Electronic structure of 3d-transition-metal compounds by analysis of the 2p core-level photoemission spectra

How to Correctly Analyze 2p X-ray Photoelectron Spectra of 3d Transition-Metal Oxides: Pitfalls and Principles

Nanomaterials based on transition-metal oxides (TMOs) that contain late 3d transition metals (e.g., Mn, Fe, Co, Ni) have diverse properties and functionality that are related to the oxidation state of constituent transition-metal (TM) cations. X-ray photoelectron spectroscopy (XPS) of TM 2p orbitals has been widely used to quantify the TM oxidation state of TMOs. However, 2p XPS spectra of late 3d TM cations usually have complicated shapes due to the charge transfer between the TM cation and oxygen ligands (anions), which makes the analysis highly nontrivial. In this article, we will examine the validity of commonly used analysis methods based on either peak fitting or the shift of binding energy (BE). The different origins of the BE shift in XPS spectra will be discussed. We will then introduce a model to reproduce the complex shapes of TM 2p spectra, based on fundamental parameters that describe the TMO electronic structure.

Visualization of oxygen vacancies and self-doped ligand holes in La3Ni2O7-δ

The recent discovery of superconductivity in La3Ni2O7-δ under high pressure with a transition temperature around 80 K (ref. 1) has sparked extensive experimental2-6 and theoretical efforts7-12. Several key questions regarding the pairing mechanism remain to be answered, such as the most relevant atomic orbitals and the role of atomic deficiencies. Here we develop a new, energy-filtered, multislice electron ptychography technique, assisted by electron energy-loss spectroscopy, to address these critical issues. Oxygen vacancies are directly visualized and are found to primarily occupy the inner apical sites, which have been proposed to be crucial to superconductivity13,14. We precisely determine the nanoscale stoichiometry and its correlation to the oxygen K-edge spectra, which reveals a significant inhomogeneity in the oxygen content and electronic structure within the sample. The spectroscopic results also reveal that stoichiometric La3Ni2O7 has strong charge-transfer characteristics, with holes that are self-doped from Ni sites into O sites. The ligand holes mainly reside on the inner apical O and the planar O, whereas the density on the outer apical O is negligible. As the concentration of O vacancies increases, ligand holes on both sites are simultaneously annihilated. These observations will assist in further development and understanding of superconducting nickelate materials. Our imaging technique for quantifying atomic deficiencies can also be widely applied in materials science and condensed-matter physics.

X-Ray Magnetic Circular Dichroism in Altermagnetic α-MnTe

Altermagnetism is a recently identified magnetic symmetry class combining characteristics of conventional collinear ferromagnets and antiferromagnets, that were regarded as mutually exclusive, and enabling phenomena and functionalities unparalleled in either of the two traditional elementary magnetic classes. In this work we use symmetry, ab initio theory, and experiments to explore x-ray magnetic circular dichroism (XMCD) in the altermagnetic class. As a representative material for our XMCD study we choose α-MnTe with compensated antiparallel magnetic order in which an anomalous Hall effect has been already demonstrated. We predict and experimentally confirm a characteristic XMCD line shape for compensated moments lying in a plane perpendicular to the light propagation vector. Our results highlight the distinct phenomenology in altermagnets of this time-reversal symmetry breaking response, and its potential utility for element-specific spectroscopy and microscopy.

In Situ X-ray Absorption Spectroscopy of LaFeO3 and LaFeO3/LaNiO3 Thin Films in the Electrocatalytic Oxygen Evolution Reaction

We study the electrocatalytic oxygen evolution reaction using in situ X-ray absorption spectroscopy (XAS) to track the dynamics of the valence state and the covalence of the metal ions of LaFeO3 and LaFeO3/LaNiO3 thin films. The active materials are 8 unit cells grown epitaxially on 100 nm conductive La0.67Sr0.33MnO3 layers using pulsed laser deposition (PLD). The perovskite layers are supported on monolayer Ca2Nb3O10 nanosheet-buffered 100 nm SiNx membranes. The in situ Fe and Ni K-edges XAS spectra were measured from the backside of the SiNx membrane using fluorescence yield detection under electrocatalytic reaction conditions. The XAS spectra show significant spectral changes, which indicate that (1) the metal (co)valencies increase, and (2) the number of 3d electrons remains constant with applied potential. We find that the whole 8 unit cells react to the potential changes, including the buried LaNiO3 film.

Electrocatalytic Properties of Oxygen-Deficient Perovskites Ca3Fe3-xMnxO8 (x = 1-2) for the Hydrogen Evolution Reaction

We have demonstrated a systematic trend in the electrocatalytic activity for the hydrogen evolution reaction (HER) and its correlations with transition-metal type, structural order, and electrical conductivity. The materials studied in this work, Ca3FeMn2O8 (CaFe1/3Mn2/3O3-1/3), Ca3Fe1.5Mn1.5O8, and Ca3Fe2MnO8, belong to the family of oxygen-deficient perovskites and show a gradual increase in the ordering of oxygen vacancies. Ca3FeMn2O8 (CaFe1/3Mn2/3O3-1/3) contains randomly distributed oxygen vacancies, which begin to order in Ca3Fe1.5Mn1.5O8, and are fully ordered in Ca3Fe2MnO8. The gradual increase in the structural order is associated with a systematic enhancement of the electrocatalytic activity for HER in acidic conditions, Ca3FeMn2O8 < Ca3Fe1.5Mn1.5O8 < Ca3Fe2MnO8. While the improvement of the HER activity is also associated with an increase in the Fe content, we have shown that the type of structural order plays a more important role. We demonstrated this effect by control experiments on an analogous material where all Mn was substituted by Fe, leading to a different type of structural order and showing an inferior HER activity compared to the above three materials. Furthermore, electrical conductivity studies in a wide range of temperatures, 25-800 °C, indicate that the trend in the electrical conductivity is the same as that of the HER activity. These findings reveal several important structure-property relationships and highlight the importance of synergistic effects in enhancing the electrocatalytic properties.

