X-ray Absorption Near Edge Spectra Spectra Research Articles

Heating up the OER: Investigation of IrO2 OER Catalysts as Function of Potential and Temperature**

AbstractDespite intensive investigations for unravelling the water splitting reaction, the catalyst behavior during the oxygen evolution reaction (OER) is still not fully understood. This is especially true under more demanding conditions like high potentials and high temperatures. Rotating disk electrode measurements show a gradual increase of OER current when increasing the temperature up to 80 °C. However, strong bubble formation at elevated temperatures makes in‐situ characterization of the catalyst challenging. Here we utilize an in‐situ electrochemical and heated flow cell, which aims at an efficient removal of bubbles from the catalyst surface and enables structural studies by X‐ray absorption spectroscopy (XAS) at temperatures up to 80 °C. Changes in the Ir L3‐edge X‐ray absorption near edge spectra (XANES) were observed with respect to the white line position and principal components related to structural changes were extracted. At temperatures of 60 °C and above, the white line position of XANES spectra reaches a steady state, which is possibly caused by an equilibrium of different Ir oxidation states. These findings provide first spectroscopic insights in the behavior of OER catalysts at elevated temperatures which are typical for industrial applications and rarely addressed until now.

Identifying Key Descriptors for the Single-Atom Catalyzed CO Oxidation

Identifying Key Descriptors for the Single-Atom Catalyzed CO Oxidation

Probing defects and electronic structure of Eu doped t-Mg2B2O5 nanocrystals using X-ray absorption near edge spectroscopy and luminescence techniques

We report here the defects and electronic structure study of Eu (1%,3%,5%) doped t-Mg2B2O5 nanocrystals synthesized using low temperature combustion method, and probed using x-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray absorption near edge spectra (XANES), photoluminescence (PL) and thermoluminescence (TL). The XRD analysis of all samples shows single phase triclinic crystal structure and average crystallite size decreases with Eu doping in Mg2B2O5. The experimental XANES spectra of each sample acquired at Eu M5,4-edges were compared with simulated absorption edges using atomic-multiplet calculations, which clearly shows the presence of Eu3+ in Mg2B2O5. The O K-edge spectra of Eu doped Mg2B2O5 predicts the formation of O and Mg defects upon increasing Eu concentrations. The photoluminescence of Mg2-xEuxB2O5 nanocrystals at an excitation of 325 nm comprises of a group of sharp peaks owing to intra-4f transitions in Eu3+ and confirms the lowering of local symmetry with Eu concentration in Mg2B2O5. The effect of Eu doping on the trapping states was probed using TL after irradiating Mg2B2O5 with ultra-violet radiations (365 nm), which shows the presence of shallow and deep level trapping states. The corresponding activation energies of trapping states were determined using glow curve deconvolution method based on Kitti's general order equation.

Understanding XANES spectra of two-temperature warm dense copper using ab initio simulations

Using ab initio molecular-dynamics simulations combined with linear-response theory, we studied the x-ray absorption near-edge spectra (XANES) of a two-temperature dense copper plasma. As the temperature increases, XANES spectra exhibit a pre-edge structure balanced by a reduction of the absorption just behind the edge. By performing systematic simulations for various thermodynamic conditions, we establish a formulation to deduce the electronic temperature Te directly from the spectral integral of the pre-edge that can be used for various thermodynamic conditions encountered in a femtosecond heating experiment where thermal non equilibrium and expanded states have to be considered.

