A-Site Layer-Ordered Double Perovskites LnBaFe ₂ O ₆ with Mixed and Unusually High Valence Fe 3.5+ : Lanthanoid-Dependent Charge Transition Behaviors

The A-site layer-ordered double perovskites, LnBaFe 2 O 6 with Ln from Pr to Dy and Y, contain mixed and unusually high valent Fe 3.5+ ions at high temperatures and upon cooling exhibit successive charge disproportionation transitions to relieve the electronic instability. The first charge disproportionation transition can be represented as 2Fe 3.5+ → Fe 3+ + Fe 4+ . This first-order transition is accompanied by a drastic change in structural and magnetic properties, signaling a strong entanglement of charge, spin, and lattice degrees of freedom. The second charge disproportionation transition, 2Fe 4+ → Fe 3+ + Fe 5+, is a second-order-like transition that occurs via intermediate states of LnBa(Fe 3+ Fe (4−δ)+ 0.5 Fe (4+δ)+ 0.5 )O 6 . The LnBa(Fe 3+ 1.5 Fe 5+ 0.5 )O 6 ground state is finally stabilized at low temperature. The size of the A-site Ln ion strongly influences the structural and magnetic properties of the compounds. As the size of the Ln 3+ ion increases, the temperature of the first charge disproportionation transition decreases almost linearly, while the temperature of the second charge disproportionation transition increases. Consequently, the temperature range over which the intermediate Fe 4+ or Fe 4+ -like Fe (4±δ)+ state is stable narrows. These results demonstrate control over the stability of mixed and unusually high valence ions.