Identifying a Universal Activity Descriptor and a Unifying Mechanism Concept on Perovskite Oxides for Green Hydrogen Production

Producing indispensable hydrogen and oxygen for social development via water electrolysis shows more prospects than other technologies. Although electrocatalysts have been explored for centuries, a universal activity descriptor for both hydrogen-evolution reaction (HER) and oxygen-evolution reaction (OER) is not yet developed. Moreover, a unifying concept is not yet established to simultaneously understand HER/OER mechanisms. Here, the relationships between HER/OER activities in three common electrolytes and over ten representative material properties on 12 3d-metal-based model oxides are rationally bridged through statistical methodologies. The orbital charge-transfer energy (Δ) can serve as an ideal universal descriptor, where a neither too large nor too small Δ (≈1 eV) with optimal electron-cloud density around Fermi level affords the best activities, fulfilling Sabatier's principle. Systematic experiments and computations unravel that pristine oxide with Δ ≈ 1 eV possesses metal-like high-valence configurations and active lattice-oxygen sites to help adsorb key protons in HER and induce lattice-oxygen participation in the OER, respectively. After reactions, partially generated metals in the HER and high-valence hydroxides in the OER dominate proton adsorption and couple with pristine lattice-oxygen activation, respectively. These can be successfully rationalized by the unifying orbital charge-transfer theory. This work provides the foundation of rational material design and mechanism understanding for many potential applications.

Unusual double ligand holes as catalytic active sites in LiNiO2

Designing efficient catalyst for the oxygen evolution reaction (OER) is of importance for energy conversion devices. The anionic redox allows formation of O-O bonds and offers higher OER activity than the conventional metal sites. Here, we successfully prepare LiNiO₂ with a dominant 3d⁸L configuration (L is a hole at O 2p) under high oxygen pressure, and achieve a double ligand holes 3d⁸L² under OER since one electron removal occurs at O 2p orbitals for Ni^III oxides. LiNiO₂ exhibits super-efficient OER activity among LiMO₂, RMO₃ (M = transition metal, R = rare earth) and other unary 3d catalysts. Multiple in situ/operando spectroscopies reveal Ni^III→Ni^IV transition together with Li-removal during OER. Our theory indicates that Ni^IV (3d⁸L²) leads to direct O-O coupling between lattice oxygen and *O intermediates accelerating the OER activity. These findings highlight a new way to design the lattice oxygen redox with enough ligand holes created in OER process.

Intermixing-Driven Surface and Bulk Ferromagnetism in the Quantum Anomalous Hall Candidate MnBi6 Te10

The recent realizations of the quantum anomalous Hall effect (QAHE) in MnBi₂ Te₄ and MnBi₄ Te₇ benchmark the (MnBi₂ Te₄ )(Bi₂ Te₃ )_n family as a promising hotbed for further QAHE improvements. The family owes its potential to its ferromagnetically (FM) ordered MnBi₂ Te₄ septuple layers (SLs). However, the QAHE realization is complicated in MnBi₂ Te₄ and MnBi₄ Te₇ due to the substantial antiferromagnetic (AFM) coupling between the SLs. An FM state, advantageous for the QAHE, can be stabilized by interlacing the SLs with an increasing number n of Bi₂ Te₃ quintuple layers (QLs). However, the mechanisms driving the FM state and the number of necessary QLs are not understood, and the surface magnetism remains obscure. Here, robust FM properties in MnBi₆ Te₁₀ (n = 2) with T_c ≈ 12 K are demonstrated and their origin is established in the Mn/Bi intermixing phenomenon by a combined experimental and theoretical study. The measurements reveal a magnetically intact surface with a large magnetic moment, and with FM properties similar to the bulk. This investigation thus consolidates the MnBi₆ Te₁₀ system as perspective for the QAHE at elevated temperatures.

Unraveling Sequential Oxidation Kinetics and Determining Roles of Multi-Cobalt Active Sites on Co3O4 Catalyst for Water Oxidation

The multi-redox mechanism involving multi-sites has great implications to dictate the catalytic water oxidation. Understanding the sequential dynamics of multi-steps in oxygen evolution reaction (OER) cycles on working catalysts is a highly important but challenging issue. Here, using quasi-operando transient absorption (TA) spectroscopy and a typical photosensitization strategy, we succeeded in resolving the sequential oxidation kinetics involving multi-active sites for water oxidation in OER catalytic cycle, with Co₃O₄ nanoparticles as model catalysts. When OER initiates from fast oxidation of surface Co²⁺ ions, both surface Co²⁺ and Co³⁺ ions are active sites of the multi-cobalt centers for water oxidation. In the sequential kinetics (Co²⁺ → Co³⁺ → Co⁴⁺), the key characteristic is fast oxidation and slow consumption for all the cobalt species. Due to this characteristic, the Co⁴⁺ intermediate distribution plays a determining role in OER activity and results in the slow overall OER kinetics. These insights shed light on the kinetic understanding of water oxidation on heterogeneous catalysts with multi-sites.

Iron L3-edge energy shifts for the full range of possible 3d occupations within the same oxidation state of iron halides

Oxidation states are integer in number but dⁿ configurations of transition metal centers vary continuously in polar bonds. We quantify the shifts of the iron L₃ excitation energy, within the same formal oxidation state, in a systematic L-edge X-ray absorption spectroscopy study of diatomic gas-phase iron(II) halide cations, [FeX]⁺,where X = F, Cl, Br, I. These shifts correlate with the electronegativity of the halogen, and are attributed exclusively to a fractional increase in population of 3d-derived orbitals along the series as supported by charge transfer multiplet simulations and density functional theory calculations. We extract an excitation energy shift of 420 meV ± 60 meV spanning the full range of possible 3d occupations between the most ionic bond in [FeF]⁺ and covalently bonded [FeI]⁺.