Automated generation and ensemble-learned matching of X-ray absorption spectra

X-ray absorption spectroscopy (XAS) is a widely used materials characterization technique to determine oxidation states, coordination environment, and other local atomic structure information. Analysis of XAS relies on comparison of measured spectra to reliable reference spectra. However, existing databases of XAS spectra are highly limited both in terms of the number of reference spectra available as well as the breadth of chemistry coverage. In this work, we report the development of XASdb, a large database of computed reference XAS, and an Ensemble-Learned Spectra IdEntification (ELSIE) algorithm for the matching of spectra. XASdb currently hosts more than 800,000 K-edge X-ray absorption near-edge spectra (XANES) for over 40,000 materials from the open-science Materials Project database. We discuss a high-throughput automation framework for FEFF calculations, built on robust, rigorously benchmarked parameters. FEFF is a computer program uses a real-space Green’s function approach to calculate X-ray absorption spectra. We will demonstrate that the ELSIE algorithm, which combines 33 weak “learners” comprising a set of preprocessing steps and a similarity metric, can achieve up to 84.2% accuracy in identifying the correct oxidation state and coordination environment of a test set of 19 K-edge XANES spectra encompassing a diverse range of chemistries and crystal structures. The XASdb with the ELSIE algorithm has been integrated into a web application in the Materials Project, providing an important new public resource for the analysis of XAS to all materials researchers. Finally, the ELSIE algorithm itself has been made available as part of veidt, an open source machine-learning library for materials science.

Chemometric analyses of XANES data collected on 99Tc bearing silicate glasses

A challenge to the incorporation of 99Tc into a glass matrix is that 99Tc is volatile at vitrification temperatures. Understanding how this volatilization occurs requires knowledge of the multiple chemistries which Tc may take during vitrification. This paper presents an overview of how the localized chemistry of 99Tc has been determined in a series of 99Tc-bearing glasses by chemometric analyses of X-ray absorption near edge spectra (XANES). Linear combination fitting and principal component analysis of the glasses’ XANES spectra suggested that the glasses contained 3–4 chemically distinct Tc environments. The identity of the detected chemistries were pertechnetates, (Tc(VII) as isolated oxyanions, or locally coordinated by Na or K), or isolated Tc(IV) species. The linear combination fitting distribution of local Tc sites agrees with reanalyzed Raman spectra, suggesting that as targeted KTcO4 incorporation increases, a significant ion exchange takes place, and speciation in the glass changes to favor Tc(VII) formation, specifically NaTcO4. Based on the statistical suggestion that not all Tc environments are accounted by the available standards, a new mechanism for the behavior of Tc during vitrification is proposed.

Calculated XANES Spectra of Cation Off-Centering in Bi(Mg0.5Ti0.5)O3

The x-ray absorption near edge spectra (XANES) of Bi, Mg and Ti in BMT for different off-centering magnitudes, associated with different structural models, were calculated by using first-principles calculations. The models studied include the high symmetric structure, two experimental proposed structures (based on an x-ray diffraction experiment) and the calculated fully relaxed structure (based on the calculated energy optimization). The features in the XANES spectra that relates with the off-center shift of cations were identified. Our calculated XANES will aid future experimental identifications of the detailed structure of BMT.

Study of XANES near Ta-L edges in LiTaO3 through thermal wave, fluorescence and first principles

X-ray absorption near-edge spectra (XANES) of Ta-L2 and L3 edges in LiTaO3 (LTO) crystals are measured by measuring amplitude and phase of thermal waves generated within the LTO crystal, using pyroelectric property of LTO. Thus, LTO crystal is used both as a sample as well as sensor material. XANES of Ta-L edges in LTO are also measured by fluorescence. XANES spectra from fluorescence and first-principles simulations agree excellently well. The onset of the pre-edge region of XANES, measured by both techniques, extends below the edge by about 50 eV. This pre-edge onset of absorption is explained in terms of the core-hole lifetime effect on near-edge absorption using density functional theory. However, detailed nature of XANES peaks near Ta-L3 and Ta-L2 absorption edges, measured by thermal waves and fluorescence, differ. Possible origins of these differences are discussed.

Sensitivity of Solvation Environment to Oxidation State and Position in the Early Actinide Period.

