X-ray Absorption Spectroscopy Spectra Research Articles

Hybridization of CaFe1−xCoxAsF (x = 0, 0.06, 0.11, 0.144) studied by x-ray absorption spectroscopy and ultraviolet photoemission spectroscopy

The parent compound CaFeAsF undergoes a structural phase transition from tetragonal to orthorhombic at T ∼ 121 K without superconductivity. The superconductivity emerged when the Fe sites are partially substituted by Co with a doping level of above 0.06 and the superconducting transition temperature is raised to 22 K in CaFe0.89Co0.11AsF. In this paper, the electronic structures of CaFe1−xCoxAsF (x = 0, 0.06, 0.11, 0.144) samples were studied by x-ray absorption spectroscopy (XAS). With the doping of Co, the Fe L3 XAS spectra shift to lower photon energy. Combined with the valence band spectra study, it is indicated that the strength of the hybridization between Fe 3d and As 4p is enhanced together with the increment of the superconducting transition temperature, after Co doping. The results suggest the enhancement of the hybridization between Fe 3d and As 4p plays an important role in the evolution of the superconductivity in Fe-based superconductors.

Bulk and surface electronic structure of MnPSe3 revealed by photoemission and x-ray absorption spectroscopy

Bulk and surface electronic states of ${\mathrm{MnPSe}}_{3}$ have been investigated by means of x-ray photoemission spectroscopy (XPS), x-ray absorption spectroscopy (XAS), and theoretical calculations. In Mn $2p$ XPS, the main peak is accompanied by a charge-transfer satellite which is commonly observed in various Mn chalcogenides. The intensity of the charge-transfer satellite is considerably reduced by oxidization of the surface. The multiplet structure of Mn $2p$ XAS indicates the high spin ${\mathrm{Mn}}^{2+}$ configuration. The cluster model analyses of the XPS and XAS spectra indicate that the Se $4p$-to-Mn $3d$ charge-transfer energy $\mathrm{\ensuremath{\Delta}}$ is negative providing the strong $p\ensuremath{-}d$ hybridization. The magnetic interaction between the third nearest neighboring Mn spins is enhanced by the smallness of $\mathrm{\ensuremath{\Delta}}$ and the unique molecular orbitals of the ${\mathrm{P}}_{2}{\mathrm{Se}}_{6}$ cluster.

Cr doping-induced ferromagnetism in the spin-glass Cd1−xMnxTe studied by x-ray magnetic circular dichroism

The prototypical diluted magnetic semiconductor Cd1−xMnxTe is a spin glass (x < 0.6) or an antiferromagnet (x > 0.6) but becomes ferromagnetic upon doping with a small amount of Cr atoms. To investigate the origin of the ferromagnetism in Cd1−x−yMnxCryTe, we have studied its element specific magnetic properties by x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD). Measured XAS and XMCD spectra indicate that both Cr and Mn atoms are divalent and that the magnetic moments of Cr and Mn are aligned parallel. The magnetization of Mn increases with increasing Cr content, suggesting that ferromagnetic interaction exists between neighboring Mn and Cr ions despite the largely antiferromagnetic interaction between Mn atoms. By analyzing the element-specific magnetization curves, we conclude that Cr doping leads to the formation of ferromagnetic or superparamagnetic clusters consisting of several Cr ions surrounded by a much larger number of Mn ions.

As-Se Pentagonal Linkers to Induce Chirality and Polarity in Mixed-Valent Fe-Se Tetrahedral Chains Resulting in Hidden Magnetic Ordering.