Charge-transfer effect in Fe 2pcore-level x-ray photoemission spectra of trivalent Fe oxides: LDA + DMFT study

Motivated by recent hard x-ray photoemission spectroscopy (XPS) experiment for trivalent Fe oxides Sr₂FeMoO₆(ferrimagnetic correlated metal) and LaFeO₃(antiferromagnetic Mott insulator) (Phuyalet al2021J. Phys. Chem.C12511249-56), we present a theoretical analysis of the Fe 2pcore-level spectra using a computational method based on local density approximation combined with dynamical mean-field theory. We find that a nonlocal screening (NLS) effect in the XPS final states is crucial for interpreting the experimental XPS result of both the Fe oxides. A close relationship between the NLS feature in core-level spectra and a long-range magnetic ordering is emphasized.

The ligand field in low-crystallinity metal-organic frameworks investigated by soft X-ray core-level absorption spectroscopy

The ligand field (LF) of transition metal ions is a crucial factor in realizing the mechanism of novel physical and chemical properties. However, the low-crystallinity state, including the amorphous state, precludes the clarification of the electronic structural relationship of transition metal ions using crystallographic techniques, ultraviolet and infrared optical methods, and magnetometry. Here, we demonstrate that soft X-ray 2p → 3d core-level absorption spectroscopy (L_2,3-edge XAS) systematically revealed the local 3d electronic states, including in the LF, of nitrogen-coordinated transition-metal ions for low-crystallinity cyanide-bridged metal-organic frameworks (MOFs) M[Ni(CN)₄] (MNi; M = Mn, Fe, Co, Ni) and Ni[Pd(CN)₄] (NiPd). In NiNi and NiPd, N-coordinated Ni ions with square-planar symmetry exhibit strong orbital hybridization and ligand-to-metal charge transfer effects. In MnNi, FeNi, and CoNi, the correlation between the crystalline electric field splitting in the LF and the transition metal-nitrogen bonding length is revealed using the multiplet LF theory. Regardless of the different local symmetries, our results indicate that L_2,3-edge XAS is a powerful tool for gaining element-specific knowledge about the transition-metal ion characterizing the functionality of low-crystallinity MOFs and will be the foundation for an attractive platform, such as adsorption/desorption materials.

Robust electronic and tunable magnetic states in Sm2 NiMnO6ferromagnetic insulator

Ferromagnetic insulators (FM-Is) are the materials of interest for the new generation quantum electronic applications. Here, we have investigated the physical observables depicting FM-I ground states in epitaxial Sm₂NiMnO₆(SNMO) double perovskite thin films fabricated under different conditions to realize the different level of Ni/Mn anti-site disorders (ASDs). The presence of ASDs immensely influence the characteristic magnetic and anisotropy behaviors in SNMO system by introducing short scale antiferromagnetic interactions in predominant long range FM ordered host matrix. Charge disproportion between cation sites, in the form of Ni²⁺+ Mn⁴⁺→ Ni³⁺+ Mn³⁺, causes mixed valency in both Ni and Mn species, which is found insensitive to ASD concentrations. Temperature dependent photo emission, photo absorption measurements duly combined with cluster model configuration interaction simulations, suggest that the eigenstates of Ni and Mn cations can be satisfactorily described as a linear combination of the unscreeneddⁿand screeneddn+1L̲(L̲: O 2phole) states. The electronic structure across the Fermi level (E_F) exhibits closely spaced Ni 3d, Mn 3dand O 2pstates. From occupied and unoccupied bands, estimated values of the Coulomb repulsion energy (U) and ligand to metal charge transfer energy (Δ), indicate charge transfer insulating nature, where remarkable modification in Ni/Mn 3d-O 2phybridization takes place across the FM transition temperature. Existence of ASD broadens the Ni, Mn 3dspectral features, whereas the spectral positions are found to be unaltered. Hereby, present work demonstrates SNMO thin film as a FM-I system, where the FM state can be tuned by manipulating ASD in the crystal structure, while the I state remains intact.

Rational Design of 3d Transition-Metal Compounds for Thermoelectric Properties by Using Periodic Trends in Electron-Correlation Modulation

The electronic structures in solid-state transition-metal compounds can be represented by two parameters: the charge-transfer energy (Δ), which is the energy difference between the p-band of an anion and an upper Hubbard band contributed by transition-metal d-orbitals, and the onsite Coulomb repulsion energy (U), which represents the energy difference between lower and upper Hubbard bands composed of split d-orbitals in transition metals. These parameters can facilitate the classification of various types of electronic structures. In this study, the dependences of anion species (N^3-, P^3-, As^3-, O^2-, S^2-, Se^2-, Te^2-, F^-, Cl^-, Br^-, and I^-) on Δ and U of 566 different binary and ternary 3d transition-metal compounds were investigated using ionic-model calculations. We were able to identify the systematic chemical trends in the variations in Δ and U values with the anion species of 11 different families of 3d transition-metal compounds in a comprehensive manner. The effective use of Δ-U diagrams given here, to facilitate the discovery and development of functional compounds, was demonstrated on thermoelectric compounds by classifying the thermoelectric properties of 3d transition-metal compounds and by predicting unrealized high-performance thermoelectric compounds.