The aqueous solvation of U-Pu in the III-VI oxidation states has been examined using density functional theory and hydrated cluster models of the form An(H2O)30(4+/3+) and AnO2(H2O)30(2+/+) embedded within a polarizable continuum model to approximate the effect of bulk water. The structural features are compared to available data from extended X-ray absorption fine structure. Then, using a multiple-scattering approach, the X-ray absorption near-edge spectra (XANES) have been simulated and compared to experiment. These structural data are complemented by a detailed thermodynamic analysis using a recently benchmarked protocol. The structural, spectroscopic, and thermodynamic information has been used to assign the primary solvation environments in water, with an emphasis upon understanding how oxidation state and position in the period modifies the hydration number and equilibrium between different solvation shell environments. Tetravalent U is proposed to exist in equilibrium between the 8- and 9-coordinate species. Moving to the right of the period, Np(IV) and Pu(IV) exist solely as the octa-aquo species. Reduction to the trivalent ions leads to thermodynamic favorability for this solvation environment, whose features reproduce the XANES spectra. The actinyl dications (AnO2(2+)) of U and Np have a preferred environment in the equatorial plane consisting of 5 solvating waters; however, changes to the ionic radius and electronic structure at Pu leads to an equilibrium between the 4- and 5-coordinate species for PuO2(2+). Reduction of the dications to form the monocations generally leads to a preference for the 4-coordinate primary solvation shell, with an equilibrium existing for uranyl, while the neptunyl and plutonyl species exist solely as AnO2(H2O)4(+). These data provide accurate thermodynamic information for several rare species and the combined thermodynamic, structural, and spectroscopic approach reveals trends in hydration behavior across actinide oxidation states and within the early actinide period.

Ab initiocalculation of x-ray absorption of iron up to 3 Mbar and 8000 K

Using ab initio simulations within the generalized gradient approximation, we calculate x-ray absorption near edge spectra (XANES) at the iron K edge throughout the high-pressure phase diagram and up to extreme density and temperature conditions that are representative of the Earth's inner core (up to 3 Mbar and 8000 K). We show that XANES spectra near the Fe K edge exhibit clear signatures for the different high-temperature, high-pressure phases of iron. This suggests that XANES spectroscopy might be used to resolve ongoing controversies regarding both the high-pressure melting curve of iron and the nature of the solid phases undergoing melting up to several Mbar. In contrast to diffraction measurements, it also offers a severe constraint for density functional theory predictions of the transport properties of iron by providing direct information on the electronic structure of iron at these extreme conditions.

Real-space finite-difference calculations of XANES spectra along the aluminum Hugoniot from molecular dynamics simulations

Individual configurations obtained from molecular dynamics simulations have been combined with a real-space finite-difference computation of x-ray absorption near edge spectra (XANES) to obtain a measurable signature of the local order in a dense (shock-)compressed material. The approach makes use of the finite-temperature neutral pseudoatom concept. K-edge XANES spectra along the principal Hugoniot of aluminum have been obtained up to the density of 7 g/cm 3. The results obtained indicate that a strong local order may exist in compressed matter even for temperatures of the order of 1 eV. We found that the red shift of the onset of absorption is only a temperature effect.

Analysis of the fine structure of XANES spectra over NiK edge in Ni(EtOCS2)2

The NiK edge X-ray absorption near edge spectra (XANES) of the Ni(EtOCS2)2 complex were measured. The theoretical NiK edge XANES spectra were calculated by the total multiple scattering and finite difference methods; the potential was calculated with a muffin-tin approximation and without it. It is shown that inclusion of the non-muffin-tin effects is important for modeling the NiK XANES spectrum for the Ni(EtOCS2)2 complex; good agreement with experiment was achieved only in the calculations with the total potential (without the muffin-tin approximation for the shape of the potential).

