X-ray Absorption Near-edge Structure Calculations Research Articles

Effect of atomic vibrations in XANES: polarization-dependent damping of the fine structure at the CuK-edge of (creat)2CuCl4.

Polarization-dependent damping of the fine structure in the Cu K-edge spectrum of creatinium tetrachlorocuprate [(creat)2CuCl4] in the X-ray absorption near-edge structure (XANES) region is shown to be due to atomic vibrations. These vibrations can be separated into two groups, depending on whether the respective atoms belong to the same molecular block; individual molecular blocks can be treated as semi-rigid entities while the mutual positions of these blocks are subject to large mean relative displacements. The effect of vibrations can be efficiently included in XANES calculations by using the same formula as for static systems but with a modified free-electron propagator which accounts for fluctuations in interatomic distances.

DFT+U calculation of the Cr K pre-edge structures in ZnCr2O4

Zinc chromite ZnCr2O4 has been investigated by X-ray absorption near edge structure (XANES) spectroscopy at the Cr K-edge, with the objective of better understanding the origin of the electronic excitations giving rise to the pre-edge features. In particular, the origin of a third feature visible only in Cr-rich compounds of the ZnAl2O4-ZnCr2O4 solid solution is studied. Standard density functional theory (DFT) XANES calculations were first performed, then a Hubbard U parameter was included in an attempt to improve the relative peak positions by increasing the localization of the Cr 3d states. The origin of the pre-edge features in ZnCr2O4 has been analyzed using DFT+U calculations, local projected densities of states and the assignment of the spectral features for the dilute ZnAl2O4:Cr3+ compound. We show that DFT+U does not overcome the limitations of standard DFT in modeling the relative peak positions, and that approaches including advanced description of the electron-hole interaction are needed.

Probing the structure of iron at extreme conditions by X-ray absorption near-edge structure calculations

We present the K-edge X-ray absorption near edge spectra of hexagonal-closed packed iron at pressure and temperature conditions relevant to Earth's mantle conditions. The calculated spectra have been obtained using the first-principles scheme based on the continued-fraction approach and norm-conserving pseudopotentials. The atomic configurations used for the X-ray absorption near edge spectroscopy calculations were obtained from classical molecular dynamics simulations, using an optimized embedded-atom potential. We compare our calculated spectra to recently available experiment results (R. Boehler, H.G. Musshoff, R. Ditz, G. Aquilanti, and A. Trapananti, Rev. Sci. Instrum. 80 (2009), pp. 045103–045108) and identify the main features of the spectra that may indicate onset of melting in iron.

Evidence for Fe2+ in Wurtzite Coordination: Iron Doping Stabilizes ZnO Nanoparticles

First-principles calculations are used to investigate the structural and electronic properties of Fe-doped ZnO nanoparticles. Based on extensive validation studies surveying various density functionals, the hybrid functional PBE0 is employed to calculate the structures, formation energies, and electronic properties of Fe in ZnO with Fe concentrations of 6.25, 12.5, and 18.75 at%. Substitution of Zn by Fe, zinc vacancies, and interstitial oxygen defects is studied. High-resolution inner-shell electron energy loss spectroscopy measurements and X-ray absorption near-edge structure calculations of Fe and O atoms are performed. The results show that Fe-doped ZnO nanoparticles are structurally and energetically more stable than the isolated FeO (rocksalt) and ZnO (wurtzite) phases. The Fe dopants distribute homogeneously in ZnO nanoparticles and do not significantly alter the host ZnO lattice parameters. Simulations of the absorption spectra demonstrate that Fe(2+) dominates in the Fe-doped ZnO nanoparticles reported recently, whereas Fe(3+) is present only as a trace.

Contribution of molecular dynamics simulations and ab initiocalculations to the interpretation of Mg K-edge experimental XANES inK2O–MgO–3SiO2 glass

The structural environment of Mg in a K-bearing silicate glass of compositionK2MgSi3O8 is investigated by x-ray absorption near-edge structure (XANES) spectroscopy at the MgK-edge. XANES calculations are performed using a plane-wave electronic structure code,and a structural model obtained by classical molecular dynamics coupled to ab initiorelaxation. Bond valence theory is used to validate plausible environments within thestructural models. Comparison between the experimental and calculated spectra enables usto conclude that the Mg atoms are located in distorted tetrahedral sites. Sitedistortions are found to be correlated to the theoretical shift of the XANES edgeposition.