Electronic Structure-Based Descriptors for Oxide Properties and Functions

ConspectusThe transition from fossil fuels to renewable energy requires the development of efficient and cost-effective energy storage technologies. A promising way forward is to harness the energy of intermittent renewable sources, such as solar and wind, to perform (electro)catalytic reactions to generate fuels, thus storing energy in the form of chemical bonds. However, current catalysts rely on the use of expensive, rare, or geographically localized elements, such as platinum. Widespread adoption of new (electro)catalytic technologies hinges on the discovery and development of materials containing earth-abundant elements, which can efficiently catalyze an array of (electro)chemical reactions.In the context of catalysis, descriptors provide correlations between fundamental physical properties, such as the electronic structure, and the resulting catalytic activity. The use of easily accessible descriptors has proven to be a powerful method to advance and accelerate discovery and design of new catalyst materials. The position of the oxygen electronic 2p band center has been proposed to capture the basic physical properties of oxides, including oxygen vacancy formation energy, diffusion barrier of oxygen ions, and work function. Moreover, the adsorption strength of relevant reaction intermediates at the surface of oxides can be strongly correlated with the energy of the oxygen 2p states, which affects the catalytic activity of reactions, such as oxygen electrocatalysis, and oxidative dehydrogenation of organic molecules. Such descriptors for catalytic activity can be used to predict the activity of new catalysts and understand trends and behavior among different catalysts.In this Account, we discuss how the energy of the oxygen 2p states can be used as a descriptor for oxide bulk and surface chemical properties. We show how the oxide redox properties vary linearly with the position of the oxygen 2p band center with respect to the Fermi level, and we discuss how this descriptor can be expanded across different materials and structural families, including possible generalizations to compounds outside oxides. We highlight the power of the oxygen 2p band center to predict the catalytic activity of oxides. We conclude with an outlook examining under which conditions this descriptor can be applied to predict oxide properties and possible opportunities for further refining and accelerating property predictions of oxides by leveraging material databases and machine learning.

Combining Valence-to-Core X-ray Emission and Cu K-edge X-ray Absorption Spectroscopies to Experimentally Assess Oxidation State in Organometallic Cu(I)/(II)/(III) Complexes

A series of organometallic copper complexes in formal oxidation states ranging from +1 to +3 have been characterized by a combination of Cu K-edge X-ray absorption (XAS) and Cu Kβ valence-to-core X-ray emission spectroscopies (VtC XES). Each formal oxidation state exhibits distinctly different XAS and VtC XES transition energies due to the differences in the Cu Z_eff, concomitant with changes in physical oxidation state from +1 to +2 to +3. Herein, we demonstrate the sensitivity of XAS and VtC XES to the physical oxidation states of a series of N-heterocyclic carbene (NHC) ligated organocopper complexes. We then extend these methods to the study of the [Cu(CF₃)₄]⁻ ion. Complemented by computational methods, the observed spectral transitions are correlated with the electronic structure of the complexes and the Cu Z_eff. These calculations demonstrate that a contraction of the Cu 1s orbitals to deeper binding energy upon oxidation of the Cu center manifests spectroscopically as a stepped increase in the energy of both XAS and Kβ_2,5 emission features with increasing formal oxidation state within the [Cun+(NHC₂)]ⁿ⁺ series. The newly synthesized Cu(III) cation [Cu^III(NHC₄)]³⁺ exhibits spectroscopic features and an electronic structure remarkably similar to [Cu(CF₃)₄]⁻, supporting a physical oxidation state assignment of low-spin d⁸ Cu(III) for [Cu(CF₃)₄]⁻. Combining XAS and VtC XES further demonstrates the necessity of combining multiple spectroscopies when investigating the electronic structures of highly covalent copper complexes, providing a template for future investigations into both synthetic and biological metal centers.

Quantifying the Steric Effect on Metal-Ligand Bonding in Fe Carbene Photosensitizers with Fe 2p3d Resonant Inelastic X-ray Scattering

Understanding the electronic structure and chemical bonding of transition metal complexes is important for improving the function of molecular photosensitizers and catalysts. We have utilized X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) at the Fe L₃ edge to investigate the electronic structure of two Fe N-heterocyclic carbene complexes with similar chemical structures but different steric effects and contrasting excited-state dynamics: [Fe(bmip)₂]²⁺ and [Fe(btbip)₂]²⁺, bmip = 2,6-bis(3-methyl-imidazole-1-ylidine)pyridine and btbip = 2,6-bis(3-tert-butyl-imidazole-1-ylidene)pyridine. In combination with charge transfer multiplet and ab initio calculations, we quantified how changes in Fe-carbene bond length due to steric effects modify the metal-ligand bonding, including σ/π donation and π back-donation. We find that σ donation is significantly stronger in [Fe(bmip)₂]²⁺, whereas the π back-donation is similar in both complexes. The resulting stronger ligand field and nephelauxetic effect in [Fe(bmip)₂]²⁺ lead to approximately 1 eV destabilization of the quintet metal-centered ⁵T_2g excited state compared to [Fe(btbip)₂]²⁺, providing an explanation for the absence of a photoinduced ⁵T_2g population and a longer metal-to-ligand charge-transfer excited-state lifetime in [Fe(bmip)₂]²⁺. This work demonstrates how combined modeling of XAS and RIXS spectra can be utilized to understand the electronic structure of transition metal complexes governed by correlated electrons and donation/back-donation interactions.

Evidence for surface effects on the intermolecular interactions in Fe(II) spin crossover coordination polymers

From X-ray absorption spectroscopy (XAS) and X-ray photoemission spectroscopy (XPS), it is evident that the spin state transition behavior of Fe(II) spin crossover coordination polymer crystallites at the surface differs from the bulk. A comparison of four different coordination polymers reveals that the observed surface properties may differ from bulk for a variety of reasons. There are Fe(II) spin crossover coordination polymers with either almost complete switching of the spin state at the surface or no switching at all. Oxidation, differences in surface packing, and changes in coordination could all contribute to making the surface very different from the bulk. Some Fe(II) spin crossover coordination polymers may be sufficiently photoactive so that X-ray spectroscopies cannot discern the spin state transition.

Operando resonant soft X-ray emission spectroscopy of LiMn2O4 cathode using an aqueous electrolyte solution

The Mn 3d electronic-structure change of LiMn2O4 cathode during Li-ion extraction/insertion in an aqueous electrolyte solution was studied by operando resonant soft X-ray emission spectroscopy (RXES). The Mn L3 RXES...

