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 Mn(4+) and unusual high valence Fe(4+) at room temperature. When cooled, the compound shows charge disproportionation followed by magnetic transition. Around 200 K, the Fe(4+) shows the charge disproportionation to Fe(3+) and Fe(5+), 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 Fe(3+) and Fe(5+) 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.

A new double perovskite Ca2FeMnO6 with a layered arrangement of Mn4+ and the unusually high-valence Fe4+ was obtained by oxidizing the precursor brownmillerite Ca2FeMnO5 with ozone at 200 °C. This low-temperature topotactic reaction kept the layered cation arrangement of the brownmillerite but oxidized Mn3+ to Mn4+ and Fe3+ to Fe4+. The crystal structure with a P21/c space group was revealed by detailed structure analysis with neutron powder diffraction data, and the layered arrangement of the cations was confirmed in an STEM-EELS elemental mapping image. The fully oxidized chemical composition was also confirmed by thermogravimetric analysis. Even with the two-dimensional layered arrangement, the instability of the unusually high-valence Fe4+ in Ca2FeMnO6, like that of the Fe4+ in the perovskites CaFeO3 and CaCu3Fe4O12 with three-dimensional arrangements of Fe4+, was relieved by charge disproportionation (2Fe4+ → Fe3+ + Fe5+) at a low temperature.

A perovskite-structure oxide containing unusually high-valence Fe was obtained by high-pressure synthesis. Instability of the Fe in Ca Bi FeO is relieved first by charge disproportionation at 250 K and then by intermetallic charge transfer between A-site Bi and B-site Fe at 200 K. These previously unobserved successive charge transitions are due to competing intermetallic and disproportionation charge instabilities. Both transitions change magnetic and structural properties significantly, indicating strong coupling of charge, spin, and lattice in the present system.