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

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

SrFe_0.5Ru_0.5O₂: Square-Planar Ru²⁺ in an Extended Oxide

Low-temperature topochemical reduction of the cation disordered perovskite phase SrFe_0.5Ru_0.5O₃ with CaH₂ yields the infinite layer phase SrFe_0.5Ru_0.5O₂. Thermo­gravimetric and X-ray absorption data confirm the transition metal oxidation states as SrFe_0.5²⁺Ru_0.5²⁺O₂; thus, the title phase is the first reported observation of Ru²⁺ centers in an extended oxide phase. DFT calculations reveal that, while the square-planar Fe²⁺ centers adopt a high-spin <i>S</i> = 2 electronic configuration, the square-planar Ru²⁺ cations have an intermediate <i>S</i> = 1 configuration. This combination of <i>S</i> = 2, Fe²⁺ and <i>S</i> = 1, Ru²⁺ is consistent with the observed spin-glass magnetic behavior of SrFe_0.5Ru_0.5O₂

Suppression of Sequential Charge Transitions in Ca_0.5Bi_0.5FeO₃ via B‑Site Cobalt Substitution

The perovskite Ca_0.5Bi_0.5FeO₃ containing unusually high-valent Fe^3.5+ undergoes sequentially charge disproportionation (CD) of the Fe centers and intersite charge transfer (CT) between Bi and Fe. From structural, magnetic, and transport property characterization, we found that substitution of Co for Fe occurs isovalently to form Ca_0.5­Bi³⁺_0.5­(Fe_1–<i>x</i>­Co_<i>x</i>)^3.5+­O₃ and destabilizes the CD state. This results in materials exhibiting only intermetallic charge transfer behavior in the region 0.01 < <i>x</i> < 0.67. The CT transitions for these materials only involve Fe^3.5+, whereas Co remains in the 3.5+ oxidation state at all temperatures. The doped Co^3.5+ ions give Pauli-paramagnetic like conducting behavior. The Co-substitution effect is very different from that observed in Ca­Fe_1–<i>x</i>­Co_<i>x</i>O₃

Topochemical Reduction of the Ruddlesden–Popper Phases Sr₂Fe_0.5Ru_0.5O₄ and Sr₃(Fe_0.5Ru_0.5)₂O₇

Reaction of the Ruddlesden–Popper phases Sr₂Fe_0.5Ru_0.5O₄ and Sr₃(Fe_0.5Ru_0.5)₂O₇ with CaH₂ results in the topochemical deintercalation of oxide ions from these materials and the formation of samples with average compositions of Sr₂Fe_0.5Ru_0.5O_3.35 and Sr₃(Fe_0.5Ru_0.5)₂O_5.68, respectively. Diffraction data reveal that both the <i>n</i> = 1 and <i>n</i> = 2 samples consist of two-phase mixtures of reduced phases with subtly different oxygen contents. The separation of samples into two phases upon reduction is discussed on the basis of a short-range inhomogeneous distribution of iron and ruthenium in the starting materials. X-ray absorption data and Mössbauer spectra reveal the reduced samples contain an Fe³⁺ and Ru^2+/3+ oxidation state combination, which is unexpected considering the Fe³⁺/Fe²⁺ and Ru³⁺/Ru²⁺ redox potentials, suggesting that the local coordination geometry of the transition metal sites helps to stabilize the Ru²⁺ centers. Fitted Mössbauer spectra of both the <i>n</i> = 1 and <i>n</i> = 2 samples are consistent with the presence of Fe³⁺ cations in square planar coordination sites. Magnetization data of both materials are consistent with spin glass-like behavior

Hexagonal Perovskite Ba₄Fe₃NiO₁₂ Containing Tetravalent Fe and Ni Ions

BaFe_<i>x</i>Ni_1–<i>x</i>O₃ with end members of BaNiO₃ (<i>x</i> = 0) and BaFeO₃ (<i>x</i> = 1), which, respectively, adopt the 2H and 6H hexagonal perovskite structures, were synthesized, and their crystal structures were investigated. A new single phase, Ba₄Fe₃NiO₁₂ (<i>x</i> = 0.75), that adopts the 12R perovskite structure with the space group <i>R</i>3̅<i>m</i> (<i>a</i> = 5.66564(7) Å and <i>c</i> = 27.8416(3) Å), was found to be stabilized. Mössbauer spectroscopy results and structure analysis using synchrotron and neutron powder diffraction data revealed that nominal Fe³⁺ occupies the corner-sharing octahedral site while the unusually high valence Fe⁴⁺ and Ni⁴⁺ occupy the face-sharing octahedral sites in the trimers, giving a charge formula of Ba₄Fe³⁺­Fe⁴⁺₂Ni⁴⁺O_11.5. The magnetic properties of the compound are also discussed