A complementary characterisation technique for spin crossover materials; the application of X-ray photoelectron spectroscopy for future device applications

Spin crossover (SCO) materials have long been studied for their inherent electronic switchability, which has been well investigated for potential application in electronic and switching devices. As the technologies for the fabrication of thin films and monolayers continue to develop at an exceedingly rapid pace, an emerging challenge for the SCO community has become the characterisation of spin transitions in the surface layers of a material, as well as understanding the origins of discrepancies observed between SCO in thin films and that of the bulk material. For the manufacture of such devices to become a reality, it is crucial to understand how spin crossover is affected by interactions with the substrate material and within thin films. As such, detailed analysis of the surface layers without interference from the substrate material emerged as a critical area of characterisation for future developments in SCO devices. In this regard, X-ray Photoelectron Spectroscopy (XPS) has emerged as a complementary technique for the analysis of SCO in the surface layers of a material, becoming an essential part of a multi-technique protocol that is driving advances in the field. Here we describe the complementary application of XPS to a variety of SCO materials, review major developments and provide illustrative examples of innovations made through surface analysis with XPS.

Variation of the electrocatalytic activity of isostructural oxides Sr2LaFeMnO7 and Sr2LaCoMnO7 for hydrogen and oxygen-evolution reactions

The effect of the electronic configurations of transition metals on electrocatalytic activity, charge transport, and magnetic properties is demonstrated through the investigation of Sr₂LaFeMnO₇ and Sr₂LaCoMnO₇. The two compounds are isostructural and contain bilayer stacks of octahedrally coordinated transition metals. Despite their structural similarity, the magnetic transition temperature of Sr₂LaCoMnO₇ is significantly lower than that of Sr₂LaFeMnO₇. The electrical charge-transport properties are also different, where Sr₂LaCoMnO₇ shows considerably improved electrical conductivity. Importantly, the electrocatalytic activities for the two half-reactions of water-splitting, i.e., the hydrogen-evolution reaction (HER) and the oxygen-evolution reaction (OER), are improved in Sr₂LaCoMnO₇ compared to Sr₂LaFeMnO₇. In addition, better kinetics for the HER and OER are observed for Sr₂LaCoMnO₇, as evaluated by the Tafel method. Furthermore, the electrochemically active surface area (ECSA) shows an enhancement for Sr₂LaCoMnO₇. Therefore, the trends in electrical charge transport, the HER and OER activities, kinetics and ECSA are all similar, indicating the improved properties of Sr₂LaCoMnO₇. These changes are explained in the context of a greater bond covalency in this material due to the higher electronegativity of Co, which results in a better overlap between the transition metal d orbital and oxygen p orbital. The relation between the electrocatalytic performance and the optimum e_g orbital occupancy in Sr₂LaCoMnO₇ is also discussed.

Chemical Sensitivity of Kβ and Kα X-ray Emission from a Systematic Investigation of Iron Compounds

K-fluorescence X-ray emission spectroscopy (XES) is receiving growing interest in all fields of natural sciences to investigate the local spin. The spin sensitivity in Kβ (Kα) XES stems from the exchange interaction between the unpaired 3p (2p) and the 3d electrons, which is greater for Kβ than for Kα. We present a thorough investigation of a large number of iron-bearing compounds. The experimental spectra were analyzed in terms of commonly used quantitative parameters (Kβ_1,3-first moment, Kα₁-full width at half-maximum, and integrated absolute difference -IAD-), and we carefully examined the difference spectra. Multiplet calculations were also performed to elucidate the underlying mechanisms that lead to the chemical sensitivity. Our results confirm a strong influence of covalency on both Kβ and Kα lines. We establish a reliable spin sensitivity of Kβ XES as it is dominated by the exchange interaction, whose variations can be quantified by either Kβ_1,3-first moment or Kβ-IAD and result in a systematic difference signal line shape. We find an exception in the Kβ XES of Fe³⁺ and Fe²⁺ in water solution, where a new difference spectrum is identified that cannot be reproduced by scaling the exchange integrals. We explain this by strong differences in orbital mixing between the valence orbitals. This result calls for caution in the interpretation of Kβ XES spectral changes as due to spin variations without a careful analysis of the line shape. For Kα XES, the smaller exchange interaction and the influence of other electron-electron interactions make it difficult to extract a quantity that directly relates to the spin.

Voltage- and time-dependent valence state transition in cobalt oxide catalysts during the oxygen evolution reaction

The ability to determine the electronic structure of catalysts during electrochemical reactions is highly important for identification of the active sites and the reaction mechanism. Here we successfully applied soft X-ray spectroscopy to follow in operando the valence and spin state of the Co ions in Li₂Co₂O₄ under oxygen evolution reaction (OER) conditions. We have observed that a substantial fraction of the Co ions undergo a voltage-dependent and time-dependent valence state transition from Co³⁺ to Co⁴⁺ accompanied by spontaneous delithiation, whereas the edge-shared Co-O network and spin state of the Co ions remain unchanged. Density functional theory calculations indicate that the highly oxidized Co⁴⁺ site, rather than the Co³⁺ site or the oxygen vacancy site, is mainly responsible for the high OER activity.

Boosting Oxygen Evolution Reaction by Creating Both Metal Ion and Lattice-Oxygen Active Sites in a Complex Oxide

Developing efficient and low-cost electrocatalysts for the oxygen evolution reaction (OER) is of paramount importance to many chemical and energy transformation technologies. The diversity and flexibility of metal oxides offer numerous degrees of freedom for enhancing catalytic activity by tailoring their physicochemical properties, but the active site of current metal oxides for OER is still limited to either metal ions or lattice oxygen. Here, a new complex oxide with unique hexagonal structure consisting of one honeycomb-like network, Ba₄ Sr₄ (Co_0.8 Fe_0.2 )₄ O₁₅ (hex-BSCF), is reported, demonstrating ultrahigh OER activity because both the tetrahedral Co ions and the octahedral oxygen ions on the surface are active, as confirmed by combined X-ray absorption spectroscopy analysis and theoretical calculations. The bulk hex-BSCF material synthesized by the facile and scalable sol-gel method achieves 10 mA cm^-2 at a low overpotential of only 340 mV (and small Tafel slope of 47 mV dec^-1 ) in 0.1 m KOH, surpassing most metal oxides ever reported for OER, while maintaining excellent durability. This study opens up a new avenue to dramatically enhancing catalytic activity of metal oxides for other applications through rational design of structures with multiple active sites.