Electronic structure of Pb1−xBaxZr0.65Ti0.35O3 ferroelectric compounds probed by soft x-ray absorption spectroscopy

O K and Ti L2,3 x-ray absorption near-edge spectra (XANES) ofPb1−xBaxZr0.65Ti0.35O3 (PBZT) ferroelectric ceramic compounds withx varying from 0.0 to 0.40 were obtained. The XANES spectra at the O K edge werecompared with ab initio calculations based on the structures obtained by x-ray diffraction,using the FEFF 8.2 code. The different features observed in the XANES spectra ofPBZT samples were quite well reproduced with the calculations, especially forx≤0.20. However,for x>0.20 the ab initio calculations do not match the experimental spectra, indicating that for theseBa concentrations, the structural model deduced from powder x-ray diffraction does notreflect the actual local structure around oxygen atoms. Based on calculations of the localpartial electron density of states (DOS), the main features of the O K-edge experimentalspectra were assigned primarily to the hybridization between O 2p–Ti 3d, Pb 6p andBa 5d orbitals. The differences in the ferroelectric character of the end-memberx = 0.0 and 0.40 sample compositions has been interpreted as due to the decrease of thehybridization between Pb 6p and O 2p orbitals caused by the substitution ofPb2+ by Ba2+ atoms.

Simulation of x-ray absorption near-edge spectra and x-ray fluorescence spectra of optically excited molecules

The x-ray absorption near-edge spectra (XANES) and fluorescence spectra of molecules in the ground state and optically excited states are computed using time-dependent density functional theory and time-dependent Hartree-Fock theory. The calculated XANES spectra of optically excited methanol, benzonitrile, hydrogen sulphide, and titanium tetrachloride and the fluorescence spectra of optically excited methanol can be used to simulate ultrafast optical pump/x-ray probe experiments.

The influence of pH on the kinetics, reversibility and mechanisms of Pb(II) sorption at the calcite-water interface

Pb(II) sorption experiments with calcite powders were conducted in suspensions equilibrated at atmospheric PCO 2(g) and ambient temperature at pH 7.3, 8.2 and 9.4. Pb fractional sorption was low at pH 7.3 and 9.4 relative to pH 8.2, and correlated well with PbCO 3 0 (aq) speciation. Desorption experiments conducted for initial sorption times ranging from 0.5 h to 12 d reveal an almost completely reversible process at pH 8.2, attributed to the dominance of an adsorption mechanism, with slight and pronounced irreversibility at pH 7.3 and 9.4 respectively. Similarities in X-ray absorption near edge spectra (XANES) for 24 h and 12 d pH 7.3 and 9.4 sorption samples indicate no effect of initial sorption time. Results from linear combination (LC) fits of XANES spectra for samples sorbed at pH 9.4 confirm ∼75% adsorbed and ∼25% coprecipitated components. The coprecipitated fraction was attributed to the non-exchangeable metal observed in desorption experiments. At pH 7.3, ∼95% adsorbed and ∼5% coprecipitated components were obtained. A comparison of results from desorption experiments and LC-XANES alludes to an irreversibly bound adsorbed component for the pH 9.4 12 d sorption sample. Extended X-ray absorption fine structure spectroscopy (EXAFS) analysis of pH 7.3 and 9.4 12 d sorption samples confirms the presence of both adsorbed and coprecipitated metal. At pH 7.3 a first-shell Pb-O bond length of 2.38 Å is intermediate between that of adsorbed (2.34 Å) and coprecipitated (2.51 Å) Pb. At pH 9.4, two first-shell Pb-O distances at 2.35 Å and 2.51 Å were obtained, indicative of the occurrence of both adsorption and coprecipitation and a larger coprecipitated fraction relative to that at pH 7.3, consistent with LC-XANES results. We propose that the disparity in the fraction of coprecipitated metal with pH may be linked to the ability of sorbed Pb to inhibit near-surface dynamic exchange of Ca and CO 3 species, which dictates step advance and retreat. Less effective inhibition of step motion at pH 9.4, due to lower fractional sorption, combined with highest rates of dynamic exchange results in a significant fraction of coprecipitated Pb at this pH. At low pH, though fractional sorption is also low, lower rates of exchange prohibit significant coprecipitation. At pH 8.2, effective inhibition of surface processes due to higher fractional sorption and lower rates of exchange compared to pH 7.3 and 9.4 preclude detectable coprecipitation. Other factors such as changes in surface speciation and solubility of the Pb-Ca solid solution with pH may also come into play. Overall, this study presents evidence for the influence of pH on Pb sorption mechanisms, and addresses the efficiency of Pb immobilization in calcitic systems.

