochemical of oxidizing A-site layer-ordered SmBaFeO in ozone at a low temperature. The compound contained unusually high and mixed valence Fe and was found to show cascade charge transitions, described as SmBaFeO → SmBa(FeFe)O → SmBa(FeFeFe)O → SmBa(FeFe)O, 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 to Fe and Fe, 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.