Successive and Site-Selective Oxygen Release from B-Site-Layer-Ordered Double Perovskite Ca₂FeMnO₆ with Unusually High Valence Fe⁴⁺

Abstract

B-site-layer-ordered double perovskite Ca2FeMnO6 with unusually high valence Fe4+ was found to exhibit unusual oxygen-release behaviors, contrasting with those of the B-site-disordered perovskite having the identical chemical composition. During heating, the B-site-layer-ordered compound shows a stepwise oxygen release with successive valence changes from Fe4+ to Fe3+ through an intermediate Fe3.5+, whereas the B-site-disordered compound releases oxygen in a single step. The oxygen in Ca2FeMnO6 is released only from the two-dimensional Fe layers, and this selective oxygen release stabilizes the intermediate Fe3.5+ phase with in-plane-oxygen-vacancy ordering. Therefore, the B-site order/disorder strongly affects the oxygen-release behaviors associated with the oxygen-vacancy ordering.

Geometrical Spin Frustration and Monoclinic-Distortion-Induced Spin Canting in the Double Perovskites Ln2LiFeO6 (Ln = La, Nd, Sm, and Eu) with Unusually High Valence Fe5+

“…Higher valence states of Fe like Fe 3.5+ and Fe 4+ can also be stabilized but have been less reported. The electronic instabilities due to such unusually high valence Fe ions often induce nontrivial phase transitions of the compounds. − For example, the A -site-ordered quadrupole perovskite LaCu 3 Fe 4 O 12 with unusually high valence Fe 3.75+ ions shows first-order intersite charge transfer between Cu and Fe ions to relieve the instability of Fe 3.75+ . The related compound NdCu 3 Fe 4 O 12 has recently been reported to show large latent heat and a barocaloric effect accompanied by first-order intersite charge transfer …”

Magnetic Ordering and Structural Transition in the Ordered Double-Perovskite Pb2NiMoO6

“…Recently, much attention has been given to the double perovskites with the general chemical formula A 2 BB′O 6 , owing to their fascinating structural and wide variety of physical properties since the report of room-temperature colossal magnetoresistance in the double-perovskite Sr 2 FeMoO 6 . Subsequently, many A 2 BB′O 6 (A = Ba, Sr, Ca, Pb, and Bi; B/B′ = Fe, Co, Ni, Cr, Mn, and Os) double perovskites with properties such as ferromagnetic, ferroelectric, and multiferroic were synthesized under ambient or high-pressure conditions. − …”