Synthesis of 4H-SrMnO_3.0 Nanoparticles from a Molecular Precursor and Their Topotactic Reduction Pathway Identified at Atomic Scale

Stoichiometric 4H-SrMnO_3.0 nanoparticles have been synthesized from thermal decomposition of a new molecular heterometallic precursor [SrMn­(edta)­(H₂O)₅]·³/₂H₂O whose crystal structure has been solved by single crystal X-ray diffraction. From this precursor, highly homogeneous 4H-SrMnO_3.0 nanoparticles, with average particle size of 70 nm, are obtained. The agglomeration of these nanoparticles maintains the sheet-assembling morphology of the metal–organic compound. Local structural information, provided by atomically resolved microscopy techniques, shows that 4H-SrMnO_3.0 nanoparticles exhibit the same general structural features as the bulk material, although structural disorder, due to edge dislocations, is observed. The nanometric particle size enables a topotactic reduction process at low temperature stabilizing a metastable 4H-SrMnO_2.82 phase. The oxygen deficiency is accommodated through extra cubic layers breaking the ...chch... 4H-sequence. These defect areas are Mn³⁺ rich, as evidenced by high energy resolution EELS data. Magnetic characterization of nano-SrMnO_3.0 shows significant variations with respect to the bulk material. Besides the dominant antiferromagnetic interactions, a weak ferromagnetic contribution as well as exchange bias and a glassy-like component are present. After the reduction process, the stabilization of Mn³⁺ in the 4H-structure gives rise to magnetic anomalies in the 40–60 K temperature range. The origin of such magnetic features is discussed

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