X-ray Absorption Spectra Spectra Research Articles

Are 4f-Orbitals Engaged in Covalent Bonding Between Lanthanides and Triphenylphosphine Oxide? An Oxygen K-Edge X-ray Absorption Spectroscopy and Density Functional Theory Study.

The bonding covalency between trivalent lanthanides (Ln = La, Pr, Nd, Eu, Gd) and triphenylphosphine oxide (TPPO) is studied by X-ray absorption spectra (XAS) and density functional theory (DFT) calculations on the LnCl3(TPPO)3 complexes. The O, P, and Cl K-edge XAS for the single crystals of LnCl3(TPPO)3 were collected, and the spectra were interpreted based on DFT calculations. The O and P K-edge XAS spectra showed no significant change across the Ln series in the LnCl3(TPPO)3 complexes, unlike the Cl K-edge XAS spectra. The experimental O K-edge XAS spectra suggest no mixing between the Ln 4f- and the O 2p-orbitals in the LnCl3(TPPO)3 complexes. DFT calculations indicate that the amount of the O 2p character per Ln-O bond is less than 0.1% in the Ln 4f-based orbitals in all of the LnCl3(TPPO)3 complexes. The experimental spectra and theoretical calculations demonstrate that Ln 4f-orbitals are not engaged in the covalent bonding of lanthanides with TPPO, which contrasts the involvement of U 5f-orbitals in covalent bonding in the UO2Cl2(TPPO)2 complex. Results in this work reinforce our previous speculation that bonding covalency is potentially responsible for the extractability of monodentate organophosphorus ligands toward metal ions.

Near-surface electronic structure in strained Ni-ferrite films: An x-ray absorption spectroscopy study

We report on the x-ray absorption spectra (XAS) and x-ray magnetic circular dichroism (XMCD) of a series of NiFe2O4 (Ni ferrite) films grown on symmetry matched substrates and measured in two geometries: out-of-plane and near in-plane. The Ni ferrite films, grown by pulsed laser deposition, are epitaxial and the substrates used (ZnGa2O4, CoGa2O4, MgGa2O4, and MgAl2O4) introduce a systematic variation in the lattice mismatch between the substrate and the film. Modeling of the XAS and XMCD spectra, both measured with the surface sensitive total electron yield mode, indicates that the Ni2+ cations reside on the octahedrally coordinated lattice sites in the spinel structure. Analyses of the Fe XAS and XMCD spectra are consistent with Fe3+ cations occupying a subset of the octahedral and tetrahedral sites in the spinel oxide lattice with the addition of a small amount of Fe2+ located on octahedral sites. The Ni2+ orbital to spin moment ratio (μℓ/μs), derived from the application of XMCD sum rules, is enhanced for the substrates with a small lattice mismatch relative to NiFe2O4. The results suggest a path for increasing the orbital moment in NiFe2O4 by applying thin film growth techniques that can maintain a highly strained lattice for the NiFe2O4 film.

Solvent Effects and pH Dependence of the X-ray Absorption Spectra of Proline from Electrostatic Embedding Quantum Mechanics/Molecular Mechanics and Mixed-Reference Spin-Flip Time-dependent Density-Functional Theory.

The accurate description of solvent effects on X-ray absorption spectra (XAS) is fundamental for comparing the simulated spectra with experiments in solution. Currently, few protocols exist that can efficiently reproduce the effects of the solute/solvent interactions on XAS. Here, we develop an efficient and accurate theoretical protocol for simulating the solvent effects on XAS. The protocol combines electrostatic embedding QM/MM based on electrostatic potential fitted operators for describing the solute/solvent interactions and mixed-reference spin-flip time-dependent density functional theory (MRSF-TDDFT) for simulating accurate XAS spectra. To demonstrate the capabilities of our protocol, we compute the X-ray absorption of neutral proline in the gas phase and ionic proline in water in all relevant K-edges, showing excellent agreement with experiments. We show that states represented by core to π* transitions are almost unaffected by the interaction with water, whereas the core to σ* transitions are more impacted by the fluctuation of proline structure and the electrostatic interaction with the solvent. Finally, we reconstruct the pH-dependent XAS of proline in solution, determining that the N K-edge can be used to distinguish its three protonation states.