Operando soft X-ray emission spectroscopy of the Fe2O3 anode to observe the conversion reaction

Drastic electronic-structure changes in an Fe2O3 thin film anode for a Li-ion battery during discharge (lithiation) and charge (delithiation) processes were observed using operando Fe 2p soft X-ray emission spectroscopy (XES). The conversion reaction forming metallic iron due to the lithiation reaction was confirmed by operando XES in combination with the analysis using full-multiplet calculation. The valence of Fe at the open-circuit voltage (OCV) before the second cycle was not Fe3+, but Fe2+ with a weak p-d hybridization, suggesting a considerable irreversibility upon the first discharge-charge cycle and a weakened Fe-O bond after the first cycle. Moreover, we revealed that the Fe 3d electronic-structure change during the second cycle was to some extent reversible as Fe2+ (2.7 V vs. Li/Li+: open circuit voltage) → Fe0 (0.1 V vs. Li/Li+: discharged) → Fe(2+δ)+ (3.0 V vs. Li/Li+: charged). This operando Fe 2p XES in combination with the full-multiplet calculation provides detailed information for redox chemistry during a discharge-charge operation that cannot be obtained by other methods such as crystal-structure and morphology analyses. XES is thus very powerful for investigating the origin and limitation of the lithiation function of anodes involving conversion reactions.

Mn 2p resonant X-ray emission clarifies the redox reaction and charge-transfer effects in LiMn2O4

High-energy-resolution soft X-ray emission spectroscopy (XES) was applied to understand the changes in the electronic structure of LiMn2O4 upon Li-ion extraction/insertion. Mn 2p-3d-2p resonant XES spectra were analyzed by configuration-interaction full-multiplet (CIFM) calculations, which reproduced both dd and charge-transfer (CT) excitations. From the resonant XES spectra it is found that Mn3+ and Mn4+ coexist in the initial state, while this changes into Mn4+ in the charged-state. For the discharged-state, the Mn3+ component appears again although the dd excitations are slightly modified from those for the initial state. Furthermore, negative CT energy is expected for the Mn4+ configuration, which suggests very strong hybridization between the Mn 3d and O 2p orbitals. The large difference in the CT effect between the Mn4+ and Mn3+ states should give mechanical stress to the Mn-O bond during charge-discharge cycling, leading to capacity fading.

Elucidating the origin of magnetic ordering in ferroelectric BaTiO3- d thin film via electronic structure modification

With the motive of unraveling the origin of native vacancy induced magnetization in ferroelectric perovskite oxide systems, here we explore the consequences of electronic structure modification in magnetic ordering of oxygen deficient epitaxial BaTiO[Formula: see text] thin films. Our adapted methodology employs state-of-the-art experimental approaches viz. photo-emission, photo-absorption spectroscopies, magnetometric measurements duly combined with first principles based theoretical methods within the frame work of density functional theory (DFT and DFT+U) calculations. Oxygen vacancy (O[Formula: see text]) is observed leading partial population of Ti 3d (t _2g ), which induces defect state in electronic structure near the Fermi level and reduces the band gap. The oxygen deficient BaTiO_2.75 film reveals Mott-Hubbard insulator characteristic, in contrast to the band gap insulating nature of the stoichiometric BaTiO₃. The observed magnetic ordering is attributed to the asymmetric distribution of spin polarized charge density in the vicinity of O[Formula: see text] site, which originates unequal magnetic moment values at first and second nearest neighboring Ti sites, respectively. Hereby, we present an exclusive method for maneuvering the band gap and on-site electron correlation energy with consequences on magnetic properties of BaTiO[Formula: see text] system, which can open a gateway for designing novel single phase multiferroic system.

Selective reduction of 1,5-diazacyclooctatetraenes: synthesis and structures of aromatic diazacyclooctatetraenyl dianions and a 2,6-bipyrrolinyl dianionic Co(ii) complex

The first structurally well-defined aromatic 1,5-diazacyclooctatetraenyl dianion and a 2,6-bipyrrolinyl dianionic Co(ii) complex were obtained selectively via reduction of 1,5-diazacyclooctatetraenes.

Electronic structure of Pr2MnNiO6 from x-ray photoemission, absorption and density functional theory

The electronic structure of double perovskite Pr₂MnNiO₆ was studied using core x-ray photoelectron spectroscopy and x-ray absorption spectroscopy. The 2p x-ray absorption spectra show that Mn and Ni are in 4+  and 2+  states respectively. Based on charge transfer multiplet analysis of the 2p XPS spectra of both ions, we find charge transfer energies [Formula: see text] of 3.5 and 2.5 eV for Ni and Mn respectively. The ground state of Ni²⁺ and Mn⁴⁺ ions reveal a higher d electron count of 8.21 and 3.38 respectively as compared to the ionic values. The partial density of states clearly show a charge transfer character of the system for U  -  J [Formula: see text] 2 eV. The O 1s edge absorption spectra reveal a band gap of 0.9 eV, which is close to the value estimated from analysis of Ni and Mn 2p photoemission and absorption spectra. The combined analysis of nature of spectroscopic data and first principles calculations reveal that the material is a p  -  d type charge transfer insulator with an intermediate covalent character according to the Zannen-Sawatzy-Allen phase diagram.

