We report on the oxygen vacancy effects controlling oxygen evolution activity of Sr2–x Ca x Fe2O6−δ perovskites. This system shows a tunable surface reactivity toward oxygen evolution reaction (OER) through changing the A-site composition (A = Sr2–x Ca x ) and the oxygen vacancy content, δ. We use solid-state synthesis to control the structural ordering and the content of oxygen vacancy. Samples with chemical compositions of x ≥ 0.75 crystallize in the brownmillerite-type structure, while those with low calcium content (x < 0.75) crystallize in either perovskite or brownmillerite structure. Our electrochemical analysis shows that compounds with mixed A-site compositions outperform Ca2Fe2O6−δ and Sr2Fe2O6−δ and reveal several compounds with higher activity than LaCoO3−δ (measured at 2 mA cm–2). The oxygen vacancy content (δ) of the compounds was quantified using X-ray photoelectron spectroscopy. The δ vs A-site composition trend shows a transition point where the structural transition was also observed, i.e., x ≃ 0.75. Interestingly, electrochemical activity correlates with δ, and compounds with δ → 1 tend to show higher activity. Our results show how short-range and long-range structural ordering and oxygen vacancy content can be employed to tune catalytic properties of Fe-based perovskites.
Geometric magnetic frustration (GMF) has attracted substantial interest due to the exotic physics and rich phase diagrams revealed by the cancellation of normally-dominant magnetic interactions, giving impetus for the search for novel frustrated systems, most often based on antiferromagnetic correlations between magnetic ions decorating triangular or tetrahedral lattices. We report here low-temperature magnetic susceptibility and muon spin relaxation results on Li4MgOsO6 and Li3Mg2OsO6, members of the A5BO6 "rock salt ordered" family of frustrated materials. In Li3Mg2OsO6 we find spin freezing below 12K. In Li4MgOsO6, which can crystallize into either orthorhombic F ddd or monoclinic C2/m crystal symmetries depending on synthesis conditions, we find magnetism consistent with glassy-like behavior dominating below 2K, with partial ordering and evidence for dynamics at somewhat higher temperatures.
In an effort to understand the structure-property relationship in magnetically frustrated systems, an orthorhombic analog of the S = 1/2 Re-based oxide LiMgReO has been successfully synthesized and its physical properties were investigated. LiMgReO had been previously synthesized in a monoclinic system in an ordered NaCl structure type. That system was shown to exhibit spin glass behavior below ∼12 K. The crystal structure of the latter phase was determined using powder X-ray diffraction data. A structural model was refined in the orthorhombic Fddd space group that resulted in cell dimensions of a = 5.84337 (7) Å, b = 8.33995 (9) Å, and c = 17.6237 (2) Å. The magnetic ions, Re (S = 1/2), consist of various arrangements of interconnected triangles and trigonal prisms that offer potential for geometric magnetic frustration. Temperature dependent magnetic susceptibility reveals an AFM transition below ∼2 K along with a ZFC/FC divergence suggestive of spin freezing. The Curie-Weiss fitting parameters to the paramagnetic regime result in θ = -124 (1) K, which is indicative of predominant AFM interactions. A frustration index of ∼62 is in accordance with a highly frustrated magnetic ground state. Zero field (ZF) μSR data provides evidence for the onset of magnetic order below 4 K, along with the evidence for dynamical fluctuations up to 5 K. Moreover, longitudinal field (LF) μSR data reveals a complete decoupling in applied field at 2 K, which is indicative of static order in most or all of the volume fraction at ∼2 K, with partial ordered volumes coexisting with dynamical fluctuations up to 5 K. Estimates of the relative strengths of various magnetic exchange pathways at the level of spin-dimer analysis for this novel system are calculated and are compared to those of the previously reported values for the monoclinic analog.
Two new transition metal oxides with the nominal chemical compositions of Li4NiOsO6 and Li3Ni2OsO6 were successfully synthesized. Both compounds crystallize in an ordered rock salt structure type in the monoclinic C2/m space group. The crystal structures were determined using both synchrotron X-ray and time-of-flight neutron, powder diffraction data. In both phases, Ni2+ ions are present while oxidation states of osmium are +6 and +5 in Li4NiOsO6 and Li3Ni2OsO6, respectively. Ni2+ ions in the hypothetical fully ordered phase form a honeycomb arrangement in the ab crystallographic plane and these hexagons are centered by osmium ions. The magnetic layers are separated along the c axis by the octahedra, which are centered by Li+ (or Li+/Ni2+, depending on the chemical compositions). Crystal structure refinements reveal that there is some degree of mixed occupancy in cationic positions. Temperature dependent magnetic susceptibility data for both phases show ferrimagnetic transitions with predominant antiferromagnetic (AFM) interactions among 3d electrons of nickel and 5d electrons of osmium. Iso-thermal magnetization loops as a function of the applied magnetic field below the transition temperatures confirm the ferrimagnetic nature in magnetic transitions. Temperature dependent heat capacity data, however, did not exhibit any anomaly in either phase, indicating the absence of long-range magnetic ordering. The lack of long-range order for both Os5+ and Os6+-based compounds was also confirmed by low temperature neutron diffraction data down to 10 K. Temperature dependent AC magnetic susceptibility data in various frequencies for both samples indicate that Li4NiOsO6 exhibits spin-glass-like behavior, while the transition temperature for Li3Ni2OsO6 is nearly frequency independent.
We report on electrochemical materials and processes with strong chemomechanical effects during electrocatalysis. We are particularly focused on anode and cathode processes for electrochemical water splitting. A combination of solid state, gas phase and thin film synthetic methods are used to prepare well-defined electrode materials. An optical laser method is used to perform in-situ surface stress measurements. We also employ Raman spectroscopy and imaging to map strain fields and inhomogeneity on surfaces. Combination of these responses provide unique insight into chemomechanics of hydrogen and oxygen evolution reactions. We will discuss the results of surface stress and strain measurements during H+ adsorption and early stages of hydrogen evolution on model Pt-M (M = Cu and Ni) surface alloys and bilayers. We will also report on the catalytic properties of perovskite-based transition metal oxides toward oxygen evolution reaction. We focus on perovskites and brownmillerites of the Sr2-xCaxFe2O6-δ system, where δ = 0-1. This system exhibits varying oxygen deficiency, vacancy ordering and volumetric strain. Our results suggest that vacancy content, ordering, and lattice oxygens control the oxygen evolution activity. Particularly, we will explain the role of lattice oxygens in the Sr2-xCaxFe2O6-δ system using in-situ surface stress results.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.