Resonant inelastic x-ray scattering in Ca3LiOsO6 from first-principles

We have investigated the electronic and magnetic properties of the transition metal oxide (TMO) Ca3LiOsO6 within the density functional theory (DFT) using the generalized gradient approximation while taking into account strong Coulomb correlations (GGA+U) in the framework of the fully relativistic spin-polarized Dirac linear muffin-tin orbital band-structure method. Ca3LiOsO6 is an insulator and forms the K4CdCl6-type crystal structure with relatively high Néel temperature TN = 117 K. Despite of the large strength of spin–orbit coupling, it has only a small effect on the electronic and magnetic properties of Ca3LiOsO6. We have also investigated theoretically the x-ray absorption spectra (XAS), x-ray magnetic circular dichroism (XMCD) and resonant inelastic x-ray scattering (RIXS) spectra at the Os L3 edge. The experimentally measured RIXS spectrum of Ca3LiOsO6 in addition to the elastic scattering peak at 0 eV possesses a sharp feature below 2 eV corresponding to transitions within the Os t2g levels. The excitation located at 4.5 eV is due to t2g→eg transitions. The third wide structure situated at 6–8 eV appears due to ligand-to-metal charge transfer excitations: d−d transitions to the Os t2g manifold from the Os 5dO states derived from the ”tails” of oxygen 2p states. The high energy structure at 10–11.5 eV is due to Os 5dO→eg interband transitions. At higher energy above 11 eV transitions from the Os 5dO states to the empty Ca 5d states are located. We have investigated the effect of SO coupling, Coulomb correlations and core-hole effect on the energy band structure, XAS, XMCD, and RIXS spectra in Ca3LiOsO6.

Highly Dispersive Metal Atoms Anchored on Carbon Matrix Obtained by Direct Rapid Pyrolysis of Metal Complexes

Highly Dispersive Metal Atoms Anchored on Carbon Matrix Obtained by Direct Rapid Pyrolysis of Metal Complexes

Eg[formula omitted]t2g Sub band splitting via crystal field and band anticrossing interaction in NixCd1-xO thin films

In the present work, full composition range of NixCd1-xO thin films, from Cadmium oxide to Nickel oxide are prepared with sol-gel chemical synthesis route on corning glass substrates. The Fourier transform infrared spectra provide a direct indication of compositional dependence for different vibrational modes with increasing Ni doping. The microscopy images reflect the reduced crystallinity and closely packed morphology with increasing Ni doping which endorses the changes reflected in electronic structure. The O K edge, soft X-ray absorption spectra (XAS) for thin films indicate a clear evidence of eg and t2g sub band splitting and has been explained via crystal field splitting and three level band anti-crossing (BAC) interaction phenomena. At higher doping fraction, the inhomogeneous electric field of neighboring ions surrounding the unfilled Ni 3d level results in rupturing of L−S coupling and further crystal field splitting. The BAC interaction brings about partially unoccupied localized Ni 3d and empty d acceptor levels above conduction band minima (CBM). As saturated Fermi level goes below CBM with increasing Ni doping, transitions to antibonding orbital [eg(π*) and t2g(σ*)] with finite energy difference, is reflected as separated eg-t2g peaks in XAS spectra. On the other hand, extended X-ray absorption fine structure for Ni K edge fitting provides a complete information for nearest (Ni-O) and next nearest (Ni-Ni) neighbor bond length.

Spin-spiral state of a Mn monolayer on W(110) studied by soft x-ray absorption spectroscopy at variable temperature

The noncollinear magnetic state of epitaxial Mn monolayers on tungsten (110) crystal surfaces is investigated by means of soft x-ray absorption spectroscopy, to complement earlier spin-polarized STM experiments. X-ray absorption spectra (XAS), x-ray linear dichroism (XLD) and x-ray magnetic circular dichroism (XMCD) Mn L23-edge spectra were measured in the temperature range from 8 to 300 K and compared to results of fully-relativistic ab initio calculations. We show that antiferromagnetic (AFM) helical and cycloidal spirals give rise to significantly different Mn L23-edge XLD signals, enabling thus to distinguish between them. It follows from our results that the magnetic ground state of a Mn monolayer on W(110) is an AFM cycloidal spin spiral. Based on temperature-dependent XAS, XLD and field-induced XMCD spectra we deduce that magnetic properties of the Mn monolayer on W(110) vary with temperature, but this variation lacks a clear indication of a phase transition in the investigated temperature range up to 300 K - even though a crossover exists around 170 K in the temperature dependence of XAS branching ratios and in XLD profiles.