First-principles calculations of Zn–K XANES in Ca-deficient hydroxyapatite

The local environment of substitutionalZn2 + inCa-deficient hydroxyapatite (HAp) was investigated using experimental and theoretical analyses ofthe x-ray absorption near edge structure (XANES). For Zn–K XANES calculations, two situations ofZn2 + were considered.One was Zn2 + substituted for Ca sites in perfect HAp, and the other was a Ca-deficient HAp model of substitutionalZn2 + associatedwith a Ca2 + vacancy charge compensated by two protons. The model ofZn2 + inperfect HAp did not reproduce the experimental Zn–K XANES spectrum. In contrast, the Ca-deficientHAp model agreed well with the experimental spectrum. This indicates that substitutionalZn2 + in Ca-deficient HApis associated with the Ca2 + vacancy complex in HAp.

Analysis of the polarization dependence of the N K edge X-ray absorption fine structure in InN

The polarization dependence of the x-ray absorption near-edge structure (XANES) of InN beyond the N K edge is calculated. The XANES calculations are performed for different values of the angle θ between the XY plane of crystalline indium nitride and the incident x-radiation (θ = 15°, 30°, 45°, 60°, 75°, and 90°). It is shown that, in the case of N K XANES for InN, a strong polarization dependence of the specific features of the spectrum is observed. The calculated spectra are compared with previously measured experimental spectra. The partial densities of the electronic states of InN are calculated near the top of the valence band and near the bottom of the conduction band.

XANES calculation with an efficient “non muffin-tin” method: application to the angular dependence of the Al K-edge in corundum

We present a reciprocal-space “non muffin-tin” scheme for calculating X-ray Absorption Near-Edge Structure (XANES). The method uses pseudopotentials and reconstructs all electron wave functions within the Projector Augmented Wave framework. The method incorporates a recursive method to compute absorption cross section as a continued fraction. The continued fraction formulation of absorption is advantageous in that it permits the treatment of core-hole-electron interaction through large supercells (hundreds of atoms). This opens new fields of applications like surfaces, molecules, small aggregates or amorphous materials, for which large supercells are required. The method is applied to the natural linear dichroism at the Al K-edge in corundum (α-Al2O3). Details about the convergence process of the calculation are given. The influence of the core-hole effects is emphasized and comparison with “muffin-tin” multiple scattering calculation is made.

Mechanism of solid-state amorphization of Se induced by mechanical milling

In this work, the solid-state amorphization process of elemental trigonal Se via mechanical milling was studied. Trigonal Se has a unique crystal structure consisting of helical [–Se–]n chains with strong intrachain covalent bonds and with weak interchain van der Waals bonds. It was found that the interchain coordination distance increased while the intrachain coordination distance decreased with increasing milling time. The crystalline Se transformed to the amorphous state once the interchain coordination distance reached a critical value. The intrachain coordination distance of amorphous Se continued to decrease with milling time, suggesting that molecular chain length decreased with milling time. Combined with the x-ray absorption near-edge structure calculations, it was concluded that the mechanical milling destroyed the interchain bonds of crystalline Se, resulting in the amorphization, while the intrachain bonds were strengthened during amorphization. The present results support a previous crystallite-destabilization model for solid-state amorphization.

Development of XAFS theory.

A major goal of theoretical simulations of X-ray absorption fine structure (XAFS) is to provide calculations for the interpretation and analysis of experimental data in terms of geometrical and electronic information. The extended region or EXAFS (50-2000 eV above an absorption edge) contains geometric information about the pair distribution function, i.e. distances to the nearest neighbors and their orientation. The theory of EXAFS is now well understood and has been recently reviewed [Rehr & Albers (2000). Rev. Mod. Phys. 72, 621-654]. The near-edge region (0-50 eV above the edge) or X-ray absorption near-edge structure (XANES) probes the states just above the Fermi level, and contains important electronic information, e.g. the electronic density of states (DOS). This data can be used to obtain the number of electrons or holes in the electronic configuration and spin and orbital moments on a particular atom via sum rules. XANES calculations with our ab initio code FEFF8 [Ankudinov et al. (1998). Phys. Rev. B, 58, 7565-7576] usually give semi-quantitative agreement with experiment, and permits the interpretation of XANES in terms of DOS. However, fully quantitative calculations remain a challenge. Several effects still need to be considered to treat the XANES region. These include non-spherical parts of the scattering potential and many-body effects such as multi-electron excitations, core-hole effects and local field effects (screening of the X-ray field).

New developments in the theory and interpretation of X-ray spectra based on fast parallel calculations.

There has been dramatic progress over the past decade both in theory and in ab initio calculations of X-ray absorption fine structure. Significant progress has also been made in understanding X-ray absorption near-edge structure (XANES). This contribution briefly reviews the developments in this field leading up to the current state. One of the key advances has been the development of several ab initio codes such as FEFF, which permit an interpretation of the spectra in terms of geometrical and electronic properties of a material. Despite this progress, XANES calculations have remained challenging both to compute and to interpret. However, recent advances based on parallel Lanczos multiple-scattering algorithms have led to speed increases of typically two orders of magnitude, making fast calculations practicable. Improvements in the interpretation of near-edge structure have also been made. It is suggested that these developments can be advantageous in structural biology, e.g. in post-genomics studies of metalloproteins.