A novel mixed-valent hybrid chiral and polar compound, Fe7As3Se12(en)6(H2O), has been synthesized by a single-step solvothermal method. The crystal structure consists of 1D [Fe5Se9] chains connected via [As3Se2]-Se pentagonal linkers and charge-balancing interstitial [Fe(en)3]2+ complexes (en = ethylenediamine). Neutron powder diffraction verified that interstitial water molecules participate in the crystal packing. Magnetic polarizability of the produced compound was confirmed by X-ray magnetic circular dichroism (XMCD) spectroscopy. X-ray absorption spectroscopy (XAS) and 57Fe Mössbauer spectroscopy showed the presence of mixed-valent Fe2+/Fe3+ in the Fe-Se chains. Magnetic susceptibility measurements reveal strong antiferromagnetic nearest neighbor interactions within the chains with no apparent magnetic ordering down to 2 K. Hidden short-range magnetic ordering below 70 K was found by 57Fe Mössbauer spectroscopy, showing that a fraction of the Fe3+/Fe2+ in the chains are magnetically ordered. Nevertheless, complete magnetic ordering is not achieved even at 6 K. Analysis of XAS spectra demonstrates that the fraction of Fe3+ in the chain increases with decreasing temperature. Computational analysis points out several competing ferrimagnetic ordered models within a single chain. This competition, together with variation in the Fe oxidation state and additional weak intrachain interactions, is hypothesized to prevent long-range magnetic ordering.

Local Coordination and Electronic Structure Ramifications of Guest-Dependent Spin Crossover in a Metal-Organic Framework: A Combined X-ray Absorption and Emission Spectroscopy Study.

The porous Hoffman-type 3D lattice Fe(pz)[NiII(CN)4] exhibits thermally induced spin-crossover (SCO) behavior that is dependent on the solvent guest species occupying the pores. Here, in situ Fe K-edge X-ray absorption spectroscopy (XAS) and both non-resonant and resonant Kβ X-ray emission spectroscopy (XES) methods are used to probe this framework under two solvent environments that yield different extremes of spin crossover temperature: acetonitrile and toluene. While the acetonitrile pore environment engenders an SCO response around room temperature, toluene guests stabilize the high spin state and effectively suppress SCO behavior throughout the ambient temperature range. The multipronged X-ray spectroscopy approach simultaneously confirmed this spin crossover behavior and provided new local coordination and electronic structural insights of the framework under these two solvent environments. Extended X-ray absorption fine structure analysis revealed spin state and solvent guest-dependent differences in coordination bond lengths and structural disorder. Resonant XES measurements produced high-resolution XAS spectra with distinct pre-edge and edge features, whose assignment was established using both simple ligand field theory and time-dependent density-functional theory calculations and further supported by their observed resonance behavior in the 2D RXES plane. Edge feature variation with the Fe spin state was interpreted to reveal changes in specific metal-linker bond covalency.

Identifying Key Descriptors for the Single-Atom Catalyzed CO Oxidation

Identifying Key Descriptors for the Single-Atom Catalyzed CO Oxidation

Manipulating Spin Polarization of Defected Co3O4 for Highly Efficient Electrocatalysis

Electrocatalytic water splitting is limited by kinetics-sluggish oxygen evolution, in which the activity of catalysts depends on their electronic structure. However, the influence of electron spin polarization on catalytic activity is ambiguous. Herein, we successfully regulate the spin polarization of Co3O4 catalysts by tuning the concentration of cobalt defects from 0.8 to 14.5%. X-ray absorption spectroscopy spectra and density functional theory calculations confirm that the spin polarization of Co3O4 is positively correlated with the concentration of cobalt defects. Importantly, the enhanced spin polarization can increase hydroxyl group absorption to significantly decrease the Gibbs free energy change value of the OER rate-determining step and regulate the spin polarization of oxygen species through a spin electron-exchange process to easily produce triplet-state O2, which can obviously increase electrocatalytic OER activity. In specific, Co3O4-50 with 14.5% cobalt defects exhibits the highest spin polarization and shows the best normalized OER activity. This work provides an important strategy to increase the water splitting activity of electrocatalysts via the rational regulation of electron spin polarization.

