Valence Band Structure and X-ray Spectra of Oxygen-Deficient Ferrites SrFeO<sub><i>x</i></sub>

Galakhov, V. R.; Kurmaev, E.Z.; Kuepper, K.; Neumann, M.; McLeod, John A.; Moewes, A.; Леонидов, И. А.; Kozhevnikov, V. L.

doi:10.1021/jp909091s

Cited by 64 publications

(69 citation statements)

References 39 publications

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“…In this thin film case, the oxygen-deficient layers can order parallel or perpendicular to the substrate (this is shown in Figure 3a and 3b, respectively, with the pseudocubic orientation shown in Figure 3c and 16,20,21 The indirect band gap of both compounds is approximately 2.0 eV, owing to a Γ-to X-point transition. 22 We find that, in agreement with experimental results of the bulk phases, the Pbcm and Pnma phase are the lowest energy (and thus equilibrium) structures for Sr 2 Fe 2 O 5…”

Section: Introductionsupporting

confidence: 88%

Crystal structure and electronic properties of bulk and thin film brownmillerite oxides

Young¹,

Rondinelli²

2015

Phys. Rev. B

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The equilibrium structure and functional properties exhibited by brownmillerite oxides, a family of perovskite-derived structures with alternating layers of BO6 octahedra and BO4 tetrahedra, viz., ordered arrangements of oxygen vacancies, is dependent on a variety of competing crystal-chemistry factors. We use electronic structure calculations to disentangle the complex interactions in two ferrates, Sr2Fe2O5 and Ca2Fe2O5, relating the stability of the equilibrium (strain-free) and thin film structures to both previously identified and newly herein proposed descriptors. We show that cation size and intralayer separation of the tetrahedral chains provide key contributions to the preferred ground state. We show the bulk ground state structure is retained in the ferrates over a range of strain values; however, a change in the orientation of the tetrahedral chains, i.e., a perpendicular orientation of the vacancies relative to the substrate, is stabilized in the compressive region. The structure stability under strain is largely governed by maximizing the intraplane separation of the 'dipoles' generated from rotations of the FeO4 tetrahedra. Lastly, we find that the electronic band gap is strongly influenced by strain, manifesting as an unanticipated asymmetric-vacancy alignment dependent response. This atomistic understanding establishes a practical route for the design of functional electronic materials in thin film geometries.

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Section: Introductionsupporting

confidence: 88%

Crystal structure and electronic properties of bulk and thin film brownmillerite oxides

Young¹,

Rondinelli²

2015

Phys. Rev. B

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“…This is expected if both CaFeO 3 and SrFeO 3 have the same formal Fe 4+ valence state because the optical parameters are derived from, and dominated by, the oxidation state‐dependent XA spectra (see Figure S3, Supporting Information), although the Bragg intensity is not fully suppressed due to the different non‐resonant contributions of Ca and Sr to the overall scattering factors. This supports the conclusion that the O ligand hole modulation is not due to oxygen vacancies, which would reduce the Fe valence and thus alter the Fe optical parameters . Second, rather than q 005 , the q 006 reflection exhibits behavior different from the other Bragg reflections.…”

supporting

confidence: 71%

Depth‐Resolved Modulation of Metal–Oxygen Hybridization and Orbital Polarization across Correlated Oxide Interfaces

