Two-Dimensional Charge Disproportionation of the Unusual High Valence State Fe⁴⁺ in a Layered Double Perovskite

The crystal and magnetic structures of charge-disproportionated Ca₂FeMnO₆ were analyzed by neutron powder diffraction. Ca₂FeMnO₆ is a layered double perovskite oxide with a two-dimensional arrangement of Mn⁴⁺ and unusual high valence Fe⁴⁺ at room temperature. When cooled, the compound shows charge disproportionation followed by magnetic transition. Around 200 K, the Fe⁴⁺ shows the charge disproportionation to Fe³⁺ and Fe⁵⁺, which are ordered in a checkerboard pattern in the two-dimensional FeO₆ octahedral layers. The magnetic transition occurs at 95 K, which is much lower than the charge disproportionation temperature. The magnetic structure is commensurate but noncollinear, and the antiferromagnetic coupling of Fe³⁺ and Fe⁵⁺ spins in the FeO₆ octahedral layers gives the ferrimagnetic moments. The unique magnetic structure is described as a result of two-dimensional localization of the ligand holes with effective spins.