Perpendicular and in-plane hole asymmetry in a strained NiFe2O4 film

Strained materials can exhibit drastically modified physical properties in comparison to their fully relaxed analogues. We report on the x-ray absorption spectra (XAS) and magnetic circular dichroism (XMCD) of a strained NiFe2O4 inverse spinel film grown on a symmetry matched single crystal MgGa2O4 substrate. The Ni XAS spectra exhibit a sizable difference in the white line intensity for measurements with the x-ray electric field parallel to the film plane (normal incidence) vs when the electric field is at an angle (off-normal). A considerable difference is also observed in the Fe L 2,3 XMCD spectrum. Modeling of the XAS and XMCD spectra indicate that the modified energy ordering of the cation 3d states in the strained film leads to a preferential filling of 3d states with out-of-plane character. In addition, the results point to the utility of x-ray spectroscopy in identifying orbital populations even with elliptically polarized x-rays.

Electronic structure and x-ray magnetic circular dichroism in ferroelectric CaMnTi2O6

We study the electronic and magnetic properties of ferroelectric ${\mathrm{CaMnTi}}_{2}{\mathrm{O}}_{6}$ within density functional theory using the generalized gradient approximation (GGA) with the consideration of strong Coulomb correlations $(\mathrm{GGA}+U)$ in the framework of the fully relativistic spin-polarized Dirac linear muffin-tin orbital band structure method. The x-ray absorption spectra (XAS) and x-ray magnetic circular dichroism (XMCD) at the Mn, Ti ${L}_{2,3}$, and O $K$ edges have been investigated theoretically. The calculated results are in good agreement with experimental data. The core-hole effect in the final state as well as the effect of the electric quadrupole ${E}_{2}$ and magnetic dipole ${M}_{1}$ transitions have been investigated. The core-hole effect has improved the agreement with the experimental XAS and XMCD spectra at the Ti and Mn ${L}_{2,3}$ edges.

Post Mortem and Operando XPEEM: a Surface-Sensitive Tool for Studying Single Particles in Li-Ion Battery Composite Electrodes

X-ray photoemission electron microscopy (XPEEM), with its excellent spatial resolution, is a well-suited technique for elucidating the complex electrode-electrolyte interface reactions in Li-ion batteries. It provides element-specific contrast images that allows the study of the surface morphology and the identification of the various components of the composite electrode. It also enables the acquisition of local X-ray absorption spectra (XAS) on single particles of the electrode, such as the C and O K-edges to track the stability of carbonate-based electrolytes, F K-edge to study the electrolyte salt and binder stability, and the transition metal L-edges to gain insights into the oxidation/reduction processes of positive and negative active materials. Here we discuss the optimal measurement conditions for XPEEM studies of Li-ion battery systems, including (i) electrode preparation through mechanical pressing to reduce surface roughness for improved spatial resolution; (ii) corrections of the XAS spectra at the C K-edge to remove the carbon signal contribution originating from the X-ray optics; and (iii) procedures for minimizing the effect of beam damage. Examples from our recent work are provided to demonstrate the strength of XPEEM to solve challenging interface reaction mechanisms via post mortem measurements. Finally, we present a first XPEEM cell dedicated to operando/in situ experiments in all-solid-state batteries. Representative measurements were carried out on a graphite electrode cycled with LiI-incorporated sulfide-based electrolyte. This measurement demonstrates the strong competitive reactions between the lithiated graphite surface and the Li2O formation caused by the reaction of the intercalated lithium with the residual oxygen in the vacuum chamber. Moreover, we show the versatility of the operando XPEEM cell to investigate other active materials, for example, Li4Ti5O12.

