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Abstract: The design of active and durable catalysts for the H2O/O2 interconversion is one of the major challenges of electrocatalysis for renewable energy. The oxygen evolution reaction (OER) is catalyzed by SrRuO3 with low potentials (ca. 1.35 VRHE), but the catalyst’s durability is insufficient. Here we show that Na doping enhances both activity and durability in acid media. DFT reveals that whereas SrRuO3 binds reaction intermediates too strongly, Na doping of ~0.125 leads to nearly optimal OER activity. Na doping increases the oxidation state of Ru, thereby displacing positively O p-band and Ru d-band centers, weakening Ru-adsorbate bonds. The enhanced durability of Na-doped perovskites is concomitant with the stabilization of Ru centers with slightly higher oxidation states, higher dissolution potentials, lower surface energy and less distorted RuO6 octahedra. These results illustrate how high OER activity and durability can be simultaneously engineered by chemical doping of perovskites.

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Abstract: The ground-state orbital occupancy of the Ru4+ ion in Ca2−xLaxRuO4[x=0, 0.05(1), 0.07(1), and 0.12(1)] was investigated by performing x-ray absorption spectroscopy (XAS) in the vicinity of the O K edge as a function of the angle between the incident beam and the surface of the single-crystal samples. A minimal model of the hybridization between the O 2p states probed at the K edge and the Ru 4d orbitals was used to analyze the XAS data, allowing the ratio of hole occupancies nxy/nyz,zx to be determined as a function of doping and temperature. For the samples displaying a low-temperature insulating ground state (x≤0.07), nxy/nyz,zx is found to increase significantly with increasing doping, with increasing temperature acting to further enhance nxy/nyz,zx. For the x=0.12 sample, which has a metallic ground state, the XAS spectra are found to be independent of temperature and not to be describable by the minimal hybridization model, while being qualitatively similar to the spectra displayed by the x≤0.07 samples above their insulating to metallic transitions. To understand the origin of the evolution of the electronic structure of Ca2−xLaxRuO4 across its phase diagram, we have performed theoretical calculations based on a model Hamiltonian, comprising electron-electron correlations, crystal field Δ, and spin-orbit coupling λ, of a Ru-O-Ru cluster, with realistic values used to parametrize the various interactions taken from the literature. Our calculations of the Ru hole occupancy as a function of Δ/λ provide an excellent description of the general trends displayed by the data. In particular they establish that the enhancement of nxy/nyz,zx is driven by significant modifications to the crystal field as the tetragonal distortion of the RuO6 octahedral changes from compressive to tensile with La doping. We have also used our model to show that the hole occupancy of the O 2p and Ru 4d orbitals displays the same general trend as a function of Δ/λ, thus validating the minimal hybridization model used to analyze the data. In essence, our results suggest that the predominant mechanism driving the emergence of the low-temperature metallic phase in La-doped Ca2RuO4 is the structurally induced redistribution of holes within the t2g orbitals, rather than the injection of free carriers.

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Abstract: We investigate the modifications of the exchange bias, effective ferromagnet-antiferromagnet (FM-AFM) coupling, and coercivity in annealed or ion-irradiated Ir−Mn/spacer layer (SL)/Co trilayers. A ferromagnetic, either Fe or permalloy (Py), thin SL or a nonmagnetic Ru one with different thicknesses (tSL) is used. The magnetic characterization is performed at room temperature via conventional magnetometry and, partly, via soft-x-ray magnetic circular dichroism. The latter shows that at the FM-AFM interface there is small uncompensated Mn magnetization coupled, preferentially, antiferromagnetically to Fe moments. This indicates the formation of small FeMn clusters that reverse their magnetizations together with those of Co, Ni, and the rest of Fe. Neither annealing nor ion irradiation of the films with tFe≥0.5 nm changes significantly the pinning part of the FM-AFM interface. In the Py-spacer films, however, the great tendency of Mn to interdiffuse with Ni leads to a decrease of the Ir−Mn anisotropy at the interface, lowering its pinning capacity. While defects created in the bulk of the AFM are mainly responsible for the changes of the magnetic characteristics of the Ir−Mn/Fe/Co films, interdiffusion and defect creation at the FM-AFM interface are the respective mechanisms determining the behavior of the Py-spacer series. These conclusions are reinforced by results for the Ru-spacer series.

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Abstract: Here we present linear and circular polarized soft x-ray absorption spectroscopy (XAS) data at the Ce M4,5 edges of the electron (Ir) and hole-doped (Re) Kondo semiconductor CeOs2Al10. Both substitutions have a strong impact on the unusual high Néel temperature TN=28.5 K, and also the direction of the ordered moment in case of Ir. The substitution dependence of the linear dichroism is weak thus validating the crystal-field description of CeOs2Al10 being representative for the Re and Ir substituted compounds. The impact of electron and hole doping on the hybridization between conduction and 4f electrons is related to the amount of f0 in the ground state and reduction of x-ray magnetic circular dichroism. A relationship of cf-hybridization strength and enhanced TN is discussed. The direction and doping dependence of the circular dichroism strongly supports the idea of strong Kondo screening along the crystallographic a direction.

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Abstract: In artificial multiferroics hybrids consisting of ferromagnetic La0.7Sr0.3MnO3 (LSMO) and ferroelectric BaTiO3epitaxial layers, net Ti moments are found from polarized resonant soft x-ray reflectivity and absorption. The Ti dichroic reflectivity follows the Mn signal during the magnetization reversal, indicating exchange coupling between the Ti and Mn ions. However, the Ti dichroic reflectivity shows stronger temperature dependence than the Mn dichroic signal. Besides a reduced ferromagnetic exchange coupling in the interfacial LSMO layer, this may also be attributed to a weak Ti-Mn exchange coupling that is insufficient to overcome the thermal energy at elevated temperatures.