Outstanding Atomic Order in Ruddlesden–Popper Oxide Microcrystals

Ruddlesden–Popper’s manganates, A_<i>n</i>+1Mn_<i>n</i>O_3<i>n</i>+1, built from the ordered intergrowth between one rock-salt and <i>n</i> perovskite blocks, display a wide variety of functionalities related to their physical-chemistry properties which can be in principle tuned by chemical modifications. Nevertheless, the poor thermodynamic stability of the high members constitutes an inherent impediment, limiting the development of new functionalities in this family. Actually, for <i>n</i> ≥ 2, defects involving disordered intergrowths between perovskite and rock-salt blocks are always present avoiding the correct characterization of their properties. For that purpose, the use of sophisticated and expensive physical methods is required. In this article, the stabilization, following a chemical strategy, of micrometric La_0.5Ca_2.5Mn₂O₇ crystalline particles exhibiting a well ordered distribution of two perovskite and one rock-salt block, according to an ideal <i>n</i> = 2 unit cell, is reported. This apparently long-range structural ordering is linked to an unconventional short-range order–disorder phenomenon of La and Ca cations, characterized at the atomic level, which allows a rational explanation of the crystallochemical and magnetic properties of this Ruddlesden–Popper compound

Experimental Evidence of the Origin of Nanophase Separation in Low Hole-Doped Colossal Magnetoresistant Manganites

While being key to understanding their intriguing physical properties, the origin of nanophase separation in manganites and other strongly correlated materials is still unclear. Here, experimental evidence is offered for the origin of the controverted phase separation mechanism in the representative La_1–<i>x</i>Ca_<i>x</i>MnO₃ system. For low hole densities, direct evidence of Mn⁴⁺ holes localization around Ca²⁺ ions is experimentally provided by means of aberration-corrected scanning transmission electron microscopy combined with electron energy loss spectroscopy. These localized holes give rise to the segregated nanoclusters, within which double exchange hopping between Mn³⁺ and Mn⁴⁺ remains restricted, accounting for the insulating character of perovskites with low hole density. This localization is explained in terms of a simple model in which Mn⁴⁺ holes are bound to substitutional divalent Ca²⁺ ions

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

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

Nickel-Doped Sodium Cobaltite 2D Nanomaterials: Synthesis and Electrocatalytic Properties

In this work we report a synthetic pathway to two-dimensional nanostructures of high oxidation state lamellar cobalt oxides with thicknesses of only few atom layers, through the combined use of precipitation in basic water at room temperature and gentle solid state topotactic transformation at 120 °C. The 2D nanomaterials are characterized by X-ray diffraction, nitrogen porosimetry, scanning electron microscopy, transmission electron microscopy and especially scanning transmission electron microscopy coupled to energy dispersive X-ray analysis and electron energy loss spectroscopy to assess the composition of the nanosheets and the oxidation state of the transition metal species. We show that the nanosheets preserve high oxidation states Co³⁺ and Co⁴⁺ of high interest for electrocatalysis of the oxygen evolution reaction (OER). By combining high Co oxidation state, surface-to-volume ratio and optimized nickel substitution, the 2D nanomaterials produced in a simple way exhibit high OER electrocatalytic activity and stability in alkaline aqueous electrolyte comparable to standard materials obtained in harsh thermal conditions

Tuning of Oxygen Electrocatalysis in Perovskite Oxide Nanoparticles by the Cationic Composition

Manganese and cobalt perovskite oxides are among the most active precious metal-free electrocatalysts for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER), respectively. Herein, we question the role of the cationic composition and charge state in manganite, cobaltite, and mixed Mn/Co perovskites in the mechanism of oxygen electrocatalysis for ORR and OER. We synthesize in molten salts a range of perovskite nanoparticles active in ORR (single B-site (LaMn)1−γO3 and (La0.7Sr0.3Mn)1−γO3), in OER (single B-site La0.67Sr0.33CoO3−δ), and in both ORR and OER (mixed B-site (LaMn0.6Co0.4)1−γO3). By using operando X-ray absorption spectroscopy coupled to ex situ electron energy loss spectroscopy, we show that Mn and Co in single B-site perovskites undergo changes in oxidation states at the steady state during electrocatalysis, while their oxidation states remain unchanged in the mixed Mn/Co perovskite during OER and ORR. We relate these distinct behaviors to modifications of the rate-determining steps of both the OER and ORR electrocatalytic cycles, triggered by an increased covalency of B–O bonds in the mixed perovskites. These results highlight how simple cationic substitutions, accompanied by a control of cationic vacancies, offer a pathway to tune oxygen electrocatalysis

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