Opportunity and challenge for study of valence electron structure in typical magnetic materials

The conventional magnetic ordering models, exchange interaction, super-exchange (SE) interaction and double exchange (DE) interaction models relating to the valence electron structure in the materials, were proposed about in or before the 1950's, the time when there was little experimental evidence. Since the 1970's, more and more experimental results for the valence electron states have been reported. These experimental results suggested that the conventional magnetic ordering models need improving. i) Many experimental results, including the electron energy-loss spectra (EELS), X-ray absorption spectra (XAS), and X-ray photoelectron spectra (XPS), indicate that there are O<sup>–</sup> anions in addition to O<sup>2–</sup> anions in oxides, and that the percentage of O<sup>–</sup> anions may reach 30% or more. This suggests that the SE model and DE model both need to improving, in which all oxygen anions are assumed to be O<sup>2–</sup> anions. ii) Several experimental results, including gamma radiation diffraction, XAS and magnetic circular dichroism spectra (XMCD), suggest that part of 4s electrons enter into 3d orbits and transit into the 3d electrons in the process of forming metals from free atoms. The effect of the orbital magnetic moment on the magnetic moment of a bulk metal is far smaller than the spin magnetic moments. These provide the evidence of exploring the relation between magnetic moment and electrical resistivity of the magnetic metal. iii) Using density function theory (DFT) to fit physical properties yields plenty of results for many materials, but there exist serious difficulties for magnetic materials. This is due to magnetic ordering energy is included in the exchange correlation energy, which has been find no phenomenological expression so far, and has to be fitted using various models in DFT calculation. These investigations provide an opportunity to improve magnetic ordering models. Therefore, our group proposed three models of magnetic ordering in typical magnetic materials, they including an O 2p itinerant electron model for magnetic oxides (IEO model), a new itinerant electron model for magnetic metal (IEM model), and a Weiss electron pair (WEP) model for the origin of magnetic ordering energy. Replacing the SE model and DE model with the IEO model, the magnetic structures of Co, Ni, Cu doped spinel ferrites as well as Cr and Ti doped spinel ferrites can be explained. The dependence of the magnetic moment on the Sr content in perovskite manganites La<sub>1–<i>x</i></sub>Sr<sub><i>x</i></sub>MnO<sub>3</sub> can also be explained, for which there have been many ongoing disputes about the cation distributions. With the IEM model, we can explain qualitatively the relation of the magnetic moment with the resitivity for each of Fe, Co, Ni, Cu metals, and fit the curves of the resistivity of NiCu alloy versus test temperature and the Cu doped level. With the WEP model, we can explain why Fe, Co, Ni metal, NiCu alloys, Fe<sub>3</sub>O<sub>4</sub> and La<sub>0.7</sub>Sr<sub>0.3</sub>MnO<sub>3</sub> oxides have different Curie temperature values. The new itinerant electron model is different from the classical model in the following three elementary characteristics. First, the s electrons in free 3d transition metal atoms are divided into two parts when they form a metal or alloy. One part of these s electrons enter into the d orbits and change into the d electrons. and the other part of those electrons are the free electrons which are no longer called the s electrons. Second, only the d electrons occupying the outer orbit of an ion core in a metal or alloy may form itinerant electrons with a certain probability, while the remaining d electrons are local electrons. Third, whether in a magnetic metal or in a magnetic oxide, the transition of the itinerant electrons is the spin-dependent transition below the Curie temperature, and the transition probability decreases with test temperature increasing. The transition of the itinerant electrons turns into the spin-independent transition when the temperature is above the Curie temperature. In this paper, first, we introduce several typical experimental results of the valence electron states. Then, we present the new magnetic ordering models proposed by our group and analyze the elementary differences between the new models and the conventional models. Finally, we point out the challenge to the future work.

The simulation of X-ray absorption spectra from ground and excited electronic states using core-valence separated DFT/MRCI.