Intervalence charge transfer and charge transport in the spinel ferrite ferromagnetic semiconductor Ru-doped CoFe2O4

Inter-valence charge transfer (IVCT) is electron transfer between two metal $M$ sites differing in oxidation states through a bridging ligand: $M^{n+1} + M'^{m} \rightarrow M^{n} + M'^{m+1}$. It is considered that IVCT is related to the hopping probability of electron (or the electron mobility) in solids. Since controlling the conductivity of ferromagnetic semiconductors (FMSs) is a key subject for the development of spintronic device applications, the manipulation of the conductivity through IVCT may become a new approach of band engineering in FMSs. In Ru-doped cobalt ferrite CoFe$_2$O$_4$ (CFO) that shows ferrimagnetism and semiconducting transport properties, the reduction of the electric resistivity is attributed to both the carrier doping caused by the Ru substitution for Co and the increase of the carrier mobility due to hybridization between the wide Ru $4d$ and the Fe $3d$ orbitals. The latter is the so-called IVCT mechanism that is charge transfer between the mixed valence Fe$^{2+}$/Fe$^{3+}$ states facilitated by bridging Ru $4d$ orbital: Fe$^{2+}$ + Ru$^{4+}$ $\leftrightarrow$ Fe$^{3+}$ + Ru$^{3+}$. To elucidate the emergence of the IVCT state, we have conducted x-ray absorption spectroscopy (XAS) and resonant photoemission spectroscopy (RPES) measurements on non-doped CFO and Co$_{0.5}$Ru$_{0.5}$Fe$_2$O$_4$ (CRFO) thin films. The observations of the XAS and RPES spectra indicate that the presence of the mixed valence Fe$^{2+}$/Fe$^{3+}$ state and the hybridization between the Fe $3d$ and Ru $4d$ states in the valence band. These results provide experimental evidence for the IVCT state in CRFO, demonstrating a novel mechanism that controls the electron mobility through hybridization between the $3d$ transition-metal cations with intervening $4d$ states.

Origin of Magnetization in Silica-coated Fe3O4 Nanoparticles Revealed by Soft X-ray Magnetic Circular Dichroism

Magnetite (Fe3O4) nanoparticles (NPs) and SiO2-coated Fe3O4 nanoparticles have successfully been synthesized using co-precipitation and modified Stöber methods, respectively. The samples were characterized using X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, high-resolution transmission electron microscopy (HRTEM), vibrating sample magnetometer (VSM) techniques, X-ray absorption spectroscopy (XAS), and X-ray magnetic circular dichroism (XMCD). XRD and FTIR data confirmed the structural configuration of a single-phase Fe3O4 and the successful formation of SiO2-coated Fe3O4 NPs. XRD also confirmed that we have succeeded to synthesize nano-meter size of Fe3O4 NPs. HRTEM images showed the increasing thickness of SiO2-coated Fe3O4 with the addition of the Tetraethyl Orthosilicate (TEOS). Room temperature VSM analysis showed the magnetic behaviour of Fe3O4 and its variations that occurred after SiO2 coating. The magnetic behaviour is further authenticated by XAS spectra analysis which cleared about the existence of SiO2 shells that have transformed the crystal as well as the local structures of the magnetite NPs. We have performed XMCD measurements, which is a powerful element-specific technique to find out the origin of magnetization in SiO2-coated Fe3O4 NPs, that verified a decrease in magnetization with increasing thickness of the SiO2 coating.Graphical Magnetite (Fe3O4) nanoparticles (NPs) and SiO2-coated Fe3O4 nanoparticles have successfully been synthesized using co-precipitation and modified Stöber methods, respectively. The samples were characterized using X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, high-resolution transmission electron microscopy (HRTEM), vibrating sample magnetometer (VSM) techniques, X-ray absorption spectroscopy (XAS), and X-ray magnetic circular dichroism (XMCD). XRD and FTIR data confirmed the structural configuration of a single-phase Fe3O4 and the successful formation of SiO2-coated Fe3O4 NPs. XRD also confirmed that we have succeeded to synthesize nano-meter size of Fe3O4 NPs. HRTEM images showed the increasing thickness of SiO2-coated Fe3O4 with the addition of the Tetraethyl Orthosilicate (TEOS). Room temperature VSM analysis showed the magnetic behaviour of Fe3O4 and its variations that occurred after SiO2 coating. The magnetic behaviour is further authenticated by XAS spectra analysis which cleared about the existence of SiO2 shells that have transformed the crystal as well as the local structures of the magnetite NPs. We have performed XMCD measurements, which is a powerful element-specific technique to find out the origin of magnetization in SiO2-coated Fe3O4 NPs, that verified a decrease in magnetization with increasing thickness of the SiO2 coating.

