Two Charge Ordering Patterns in the Topochemically Synthesized Layer-Structured Perovskite LaCaFeO with Unusually High Valence Fe.

A-site-ordered layer-structured perovskite LaCaFeO with unusually high valence Fe was obtained by low-temperature topochemical oxidation of the A-site layer-ordered LaCaFeO. The unusually high valence Fe in LaCaFeO shows charge disproportionation of Fe and Fe first along the layer-stacking ⟨010⟩ direction below 230 K. Fe is located between the La and Ca layers, while Fe is between the Ca 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 Fe and Fe ions are ordered along the ⟨111⟩ direction. The ground-state charge ordering pattern is stabilized primarily by the electrostatic lattice energy, and the Fe ions are arranged to make the distances between the nearest neighboring Fe as large as possible.