Ligand-hole localization in oxides with unusual valence Fe

Unusual high-valence states of iron are stabilized in a few oxides. A-site-ordered perovskite-structure oxides contain such iron cations and exhibit distinct electronic behaviors at low temperatures, e.g. charge disproportionation (4Fe4+ → 2Fe3+ + 2Fe5+) in CaCu3Fe4O12 and intersite charge transfer (3Cu2+ + 4Fe3.75+ → 3Cu3+ + 4Fe3+) in LaCu3Fe4O12. Here we report the synthesis of solid solutions of CaCu3Fe4O12 and LaCu3Fe4O12 and explain how the instabilities of their unusual valence states of iron are relieved. Although these behaviors look completely different from each other in simple ionic models, they can both be explained by the localization of ligand holes, which are produced by the strong hybridization of iron d and oxygen p orbitals in oxides. The localization behavior in the charge disproportionation of CaCu3Fe4O12 is regarded as charge ordering of the ligand holes, and that in the intersite charge transfer of LaCu3Fe4O12 is regarded as a Mott transition of the ligand holes

Selective reduction of layers at low temperature in artificial superlattice thin films

Reduction and oxidation in transition-metal oxides are keys to develop technologies related to energy and the environment. Here we report the selective topochemical reduction observed when artificial superlattices with transition-metal oxides are treated at a temperature below 300 °C with CaH2. [CaFeO2]m/[SrTiO3]n infinite-layer/perovskite artificial superlattice thin films were obtained by low-temperature reduction of [CaFeO2.5]m/[SrTiO3]n brownmillerite/perovskite artificial superlattice thin films. By the reduction only the CaFeO2.5 layers in the artificial superlattices were reduced to the CaFeO2 infinite layers whereas the SrTiO3 layers were unchanged. The observed low-temperature reduction behaviors strongly suggest that the oxygen ion diffusion in the artificial superlattices is confined within the two-dimensional brownmillerite layers. The reduced artificial superlattice could be reoxidized, and thus, the selective reduction and oxidation of the constituent layers in the perovskite-structure framework occur reversibly