Uncovering temperature-tempted coordination of inclusions within ultra-high-strength-steel via in-situ spectro-microscopy

Despite the common challenge of investigating non-metallic inclusions within ultra-high-strength-steel (UHSS) at sub-micrometer scale via conventional methods, probing nitride inclusions at elevated temperatures is vital for guiding steel’ performance. Herein, an in-situ spectro-microscopic determination using advanced Synchrotron X-ray absorption spectroscopy (XAS) coupled with photoelectron emission microscopy (PEEM) is employed to explore the local structure and electronic properties of selective h-boron nitride (h-BN) containing inclusions (A1 and A2) embedded within steel matrix. While the variation in the relative intensity of π∗/σ∗ excitonic peaks at spatially different locations refers to the polarization and or thickness effects. Several minute features observed in the 192–195 eV energy range show oxygen (O) substituted nitrogen (N) defects (ON,2N,3N), which are more dominant in A2 inclusion. The observed dominance further explains the relatively high intense π∗ peak in A2 due to increased localization. Weak shoulder on the left side of π∗ peak in both room and high-temperature XAS spectra is ascribed to the interaction between h-BN and the local environment, such as Ca-based inclusion or steel matrix. Defects are commonly found in h-BN, and precise identification of the same is vital as they affect the overall physical, chemical, and mechanical properties. Moreover, significant changes in high-temperature B K-edge XAS spectra, such as relative intensity of π∗/σ∗ excitonic peaks at the same location and reduced intensity of defects, suggest the adjusting nature of BN inclusion, complicating their precise prediction and control towards clean steel production.

Hydrogen Adsorption/Desorption Isotherms on Supported Platinum Nanoparticles Determined by in‐situ XAS and ΔXANES Analysis

AbstractHydrogen atoms on supported metal nanoparticles (NPs) play a significant role in heterogeneous catalysis. Despite the extensive previous research, however, the adsorption of hydrogen onto practical supported NP catalysts is still unclear especially in the atmospheric pressure conditions. In this study, adsorption/desorption isotherms of hydrogen on Pt NPs supported on alumina (Al2O3) were obtained in gas‐flow conditions using the analysis of the difference spectra of Pt LIII‐edge X‐ray absorption spectroscopy (XAS) at 303 K. From the analysis of extended X‐ray absorption fine structure (EXAFS), a positive linear relationship was observed between the hydrogen coverage and Pt−Pt bond distance. Additionally, the re‐adsorption experiment of hydrogen after the first adsorption/desorption cycle and the transient change of dispersive XAS spectra in the hydrogen adsorption suggests that the hydrogen adsorption induces the surface reconstruction of Pt NPs on Al2O3 at around room temperature and atmospheric pressure.

X-ray absorption spectra of aqueous cellobiose: Experiment and theory

The hydration structure of cellulose is very important for understanding the hydrolysis of cellulose at the molecular level. In this paper, we report a joint experimental and theoretical study on x-ray absorption spectroscopy (XAS) of aqueous cellobiose, a disaccharide unit of cellulose. In the experimental part, high resolution measurements of the carbon K-edge XAS spectra were taken. In the theoretical part, ab initio molecular dynamics simulations and ensemble calculations of electronic excited states were performed to obtain the continuous XAS spectra. The XAS spectra were found to have three characteristic peaks at 289.3, 290.7, and 293.6eV, each representing the absorption by carbon atoms of the alcohol group, the hemiacetal group, and both of these functional groups. It was found that the peak heights in the spectrum change considerably over the temperature range of 25-60 °C, which is a reflection of the number of hydrogen bonds between cellobiose and water. We suggest that this spectral change could be useful information for identifying the hydration of cellulose in various environments.