Synergistic enhancement in photocatalytic performance of Ce (IV) and Cr (III) co-substituted magnetite nanoparticles loaded on reduced graphene oxide sheets

Effective utilization of visible-light by a photocatalyst is of great significance in photocatalytic processes. Herein, magnetite structure was modified by co-incorporation of Ce⁴⁺ and Cr³⁺ cations, and deposition on reduced graphene oxide sheets. The as-prepared Fe_2.8Cr_0.2O₄#rGO and Fe_2.5Cr_0.2Ce_0.3O₄#rGO nanocomposites were characterized by XRD, SEM, X-ray Dot mapping, EDX, BET, DRS, XPS, FT-IR and VSM techniques and assessed for their photocatalytic performance under visible light irradiation for treatment of methylene blue. The results confirmed the central role of the incorporated Cr and Ce in improving the photocatalytic performance of magnetite through enhanced light harvesting, and the role of Ce³⁺/Ce⁴⁺ redox pair, and rGO sheets in extending the life span of photo-induced e^-/h⁺. Moreover, the influence of enhancers and scavengers were evaluated and oxidation path and generated byproducts were estimated. The results established the Fe_2.5Cr_0.2Ce_0.3O₄#rGO nanocomposite as a visible-light-driven photocatalyst for effective degradation of recalcitrant compounds.

Large Charge-Transfer Energy in LiFePO4 Revealed by Full-Multiplet Calculation for the Fe L3 -edge Soft X-ray Emission Spectra

We analyzed the Fe 3d electronic structure in LiFePO₄ /FePO₄ (LFP/FP) nanowire with a high cyclability by using soft X-ray emission spectroscopy (XES) combined with configuration-interaction full-multiplet (CIFM) calculation. The ex situ Fe L_2,3 -edge resonant XES (RXES) spectra for LFP and FP are ascribed to oxidation states of Fe²⁺ and Fe³⁺ , respectively. CIFM calculations for Fe²⁺ and Fe³⁺ states reproduced the Fe L₃ RXES spectra for LFP and FP, respectively. In the calculations for both states, the charge-transfer energy was considerably larger than those for typical iron oxides, indicating very little electron transfer from the O 2p to Fe 3d orbitals and a weak hybridization on the Fe-O bond during the charge-discharge reactions.

Rare-earth nickelates RNiO3: thin films and heterostructures

This review stands in the larger framework of functional materials by focussing on heterostructures of rare-earth nickelates, described by the chemical formula RNiO₃ where R is a trivalent rare-earth R  =  La, Pr, Nd, Sm, …, Lu. Nickelates are characterized by a rich phase diagram of structural and physical properties and serve as a benchmark for the physics of phase transitions in correlated oxides where electron-lattice coupling plays a key role. Much of the recent interest in nickelates concerns heterostructures, that is single layers of thin film, multilayers or superlattices, with the general objective of modulating their physical properties through strain control, confinement or interface effects. We will discuss the extensive studies on nickelate heterostructures as well as outline different approaches to tuning and controlling their physical properties and, finally, review application concepts for future devices.

Charge-Spin Correlation in van der Waals Antiferromagnet NiPS_{3}

Strong charge-spin coupling is found in a layered transition-metal trichalcogenide NiPS_{3}, a van der Waals antiferromagnet, from studies of the electronic structure using several experimental and theoretical tools: spectroscopic ellipsometry, x-ray absorption, photoemission spectroscopy, and density functional calculations. NiPS_{3} displays an anomalous shift in the optical spectral weight at the magnetic ordering temperature, reflecting strong coupling between the electronic and magnetic structures. X-ray absorption, photoemission, and optical spectra support a self-doped ground state in NiPS_{3}. Our work demonstrates that layered transition-metal trichalcogenide magnets are useful candidates for the study of correlated-electron physics in two-dimensional magnetic materials.

Investigation of the multiplet structures and crystal field effects of a TiO6 3d 1 cluster based on configuration interaction calculations

Configuration interaction cluster calculation can effectively reproduce the experimentally measured Ti L 23-edge absorption spectrum for the TiO6 cluster LaTiO3. A further investigation of the hybridization strength and charge-transfer energy effects on the multiplet structures suggests that LaTiO3 should be classified as an intermediate state between the charge-transfer and Mott–Hubbard regimes. Detailed temperature-dependent simulations of absorption spectra support the lifting of Ti t 2g orbital degeneracy and crystal field splitting. The spin–orbit coupling scenario is ruled out, even though 3d spin–orbit coupling can reproduce the experimental spectrum without including temperature. A combined polarization- and crystal-field-splitting-dependent analysis indicates asymmetric ΔCF–orbital interactions for the TiO6 cluster [Ti3+:3d 1(t 2g 1)], different from the orbital–lattice interactions reported for the NiO6 cluster [Ni3+:3d 7(t 2g 6 eg 1)]. The orbital polarization is defined in terms of the normalized electron occupancies in orbitals with xy and xz(yz) symmetries, and nearly complete orbital polarization (more than 75%) is observed, indicating strongly reduced orbital fluctuations due to the correlation effects. This is consistent with the density of states for titanates based on local density approximation plus dynamical mean-field theory calculations.

High resolution x-ray absorption and emission spectroscopy of Li x CoO2 single crystals as a function delithiation

The effect of delithiation in Li _x CoO₂ is studied by high resolution Co K-edge x-ray absorption and x-ray emission spectroscopy. Polarization dependence of the x-ray absorption spectra on single crystal samples is exploited to reveal information on the anisotropic electronic structure. We find that the electronic structure of Li _x CoO₂ is significantly affected by delithiation in which the Co ions oxidation state tending to change from 3+  to 4+. The Co intersite (intrasite) 4p-3d hybridization suffers a decrease (increase) by delithiation. The unoccupied 3d t _2g orbitals with a _1g symmetry, containing substantial O 2p character, hybridize isotropically with Co 4p orbitals and likely to have itinerant character unlike anisotropically hybridized 3d e _g orbitals. Such a peculiar electronic structure could have significant effect on the mobility of Li in Li _x CoO₂ cathode and hence the battery characteristics.

