Stabilities of Charge Disproportionated States by Successive Charge Transitions of Mixed and Unusually High Valence Fe3.5+ in LnBaFe₂O₆ (Ln = Pr, Sm)

A-site layer-ordered double perovskites LnBaFe2O6 (Ln = Pr, Sm) were synthesized by topotactic ozone oxidation, and their successive phase transition behaviors were investigated. The results of differential scanning calorimetry, X-ray powder diffraction, Mössbauer spectroscopy, and magnetization measurements suggested that both compounds exhibited successive charge transitions accompanied by complicated changes in physical properties, involving the charge, lattice, and spin degrees of freedom. For SmBaFe2O6, the first-order structural and magnetic transition related to the first charge disproportionation transition (2Fe3.5+ → Fe3+ + Fe4+) occurred at a higher temperature than that of PrBaFe2O6, while the second-order magnetic transition, which was induced by the second charge disproportionation (Fe4+ → 0.5Fe3+ + 0.5Fe5+) was observed at a lower temperature. The obtained results suggest that the metastable charge-disproportionated state (Fe3+ + Fe4+) of LnBaFe2O6 is more stabilized when the lanthanoid ion at the A-site is smaller.