We present an extension of the combined density functional theory (DFT) and multireference configuration interaction (MRCI) method (DFT/MRCI) [S. Grimme and M. Waletzke, J. Chem. Phys. 111, 5645 (1999)] for the calculation of core-excited states based on the core-valence separation (CVS) approximation. The resulting method, CVS-DFT/MRCI, is validated via the simulation of the K-edge X-ray absorption spectra of 40 organic chromophores, amino acids, and nucleobases, ranging in size from CO2 to tryptophan. Overall, the CVS-DFT/MRCI method is found to yield accurate X-ray absorption spectra (XAS), with consistent errors in peak positions of ∼2.5-3.5 eV. Additionally, we show that the CVS-DFT/MRCI method may be employed to simulate XAS from valence excited states and compare the simulated spectra to those computed using the established wave function-based approaches [ADC(2) and ADC(2)x]. In general, each of the methods yields excited state XAS spectra in qualitative and often quantitative agreement. In the instances where the methods differ, the CVS-DFT/MRCI simulations predict intensity for transitions for which the underlying electronic states are characterized by doubly excited configurations relative to the ground state configuration. Here, we aim to demonstrate that the CVS-DFT/MRCI approach occupies a specific niche among numerous other electronic structure methods in this area, offering the ability to treat initial states of arbitrary electronic character while maintaining a low computational cost and comparatively black box usage.

Probing and Tuning Lattice Oxygen Redox in Li2RuxM(1-x)O3 for Design of High-Energy Li-Ion Batteries

The participation of anion redox in lithium transition metal oxides such as Li2RuO3 and Li2MnO3 for positive electrodes have opened a new avenue to boost up the energy density of current lithium-ion batteries. There are many schools of thoughts for the physical origin proposed to be responsible for anion redox and more direct and clear experimental evidence of anion redox is needed to further understand and utilize this phenomena. In the first part of this work, we demonstrated the electronic signature of oxygen-oxygen coupling, direct evidence central to lattice oxygen redox (O2-/(O2)n-) in charged Li2-xRuO3 after Ru oxidation (Ru4+/Ru5+) upon first-electron removal with lithium de-intercalation. Experimental and simulated Ru L3-edge X-ray absorption spectra (XAS) revealed that the increase in the high-energy shoulder intensity upon lithium de-intercalation resulted from increased O-O coupling, inducing (O-O) σ* with π overlap with Ru d-manifolds, in agreement with the similar changes in the O K-edge XAS spectra. Further support came from experimental and simulated O K-edge X-ray emission spectra (XES), where the broadening of the oxygen nonbonding feature upon charging also originated from creation of (O-O) σ* states. By combining differential electrochemistry mass spectrometry (DEMS) with core-level spectroscopies, we are able to quantify the percentage of this lattice oxygen redox across different Li2RuxM(1-x)O3 (M=Ti, Cr, Mn, Fe, Ir, Sn) chemistries, through which we proposed a preliminary electronic descriptor for reversible bulk oxygen redox activity. This study constructed a transferrable framework to rationally interpret the spectroscopic features by combining experiments and theory, and pinpointed the key spectra footprints related to lattice oxygen redox from a molecular level. We also proposed a simple descriptor to rationalize and facilitate novel positive electrode materials design for reversible anion redox activities in high-energy Li-ion batteries.

Structural studies of spray pyrolysis synthesized oxygen deficient anatase TiO2 thin films by using X-ray absorption spectroscopy.

The present work focuses on synthesis and X-ray absorption studies of single phase oxygen deficient anatase TiO2 thin films. These films are prepared in a two-step method viz. the synthesis of near stoichiometric anatase TiO2 films using an open atmospheric spray pyrolysis method followed by vacuum annealing at their corresponding synthesis temperatures (Ts = 450 °C, 500 °C) for different time durations (t = 2, 4, 6, 8 hours). XRD and Raman studies of these films ascertained the formation and retention of the anatase phase post annealing, indicating that there was no phase change due to prolonged annealing. Extended X-ray absorption spectra (EXAFS) and X-ray absorption near edge spectra (XANES) revealed the presence of an oxygen vacancy and its effect on the local co-ordination. The co-ordination number obtained from EXAFS analysis revealed that the number density of the oxygen vacancy is higher in the case of thin films synthesized at 450 °C than in the case of the films synthesized at 500 °C. As the oxygen vacancy leads to changes in local co-ordination, which in turn have a profound effect on the pre-edge features of the X-ray absorption spectra (XAS), theoretically simulated XAS spectra of pure anatase TiO2 and oxygen deficient anatase TiO2 were generated using FEFF and were found to match with the experimentally observed spectra. In addition, the ambiguities in whether a change in the metal-oxygen bond length has any effect on the pre-edge features or not were delineated in the present study by comparing the pre-edge peak positions of the oxygen deficient TiO2 films. Our results matched with those of some of the researchers who have studied the rutile phase TiO2, wherein it was concluded that in the case of the titanium-oxygen system, the mean Ti-O bond length does have an effect on the pre-edge peak position. It was observed that as the Ti-O mean bond length increases, the pre-edge peak positions shift towards lower energy, which is in concurrence with the literature available for other Ti-O systems with similar geometry. The second pre-edge peak intensity, which is a measure of disorder, is higher for TiO2450 °C-2 h and TiO2500 °C-2 h thin films. This is attributed to the annealing effect, which suggests that TiO6 octahedrons are the most disordered for thin films annealed for 2 h and become more ordered upon annealing for longer times.