Heteroleptic, two-coordinate [M(NHC){N(SiMe3)2}] (M = Co, Fe) complexes: synthesis, reactivity and magnetism rationalized by an unexpected metal oxidation state

The linear, two-coordinate and isostructural heteroleptic [M(IPr){N(SiMe₃)₂}] (IPr = 1,3-bis(diisopropylphenyl)-imidazol-2-ylidene), formally M^I complexes (M = Co, 3; Fe, 4) were obtained by the reduction of [M(IPr)Cl{N(SiMe₃)₂}] with KC₈, or [Co(IPr){N(SiMe₃)₂}₂] with mes*PH₂, mes* = 2,4,6-tBu₃C₆H₂. The magnetism of 3 and 4 implies Co^II and Fe^II centres coupled to one ligand-delocalized electron, in line with XPS and XANES data; the ac susceptibility of 4 detected a pronounced frequency dependence due to slow magnetization relaxation. Reduction of [Fe(IPr)Cl{N(SiMe₃)₂}] with excess KC₈ in toluene gave the heteronuclear 'inverse-sandwich' Fe-K complex 7, featuring η⁶-toluene sandwiched between one Fe⁰ and one K⁺ centre.

Incommensurate Spiral Order from Double-Exchange Interactions

The double-exchange model describing interactions of itinerant electrons with localized spins is usually used to explain ferromagnetism in metals. We show that for a variety of crystal lattices of different dimensionalities and for a wide range of model parameters, the ferromagnetic state is unstable against a noncollinear spiral magnetic order. We revisit the phase diagram of the double-exchange model on a triangular lattice and show in a large part of the diagram the incommensurate spiral state has a lower energy than the previously discussed commensurate states. These results indicate that double-exchange systems are inherently frustrated and can host unconventional spin orders.

Significance of the electron-density of molecular fragments on the properties of manganese(iii) β-diketonato complexes: an XPS and DFT study

The group electronegativity of the R-groups of the ligand influences the XPS binding energies and the amount of charge transferred in the Mn 2p_3/2 photoelectron lines. DFT studies illustrated different Jahn–Teller elongation bond stretch isomers.

Correlation between the O 2p Orbital and Redox Reaction in LiMn0.6 Fe0.4 PO4 Nanowires Studied by Soft X-ray Absorption

The changes in the electronic structure of LiMn_0.6 Fe_0.4 PO₄ nanowires during discharge processes were investigated by using ex situ soft X-ray absorption spectroscopy. The Fe L-edge X-ray absorption spectrum attributes the potential plateau at 3.45 V versus Li/Li⁺ of the discharge curve to a reduction of Fe³⁺ to Fe²⁺ . The Mn L-edge X-ray absorption spectra exhibit the Mn²⁺ multiplet structure throughout the discharge process, and the crystal-field splitting was slightly enhanced upon full discharge. The configuration-interaction full-multiplet calculation for the X-ray absorption spectra reveals that the charge-transfer effect from O 2p to Mn 3d orbitals should be considerably small, unlike that from the O 2p to Fe 3d orbitals. Instead, the O K-edge X-ray absorption spectrum shows a clear spectral change during the discharge process, suggesting that the hybridization of O 2p orbitals with Fe 3d orbitals contributes essentially to the reduction.

Superconductivity in alkali metal intercalated iron selenides

Alkali metal intercalated iron selenide superconductors A x Fe2-y Se2 (where A  =  K, Rb, Cs, Tl/K, and Tl/Rb) are characterized by several unique properties, which were not revealed in other superconducting materials. The compounds crystallize in overall simple layered structure with FeSe layers intercalated with alkali metal. The structure turned out to be pretty complex as the existing Fe-vacancies order below ~550 K, which further leads to an antiferromagnetic ordering with Néel temperature fairly above room temperature. At even lower temperatures a phase separation is observed. While one of these phases stays magnetic down to the lowest temperatures the second is becoming superconducting below ~30 K. All these effects give rise to complex relationships between the structure, magnetism and superconductivity. In particular the iron vacancy ordering, linked with a long-range magnetic order and a mesoscopic phase separation, is assumed to be an intrinsic property of the system. Since the discovery of superconductivity in those compounds in 2010 they were investigated very extensively. Results of the studies conducted using a variety of experimental techniques and performed during the last five years were published in hundreds of reports. The present paper reviews scientific work concerning methods of synthesis and crystal growth, structural and superconducting properties as well as pressure investigations.

Passivation Layer and Cathodic Redox Reactions in Sodium-Ion Batteries Probed by HAXPES

The cathode material P2-Nax Co2/3 Mn2/9 Ni1/9 O2, which could be used in Na-ion batteries, was investigated through synchrotron-based hard X-ray photoelectron spectroscopy (HAXPES). Nondestructive analysis was made through the electrode/electrolyte interface of the first electrochemical cycle to ensure access to information not only on the active material, but also on the passivation layer formed at the electrode surface and referred to as the solid permeable interface (SPI). This investigation clearly shows the role of the SPI and the complexity of the redox reactions. Cobalt, nickel, and manganese are all electrochemically active upon cycling between 4.5 and 2.0 V; all are in the 4+ state at the end of charging. Reduction to Co(3+), Ni(3+), and Mn(3+) occurs upon discharging and, at low potential, there is partial reversible reduction to Co(2+) and Ni(2+). A thin layer of Na2 CO3 and NaF covers the pristine electrode and reversible dissolution/reformation of these compounds is observed during the first cycle. The salt degradation products in the SPI show a dependence on potential. Phosphates mainly form at the end of the charging cycle (4.5 V), whereas fluorophosphates are produced at the end of discharging (2.0 V).

Dual-doping to suppress cracking in spinel LiMn2O4: a joint theoretical and experimental study

Bonding Layout (purple line) of Mn³⁺ (green balls) results in different unit cells (blue box), correlating to distinct cracking tendency in long-term charging-discharging cycling.

Tuning the electronic structures and magnetism of two-dimensional porous C2N via transition metal embedding

Based on first-principles calculations, the electronic structures and magnetism are investigated in 3d transition metal (TM)-embedded porous two-dimensional (2D) C₂N monolayers.

Tuning electronic structures of the stanene monolayer via defects and transition-metal-embedding: spin–orbit coupling

The magnetic moments of a transition metal embedded into single and double vacancies of stanene nanosheets.