Two Charge Ordering Patterns in the Topochemically Synthesized Layer-Structured Perovskite LaCa2Fe3O9 with Unusually High Valence Fe3.67.

A-site-ordered layer-structured perovskite LaCa2Fe3O9 with unusually high valence Fe3.67+ was obtained by low-temperature topochemical oxidation of the A-site layer-ordered LaCa2Fe3O8. The unusually high valence Fe3.67+ in LaCa2Fe3O9 shows charge disproportionation of Fe3+ and Fe5+ first along the layer-stacking ⟨010⟩ direction below 230 K. Fe3+ is located between the La3+ and Ca2+ layers, while Fe5+ is between the Ca2+ layers. The two-dimensional electrostatic potential due to the A-site layered arrangement results in the quasi-stable ⟨010⟩ charge ordering pattern. Below 170 K, the charge ordering pattern changes, and the 2:1 charge-disproportionated Fe3+ and Fe5+ ions are ordered along the ⟨111⟩ direction. The ground-state charge ordering pattern is stabilized primarily by the electrostatic lattice energy, and the Fe5+ ions are arranged to make the distances between the nearest neighboring Fe5+ as large as possible.