Orbital localization and the role of the Fe and As 4p orbitals in BaFe2As2 probed by XANES

The polarization dependence of the near edge x-ray absorption spectroscopy (XANES) is an element specific probe to the real-space distribution of the density of unoccupied states in solid-state materials. In this paper, we present Fe and As $K$-edge experiments of Ba(Fe$_{1-x}$$M_{x}$)$_{2}$As$_{2}$ ($M=$ Mn, Co and $x=0.0$ and $0.08$). The experiments reveal a strong polarization dependence of the probed XANES spectra, which concerns mainly an increase of the intensity of electronic transitions when the beam polarization is set out of the sample's $ab$ crystallographic plane. The results show that states with $p_{z}$-orbital character dominate the density of unoccupied states close to the Fermi level. Partial substitution of Fe by Co is shown to decrease the intensity anisotropy, suggesting that Co promotes electronic transfer preferentially to states with $p_{z}$-orbital character. On the other hand, Mn substitution causes the increase of the spectra $p_{z}$-orbital anisotropy, which is proposed to take place by means of an enhanced local Fe $3d4p$ mixing, unveiling the role of Fe $4p$ states in the localization of the Fe $3d$ orbitals. Moreover, by comparing our results to previous experiments, we identify the relative mixing between Fe and the pnictide $4p_{x,y,z}$ orbitals as a clear divide between the electronic properties of iron arsenides and selenides. Our conclusions are supported by multiple-scattering theory calculations of the XANES spectra and by quantum chemistry calculations of Fe coordination electronic structure.

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 5T2g excited state compared to [Fe(btbip)2]2+, providing an explanation for the absence of a photoinduced 5T2g 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.

Covalency between the uranyl ion and dithiophosphinate by sulfur K-edge X-ray absorption spectroscopy and density functional theory.

The dithiophosphinic acids (HS2PR2) have been used for the selective separation of trivalent actinides (AnIII) from lanthanides (LnIII) over the past decades. The substituents on the dithiophosphinic acids dramatically impact the separation performance, but the mechanism is still open for debate. In this work, two dithiophosphinic acids with significantly different AnIII/LnIII separation performance, i.e. diphenyl dithiophosphinic acid (HS2PPh2) and bis(ortho-trifluoromethylphenyl) dithiophosphinic acid [HS2P(o-CF3C6H4)2], are employed to understand the substituent effect on the bonding covalency between the S2PR2- anions (R = Ph and o-CF3C6H4) and the uranyl ion by sulfur K-edge X-ray absorption spectroscopy (XAS) in combination with density functional theory calculations. The two UO2(S2PR2)(EtOH) complexes display similar XAS spectra, in which the first pre-edge feature with an intensity of 0.16 is entirely attributed to the transitions from S 1s orbitals to the unoccupied molecular orbitals due to the mixing between U 5f and S 3p orbitals. The Mulliken population analysis indicates that the amount of \% S 3p character inthese orbitals is essentially identical for the UO2(S2PPh2)2(EtOH) and UO2[S2P(o-CF3C6H4)2]2(EtOH) complexes, which is lower than that in the U 6d-based orbitals. The essentially identical covalency in U-S bonds for the two UO2(S2PR2)2(EtOH) complexes are contradictory to the significantly different AnIII/LnIII separation performance of the two dithiophosphinic acids, thus the covalency seems to be unable to account for substituent effects in the AnIII/LnIII separation by the dithiophosphinic acids. The results in this work provide valuable insight into the understanding of the mechanism in the AnIII/LnIII separation by the dithiophosphinic acids.

Fe(II) Induced Reduction of Incorporated U(VI) to U(V) in Goethite.