Fe oxidation state in heat-treated basaltic blue sapphire samples and its implication to the 3309 cm-1-series peaks in infrared absorption spectra

Ti atoms bond with Fe atoms and exchange their electron particles in the adjacent octahedral sites. This process produces the blue color on the blue sapphire. The Ti content is an important key for the alteration of the 3309, 3232, and 3185 cm-1 (3309 cm-1-series peaks) in infrared absorption (FTIR) spectra of blue sapphire samples. To study the effect of Fe and Ti oxidation state and the behavior of the Ti-related peaks in FTIR spectra during the heating process, the blue sapphire samples from Phrae and Kanchanaburi, Thailand were collected regarding their basaltic origins. The heating experiment was performed. The samples were step-heated at 800, 1000, 1200, 1400, and 1650 °C under an oxidizing atmosphere, with 1 hour soaking time at each temperature. The FTIR spectra and the X-ray absorption spectra (XAS) were measured before and after heating. Before heating, the FTIR spectra showed only the peak at 3309 cm-1 while the side peaks at 3232 cm-1 and 3185 cm-1 start to appear after heating at 800 °C. The 3309 cm-1-series peaks were assigned as -Ti-OH stretching. The intensity of those peaks was slightly decreased after heat-treated. The XAS spectra revealed that the oxidation state of Fe is still Fe3+ while Ti is still Ti4+ in every heating temperature. The results pointed that the alteration of the 3309 cm-1-series peaks does not depend on the Fe and/or Ti oxidation state.

Ab-initio study of X-ray absorption and X-ray magnetic circular dichroism spectra of [formula omitted]PtGa and [formula omitted]PtGa alloys

Using first-principles calculations based on density functional theory, we have studied the X-ray absorption spectra (XAS) and X-ray Magnetic Circular Dichroism (XMCD) spectra of Co2PtGa and Mn2PtGa alloys. Using relativistic calculations, we have estimated and analyzed the contribution of the orbital and spin moments to the total magnetic moment for the transition metal atoms, Pt, Co and Mn. It has been observed that while the ratio of orbital and spin magnetic moments of Pt atom in Co2PtGa in cubic phase is substantial, this is an order of magnitude lower in Mn2PtGa. On the other hand, this difference reduces drastically in case of the martensite phase. This signifies the crucial role of the neighborhood in determining the magnetic properties of these materials. Further, the XAS and XMCD spectra at the L2,3 edges of different constituent atoms in various materials have been analyzed. The results (peaks and shoulders present in these XAS and XMCD spectra) have been corroborated with the atom-projected unoccupied density of states of the respective materials.

Nanometer-scale local probing of X-ray absorption spectra of Co/Pt multilayer film

We report our local X-ray absorption spectra (XAS) measurement mapping for a Co/Pt multilayer using scanning transmission microscopy with 25-nm spatial resolution and 0.1-eV spectral resolution. We have systematically analyzed the two-dimensional XAS intensity variation over the corresponding magnetic domain patterns, revealing a XAS profile across the magnetic domain wall as well as the simultaneous high-throughput measurement of local XAS spectra.