et al. 2019

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orbital degrees of freedom and produce emergent electronic and magnetic properties in TMOs, [1][2][3][4] a key question is what role do the underlying changes in metaloxygen hybridization play? This uncertainty remains because while electronic and magnetic properties can be measured by existing techniques, metal-oxygen hybridization cannot be thus far probed in a quantitative manner across interfaces. Here, we deploy resonant soft X-ray reflectivity to depth resolve the oxygen ligand hole density arising from metal-oxygen hybridization and reveal that a superlattice consisting of two TMOs with similar electronic structures unexpectedly exhibits large changes in orbital character. By resolving the orbital character at the unit cell level, we find that the disparate orbital characters reconstruct at the interface, thus demonstrating a new class of oxide interfacial reconstructions, that of metal-oxygen hybridization.The important role of metal-oxygen hybridization is highlighted by the anomalous behavior of TMOs with strong metal-oxygen covalency. These materials have metal d and O p bands that directly overlap in energy-leading to a small or negative charge transfer energy-and gives rise to self-doped oxygen ligand holes due to the energetically favorable transfer Interface-induced modifications of the electronic, magnetic, and lattice degrees of freedom drive an array of novel physical properties in oxide heterostructures. Here, large changes in metal-oxygen band hybridization, as measured in the oxygen ligand hole density, are induced as a result of interfacing two isovalent correlated oxides. Using resonant X-ray reflectivity, a superlattice of SrFeO 3 and CaFeO 3 is shown to exhibit an electronic character that spatially evolves from strongly O-like in SrFeO 3 to strongly Fe-like in CaFeO 3 . This alternating degree of Fe electronic character is correlated with a modulation of an Fe 3d orbital polarization, giving rise to an orbital superstructure. At the SrFeO 3 /CaFeO 3 interfaces, the ligand hole density and orbital polarization reconstruct in a single unit cell of CaFeO 3 , demonstrating how the mismatch in these electronic parameters is accommodated at the interface. These results provide new insight into how the orbital character of electrons is altered by correlated oxide interfaces and lays out a broadly applicable approach for depth-resolving band hybridization. Band HybridizationThe hybridization between transition metal d states and O p states is one of the defining features of transition metal oxides (TMOs) compared to conventional band insulators and metals, and the resulting mixed metal/oxygen character of the electronic structure plays a key role in their electronic properties. While it has been thoroughly demonstrated that formation of heterostructures can alter the charge, spin, and

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“…The values of δ (i.e. the amount of oxygen vacancy) in SrFeO 3-δ were determined by comparing the X-ray adsorption spectra with previous references 33 , as shown in Figure S1 in the Supporting Information (SI). All oxides examined in this study were single phase, as revealed by X-ray diffraction (XRD), with lattice parameters listed in Table S1 in the SI.…”

Section: Oxides Synthesis and Bulk Characterizationmentioning

confidence: 99%

Iron-Based Perovskites for Catalyzing Oxygen Evolution Reaction

et al. 2018

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The slow kinetics of the oxygen evolution reaction (OER) is the main cause of energy loss in many low-temperature energy storage techniques, such as metal-air batteries and water splitting. A better understanding of both the OER mechanism and the degradation mechanism on different transition metal oxides is critical for the development of the next generation of oxides as OER catalysts. In this paper, we systematically investigated the catalytic mechanism and lifetime of ABO 3-δ perovskite catalysts for OER, where A = Sr or Ca and B = Fe or Co. During the OER process, the Fe-based AFeO 3-δ oxides with δ ≈ 0.5 demonstrate no activation of lattice oxygen or pH dependence of OER activity, which is different from the SrCoO 2.5 with similar oxygen 2p-band position relative to the Fermi level. The difference was attributed to the larger changes in the electronic structure during the transition from the oxygen-deficient brownmillerite structure to the fully-oxidized perovskite structure and the poor conductivity in Fe-based oxides, which hinders the uptake of oxygen from the electrolyte to the lattice under oxidative potentials. The low stability of Fe-based perovskites under OER conditions in basic electrolyte also contribute to the different OER mechanism compared with the Co-based perovskites. This work reveals the influence of transition metal composition and electronic structure on the catalytic mechanism and operational stability of perovskite OER catalysts.

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Valence Band Structure and X-ray Spectra of Oxygen-Deficient Ferrites SrFeO_x

Cited by 64 publications

References 39 publications

Crystal structure and electronic properties of bulk and thin film brownmillerite oxides

Crystal structure and electronic properties of bulk and thin film brownmillerite oxides

Depth‐Resolved Modulation of Metal–Oxygen Hybridization and Orbital Polarization across Correlated Oxide Interfaces

Iron-Based Perovskites for Catalyzing Oxygen Evolution Reaction

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Valence Band Structure and X-ray Spectra of Oxygen-Deficient Ferrites SrFeOx

Cited by 64 publications

References 39 publications

Crystal structure and electronic properties of bulk and thin film brownmillerite oxides

Crystal structure and electronic properties of bulk and thin film brownmillerite oxides

Depth‐Resolved Modulation of Metal–Oxygen Hybridization and Orbital Polarization across Correlated Oxide Interfaces

Iron-Based Perovskites for Catalyzing Oxygen Evolution Reaction

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Product

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Valence Band Structure and X-ray Spectra of Oxygen-Deficient Ferrites SrFeO_x