Over 60 years of nuclear activities have resulted in a global legacy of radioactive wastes, with uranium considered a key radionuclide in both disposal and contaminated land scenarios. With the understanding that U has been incorporated into a range of iron (oxyhydr)oxides, these minerals may be considered a secondary barrier to the migration of radionuclides in the environment. However, the long-term stability of U-incorporated iron (oxyhydr)oxides is largely unknown, with the end-fate of incorporated species potentially impacted by biogeochemical processes. In particular, studies show that significant electron transfer may occur between stable iron (oxyhydr)oxides such as goethite and adsorbed Fe(II). These interactions can also induce varying degrees of iron (oxyhydr)oxide recrystallization (<4% to >90%). Here, the fate of U(VI)-incorporated goethite during exposure to Fe(II) was investigated using geochemical analysis and X-ray absorption spectroscopy (XAS). Analysis of XAS spectra revealed that incorporated U(VI) was reduced to U(V) as the reaction with Fe(II) progressed, with minimal recrystallization (approximately 2%) of the goethite phase. These results therefore indicate that U may remain incorporated within goethite as U(V) even under iron-reducing conditions. This develops the concept of iron (oxyhydr)oxides acting as a secondary barrier to radionuclide migration in the environment.

Electrochemical performance of manganese hexacyanoferrate cathode material in aqueous Zn-ion battery

Manganese hexacyanoferrate (MnHCF) has attracted much attention as promising cathode material for Li and Na ion batteries, owning to its low cost, environmental friendliness, high specific capacity and voltage plateau. Here, the electrochemical performance and electronic structure information of MnHCF were studied in aqueous Zn-ion batteries (ZIBs). Based on the cyclic voltammetry and galvanostatic charge/discharge results, an activation of Fe-sites during beginning cycles was observed, and the capacity contribution of Fe-sites increases from 30 to 86% at C/20 during the first 10 cycles. The local geometric and electronic structure information of MnHCF was investigated by X-ray absorption spectroscopy (XAS) in a set of ex-situ electrodes. From Fe K-edge spectra, it shows a consistent oxidation and reduced state in charged and discharged electrodes, and this indicates that there is no apparent change for the local Fe-sites environment. However, the XAS spectra of Mn K-edge show apparent change after 10 cycles. Compared to the rhombohedral phase of Zinc hexacyanoferrate (ZnHCF), a -Zn-CN-Fe- structural framework was detected in the cycled MnHCF samples, and this indicates that a part of Zn replaced Mn-sites, because of the dissolution of the Mn-sites. The gradual activation of Fe-sites at the beginning cycles can be attributed to the alleviation spatial resistance with the dissolution of Mn-sites, and the replacement of Zn for Mn explains the decreasing capacity during cycling.

Uniting Experiment and Computational Modeling to Elucidate Structure-Property Relationships of Water Splitting Materials

The layered perovskite BaCe0.25Mn0.75O3 (BCM) is a proven solar thermochemical water splitting material demonstrating higher hydrogen generation and improved redox kinetics relative to current state of the art materials. However, fundamental understanding of the underlying mechanisms of BCM’s efficacy is necessary for solar thermochemical water splitting in related perovskite-type materials to become a viable pathway for green hydrogen generation. To this end, a family of BXM (X = Ce, Nb, Pr) materials has been synthesized and undergone in-depth, multi-faceted characterization. Despite the three materials having identical structures, and differing only in 1/8 of their cation species, their water splitting performances are markedly different. X-ray absorption spectroscopy (XAS) analysis reveals substantial differences in the electronic structure of oxygen between the three samples, both when fully oxidized and at quantified degrees of reduction. However, interpreting the changes in XAS spectra of these complex materials via conventional means has proven to be challenging. The Molecular Foundry has successfully simulated the XAS spectra of the oxidized and defected structures allowing specific peaks in the XAS spectra to be correlated to molecular orbitals and providing invaluable insights into the underlying electronic changes observed experimentally. Additionally, both in situ and ex situ electron energy loss spectroscopy (EELS) analysis of the BXM materials allowed changes in the O K-edge and the Mn L-edge to be monitored with increasing extent of reduction and directly compared to XAS. The unique advantage of performing STEM-EELS analysis of these materials during in situ thermal reduction is the ability to observe underlying structural modifications, such as phase changes or defect formations, in tandem with the electronic changes in the material. Taken together, the results reported here provide a key component towards elucidating structure-property relationships for this class of solar thermochemical hydrogen production (STCH) materials.