Electronic structure and X-ray magnetic circular dichroism in the Ni-Mn-Ga Heusler alloys

The electronic structure and X-ray magnetic circular dichroism (XMCD) spectra of the Ni2MnGa and Cu doped Ni2MnGa Heusler alloys were investigated theoretically from first principles, using the fully relativistic Dirac linear MT-orbital (LMTO) band structure method. Densities of valence states, orbital and spin magnetic moments are analyzed and discussed. The electronic and magnetic structure of Ni-Mn-Ga Heusler alloys were investigated for the cubic austenitic and modulated 7M-like incommensurate martensitic phases. The X-ray absorption spectra (XAS) and XMCD at the Mn, Ni, Ga, and Cu L2,3, and Mn, Ni, and Ga K edges were investigated theoretically from first principles. The origin of the XMCD spectra in the Ni2MnGa compound is examined. The ab initio calculations reproduce well experimental XAS and XMCD spectra. The XAS at the Mn L2,3 edges remains mostly unchanged through martensitic phase transition. A fingerprint of the martensitic phase transition has been found in the Ni L2,3 XAS spectra. The experimental Ni L3 XAS has a pronounced shoulder at the L3 peak at around 853 eV. This peak is nearly suppressed in the martensitic state in comparison with the austenitic phase due to the lifting of the degeneracy in the Ni 3d related unoccupied electronic states. The XMCD of the martensitic phase is increased compared to the XMCD of the austenite phase at the Ni and Mn L2,3 edges.

Spin-orbit coupling induced band structure change and orbital character of epitaxialIrO2films

We investigated the electronic structure of $\mathrm{Ir}{\mathrm{O}}_{2}$ to address the controversy regarding spin-orbit coupling (SOC) effects in metallic $5d$ transition-metal oxides. Two issues have come to the forefront: (1) SOC effects on electronic structure and physical properties of $\mathrm{Ir}{\mathrm{O}}_{2}$ and (2) the possible formation of a novel ground state in this material, the ${J}_{\mathrm{eff}}=1/2$ state. To better understand the SOC mechanism, we grew epitaxial $\mathrm{Ir}{\mathrm{O}}_{2}$ films whose $dc$ resistivity values were comparable with those of a single crystal. We obtained polarization-dependent optical and x-ray absorption spectra (XAS) and compared these results with those acquired using the generalized gradient approximation (GGA) and $\mathrm{GGA}+\mathrm{SOC}$ calculations. From the optical spectra, peak structures were identified at 0.4 and 2.0 eV, which could only be explained using the $\mathrm{GGA}+\mathrm{SOC}$ calculation. This suggests that SOC plays an important role in the electronic structure of $\mathrm{Ir}{\mathrm{O}}_{2}$. From the polarization-dependent $\mathrm{O}\phantom{\rule{0.16em}{0ex}}1s$ XAS spectra, we observed that the empty state near the Fermi level lacks involvement of an Ir ${d}_{xy}$ orbital. Despite the importance of SOC in $\mathrm{Ir}{\mathrm{O}}_{2}$, the ${J}_{\mathrm{eff}}=1/2$ state does not form in metallic $\mathrm{Ir}{\mathrm{O}}_{2}$.

What is the correct Fe L23 X-ray absorption spectrum of magnetite?

Various groups have reported Fe L23 X-ray absorption spectra (XAS) of magnetite (Fe3O4), each claiming to be that of magnetite, but which contradict each other. Here we report an XAS study of two kinds of magnetite: one is biogenic magnetite nanocrystals extracted from the magnetotactic bacterium Magnetovibrio blakemorei strain MV-1; the other is synthetic, abiogenically produced nano-magnetite. We see significantly different XAS spectra of these two materials. Only when the abiogenic magnetite was reduced under H2 did it give the same spectrum as the biogenic sample. Extensive heating of the biogenic magnetite in air produced spectra similar to that of the abiogenic magnetite. These two spectra are typical of the range of published Fe L23 spectra of magnetite. X-ray diffraction confirmed that the biogenic material is stoichiometric Fe3O4, and showed that the as-received or partly reduced abiogenic material is a non-stoichiometric oxide, intermediate between magnetite and maghemite (γ-Fe2O3). When the membrane which surrounds magnetosome chains was intact, the biotic magnetite single crystals were surprisingly resistant to oxidation. This study clarifies a significant confusion existing in the literature as to the correct Fe L23 X-ray absorption spectra of magnetite and maghemite.