Two Charge Ordering Patterns in the Topochemically Synthesized Layer-Structured Perovskite LaCa₂Fe₃O₉ with Unusually High Valence Fe^3.67+

Abstract

A-site-ordered layer-structured perovskite LaCa₂Fe₃O₉ with unusually high valence Fe^3.67+ was obtained by low-temperature topochemical oxidation of the A-site layer-ordered LaCa₂Fe₃O₈. The unusually high valence Fe^3.67+ 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.