Ground electronic state description of thiourea coordination in homoleptic Zn2+, Ni2+ and Co2+ complexes using sulfur K-edge X-ray absorption spectroscopy.

Sulfur K-edge X-ray absorption spectroscopy (XAS) was employed to experimentally characterize the coordinative bond between the thiourea (TU) or thiocarbamide ligand and transition metal (TM) ions Zn2+, Co2+ and Ni2+ in distorted tetrahedral and octahedral homoleptic coordination environments. Comparisons of XAS spectra of the free TU ligand and [Zn(TU)4]2+, [Co(TU)4]2+ and [Ni(TU)6]2+ complexes clearly identify spectral features unique to TM2+-S(TU) bonding. Quantitative analysis of pre-edge intensities describes the covalency of Ni2+-S(TU) and Co2+-S(TU) bonding to be at most 21% and 9% as expressed by the S 3p contributions per TM 3d electron hole. Using relevant Ni2+ complexes with dithiocarbamate and thioether ligands, we evaluated the empirical S 1s → 3p transition dipole integrals developed for S-donor ligands and their dependence on heteroatom substitutions. With the aid of density functional theory-based ground electronic state calculations, we found evidence for the need of using a transition dipole that is dependent on the presence of conjugated heteroatom (N) substitution in these S-donor ligands.

Enhanced orbital anisotropy through the proximity to a SrTiO3 layer in the perovskite iridate superlattices

We have used angle-dependent soft x-ray absorption spectroscopy (XAS) at the O $K$ edge and first-principles calculations to investigate the electronic structures of iridate-based superlattices ${(\mathrm{SrIr}{\mathrm{O}}_{3})}_{m}/(\mathrm{SrTi}{\mathrm{O}}_{3})$ ($m=1$, 2, 3, and \ensuremath{\infty}). We focus on the pre-edge Ir $5d {t}_{2g}--\mathrm{O} 2p$ orbital hybridization feature in the XAS spectra. By varying the measurement geometry relative to the incident photon polarization, we are able to extract the dichroic contrast and observe the systematic increase in the anisotropy of Ir $5d$ orbitals as $m$ decreases. First-principles calculations elucidate the orbital anisotropy coming mainly from the enhanced out-of-plane compression of $\mathrm{Ir}{\mathrm{O}}_{6}$ octahedra in the $\mathrm{SrIr}{\mathrm{O}}_{3}$ layers that are adjacent to the inserted $\mathrm{SrTi}{\mathrm{O}}_{3}$ layers. As $m$ decreases, the increased volume fraction of these interfacial $\mathrm{SrIr}{\mathrm{O}}_{3}$ layers and their contact with the $\mathrm{SrTi}{\mathrm{O}}_{3}$ layers within the ${(\mathrm{SrIr}{\mathrm{O}}_{3})}_{m}/(\mathrm{SrTi}{\mathrm{O}}_{3})$ supercell lead to enhanced orbital anisotropy. Furthermore, the tilt and rotation of $\mathrm{Ir}{\mathrm{O}}_{6}$ octahedra are shown to be essential to understand the subtle orbital anisotropy in these superlattices, and constraining these degrees of freedom will give an incorrect trend. Our results demonstrate that the structural constraint from the inserted $\mathrm{SrTi}{\mathrm{O}}_{3}$ layers, in addition to other electronic means such as polar interface and charge transfer, can serve as a knob to control the orbital degree of freedom in these iridate-based superlattices.