Unusual layered order and charge disproportionation in double perovskite Ca2FeMnO6

TL;DR

This study reveals that charge disproportionation in Ca2FeMnO6 stabilizes its unusual layered structure, contrasting with similar compounds that prefer a rock-salt arrangement, highlighting the interplay between charge states and ion ordering.

Contribution

It demonstrates, through density functional calculations, that charge disproportionation stabilizes the layered ordering in Ca2FeMnO6, a structure atypical for double perovskites.

Findings

Charge disproportionation stabilizes layered structure in Ca2FeMnO6.

Ca2TiMnO6 prefers rock-salt structure without charge disproportionation.

Layered order is linked to Fe3+-Fe5+ charge disproportionation.

Abstract

While double perovskites A2BB'O6, if ordered, usually form a rock-salt-type structure with a checkerboard B/B' ordering, it is surprising that Ca2FeMnO6 has alternate FeO2 and MnO2 layers in its perovskite structure. Here we demonstrate, using density functional calculations, that this unusual layered ordering facilitates, and is largely helped by, the Fe3+-Fe5+ charge disproportionation (CD) of the formal Fe4+ ions, which would otherwise be frustrated in the common rock salt structure. To further verify the important role of the CD for stabilization of this layered ordering, we carry out a comparative study for the isostructural Ca2TiMnO6 which has a simple Ti4+-Mn4+ state free of the CD. Our calculations indicate that Ca2TiMnO6 instead prefers the standard rock salt structure to the layered one. Thus our study shows a nontrivial interplay between the CD and the type of ion ordering, and proves that the CD is strongly involved in stabilizing the unusual layered order of Ca2FeMnO6.