Chlorine Insertion Promoting Iron Reduction in Ba–Fe Hexagonal Perovskites: Effect on the Structural and Magnetic Properties

BaFeCl_0.13(2)O_2.48(2) has been synthesized and studied. A proper tuning of the synthetic route has been designed to stabilize this compound as a single phase. The thermal stability and evolution, along with the magnetic and structural properties are reported here. The crystal structure has been refined from neutron powder diffraction data, and it is of the type (hhchc)₂-10H. It is stable up to a temperature of 900 °C, where the composition reads BaFeCl_0.13(2)O_2.34(2). The study by electron microscopy shows that the crystal structure suffers no changes in the whole BaFeCl_0.13(1)O_3–<i>y</i> (2.34 ≤ 3 – <i>y</i> ≤ 2.48) compositional range. Refinement of the magnetic structure shows that the Fe is antiferromagneticaly ordered, with the magnetic moment parallel to the <i>ab</i> plane of the hexagonal structure. At higher temperature, a nonreversible phase transition into a (hchc)-4H structure type takes place with overall composition BaFeCl_0.13(1)O_2.26(1). Microstructural characterization shows that, in some crystals, this phase intergrows with a seemingly cubic related phase. Differences between these two crystalline phases reside in the chlorine content, which keeps constant through the phase transition for the former and disappears for the latter

Direct Atomic Observation in Powdered 4H-Ba_0.8Sr_0.2Mn_0.4Fe_0.6O_2.7

A new hexagonal polytype in the BaMn_1‑<i>x</i>Fe_<i>x</i>O_3‑δ system has been stabilized. Powdered Ba_0.8Sr_0.2Mn^IV_0.4Fe^III_0.6O_2.70 crystallizes in the 4H hexagonal polytype (space group <i>P</i>6₃/<i>mmc</i>) according to X-ray diffraction. HAADF images and chemical maps with atomic resolution have been obtained by combining Cs-corrected electron microscopy and EELS spectroscopy. The structure is formed by dimers of face-sharing octahedra linked by corners. EELS data show a random distribution of the transition metals ions identified by Fe and Mn-L2,3 chemical maps. A systematic difference in contrast observed in the O–K signal mapping suggests that anion deficiency is randomly located along the hexagonal layers in agreement with ND data. The magnetic structure consists of ferromagnetic sheets with the magnetic moments aligned along the <i>x</i>-axis and coupled antiferromagnetically along the <i>c</i>-axis

SrMnO₃ Thermochromic Behavior Governed by Size-Dependent Structural Distortions

The influence of particle size in both the structure and thermochromic behavior of 4H-SrMnO₃ related perovskite is described. Microsized SrMnO₃ suffers a structural transition from hexagonal (<i>P</i>6₃/<i>mmc</i>) to orthorhombic (<i>C</i>222₁) symmetry at temperature close to 340 K. The orthorhombic distortion is due to the tilting of the corner-sharing Mn₂O₉ units building the 4H structural type. When temperature decreases, the distortion becomes sharper reaching its maximal degree at ∼125 K. These structural changes promote the modification of the electronic structure of orthorhombic SrMnO₃ phase originating the observed color change. nano-SrMnO₃ adopts the ideal 4H hexagonal structure at room temperature, the orthorhombic distortion being only detected at temperature below 170 K. A decrease in the orthorhombic distortion degree, compared to that observed in the microsample, may be the reason why a color change is not observed at low temperature (77 K)

Critical Influence of Redox Pretreatments on the CO Oxidation Activity of BaFeO_3−δ Perovskites: An in-Depth Atomic-Scale Analysis by Aberration-Corrected and in Situ Diffraction Techniques

A BaFeO_3−δ (δ ≈ 0.22) perovskite was prepared by a sol–gel method and essayed as a catalyst in the CO oxidation reaction. BaFeO_3−δ (0.22 ≤ δ ≤ 0.42) depicts a 6H perovskite hexagonal structural type with Fe in both III and IV oxidation states and oxygen stoichiometry accommodated by a random distribution of anionic vacancies. The perovskite with the highest oxygen content, BaFeO_2.78, proved to be more active than its lanthanide-based counterparts, LnFeO₃ (Ln = La, Sm, Nd). Removal of the lattice oxygen detected in both temperature-programmed oxidation (TPO) and reduction (TPR) experiments at around 500 K and which leads to the complete reduction of Fe⁴⁺ to Fe³⁺, i.e. to BeFeO_2.5, significantly decreases the catalytic activity, especially in the low-temperature range. The analysis of thermogravimetric experiments performed under oxygen and of TPR studies run under CO clearly support the involvement of lattice oxygen in the CO oxidation on these Ba-Fe perovskites, even at the lowest temperatures. Atomically resolved images and chemical maps obtained using different aberration-corrected scanning transmission electron microscopy techniques, as well as some in situ type experiments, have provided a clear picture of the accommodation of oxygen nonstoichiometry in these materials. This atomic-scale view has revealed details of both the cation and anion sublattices of the different perovskites that have allowed us to identify the structural origin of the oxygen species most likely responsible for the low-temperature CO oxidation activity