Bayes–Turchin approach to XAS analysis

Modern analysis of X-ray absorption fine structure (XAFS) is usually based on a traditional least-squares fitting procedure. Here an alternative Bayes-Turchin method is discussed which has a number of advantages. In particular the method takes advantage of a priori estimates of the model parameters and their uncertainties and avoids the restriction on the size of the model parameter space or the necessity for Fourier filtering. Thus the method permits the analysis of the full X-ray absorption spectra (XAS), including both XAFS and X-ray absorption near-edge spectra (XANES). The approach leads to a set of linear equations for the model parameters, which are regularized using the 'Turchin condition'. Also, the method naturally partitions parameter space into relevant and irrelevant subspaces which are spanned by the experimental data or the a priori information, respectively. Finally we discuss how the method can be applied to the analysis of XANES spectra based on fits of experimental data to full multiple-scattering calculations. An illustrative application yields reasonable results even for very short data ranges.

Experimental and theoretical study of the electronic structures of Y 1− xR xPd 2B 2C (R = Gd, Dy, Ho, and Er) superconductors

The electronic structures of four polycrystalline borocarbide superconductors Y 1− x R x Pd 2B 2C (R = Gd, Dy, Ho, and Er) were studied by photoabsorption spectroscopy and theoretical calculation. The Pd and rare-earth L 3-edge X-ray absorption near edge spectra (XANES) are compared with the calculated XANES spectra for these intermetallic compounds. The decrease of the superconducting transition temperatures with addition of rare-earth dopant in these compounds is not due to a decrease of the total density of states at the Fermi energy.

Experimental and theoretical study of the electronic structures of intermetallic compounds and alloys containing Fe or Ni

The electronic structures of two groups of intermetallic compounds and alloys containing Fe, V, or Ni are studied by X-ray absorption near-edge spectra (XANES) at the Fe, V, and Ni L2,3 edges and the Fe K edge. The XANES spectra are compared with those calculated with two theoretical methods. The differences in the XANES spectra of compounds containing Ni calculated by these two methods are discussed. Except for NiGa, a reasonable good agreement between theory and experiment was found for all materials. The density of states at the Fermi energy at different atomic sites are also calculated for the compounds and alloys containing Fe.

Experimental and theoretical study of the electronic structures of Ni3Al, Ni3Ga, Ni3In, and NiGa

The electronic structures of Ni3Al, Ni3Ga, Ni3In, and NiGa are studied by x-ray absorption near-edge spectra (XANES) at the Ni and Ga K edges. The XANES spectra are compared with those calculated with theory. The experimental XANES features for these compounds reflect the Ni- and Ga-p unoccupied density of states. The calculated magnetic moments for Ni3Al, Ni3Ga, and Ni3In are between 0.7–0.8 μB/cell. The number of 3d holes per Ni atom is calculated for Ni3Al, Ni3Ga, and Ni3In. These numbers show correlation with heats of formation of the bulk compounds.

Modeling And Photoabsorption Study of YPd2-xRhxB2C Superconductors

AbstractThe electronic structures of six polycrystalline borocarbide superconductors YPd5-xRhxB3C0.4 (x=0, 0.05, 0.1, 0.15, 0.2, and 0.25) were studied by photoemission and photoabsorption spectroscopies and theoretical calculations. The valence-band (VB) photoemission spectrum is compared with the theoretical total and partial density-of-states (DOS) curves. The Pd L3-edge x-ray absorption near edge spectra (XANES) are compared with the calculated XANES spectra for these intermetallic compounds. The decrease of the superconducting transition temperatures (Tc) with addition of Rh dopant in these compounds is due to a decrease of the total DOS at the Fermi level (EF).