Rapid calculation of x‐ray absorption near edge structure using parallel computation

AbstractModeling x‐ray absorption near edge structure (XANES) requires computationally intensive calculations. We show that parallel processing can reduce the time required for XANES calculations by a factor of up to 50 over standard desktop computers. Parallel processing is implemented in our codes using the Message Passing Interface (MPI) and is portable across most hardware and operating systems. We demonstrate the inverse scaling of the parallel algorithm with the number of processors, and discuss how this approach to parallel processing could be implemented in other multiple‐scattering calculations. Faster calculations should improve the applicability of ab initio XANES studies to many materials science problems. Published in 2001 by John Wiley & Sons, Ltd.

X-ray absorption near-edge structure calculations beyond the muffin-tin approximation

A new scheme for calculating the x-ray absorption near edge structure (XANES) based on the finite-difference method is proposed. It allows completely free potential shape and, in particular, is not constrained to the muffin-tin approximation. In our approach, the calculation of the final states is performed in real space. The Schr\odinger equation is solved in a discrete form on the node points of a three-dimensional grid. The unknowns are the values of the wave functions at the grid points. The validity of the method is shown on two different systems the metallic copper and the carbon monoxide molecule; then, the differences resulting from muffin-tin and non-muffin-tin calculations are shown on different typical molecules.

Relativistic XANES calculations of Pu hydrates

A theoretical analysis of x-ray absorption near edge structure (XANES) at L{sub 3} and L{sub 1} edges for Pu hydrates with a formal oxidation state of Pu ranging from +3 to +6, using the {ital ab initio} multiple scattering code FEFF7, is presented. For each hydrate our calculations reproduce well the white line intensity and relative peak positions, which are the features commonly used for formal valence identification. In order to achieve such a degree of coincidence between theory and experiment, it was necessary to use a relativistic Dirac-Fock treatment of atomic densities and mixed Dirac-Fock{endash}local-density-approximation exchange-correlation potentials, not considered in previous actinide studies. We find that most of the white line intensity at L{sub 3} edges originates from scattering of the photoelectron, i.e., x-ray absorption fine structure. We also show that the white line shoulder peak, present in the case of plutonyl compounds (Pu{sup 5+} and Pu{sup 6+}), is due largely to constructive interference of scattering paths containing axial oxygens. Limitations of the FEFF7 code and ways to improve the quantitative agreement between XANES calculations and experimental spectra are also discussed. {copyright} {ital 1998} {ital The American Physical Society}

Ti K-Edge XANES Studies of Oxides: Theory vs. Experiment

Ti K-edge x-ray absorption near edge structure (XANES) spectra for a variety of Ti(IV)-bearing crystalline oxide model compounds have been measured and compared to ab-initio multiple-scattering XANES calculations using the FEFF6 code. Depending on Ti-coordination (4, 5, or 6), the pre-edge features are found to vary in both position (by -2 ± 0.1 eV) and normalized height (from - 0.04 1.0), in agreement with the calculations. In compounds where the Ti-coordination is unknown (e.g., glasses and melts), both pre-edge position and height must be used to derive reliable coordination information for Ti. Use of both pre-edge parameters allows one to differentiate, for instance, between 100% of 5-coordinated Ti and a 50:50 mixture of 4- and 6-Ti. Finally, the intensity of the main-edge features can be used as a direct probe of disorder in the short- and medium-range environment of Ti, as exemplified by the comparison of the theoretical and measured spectra obtained for several crystals and their amorphous analogs such as crystalline and radiation-damaged CaTiSiO 5 and glassy and molten titanosilicates in the 4.5-1550 K range.

Ab Initio XAFS and XANES Standards

ABSTRACTAb initio x-ray-absorption fine structure (XAFS) and x-ray-absorption near edge structure (XANES) standards are developed for molecules and solids. These standard XAFS spectra are obtained from ab initio XAFS calculations, using an automated code, FEFF, which we have extended to include multiple-scattering (MS) and XANES calculations. Our treatment is based on state-of-the-art curved-wave MS theory. Sample results are presented and compared with experiment. We find that these theoretical standards are comparable with experimental measurements, yielding distance determinations better than 0.02 Å and coordination numbers better than 20%. Our approach also gives a new MS interpretation of the σ* shape-resonances in XANES.

X-ray-absorption near-edge structure of transition-metal zinc-blende semiconductors: Calculation versus experimental data and the pre-edge feature.

X-ray-absorption near-edge structure (XANES) data were collected for Zn in sphalerite (ZnS), and for Cu and Fe in chalcopyrite (${\mathrm{CuFeS}}_{2}$), where all three cations are in nearly identical coordination environments. The data have similar features, except near the edge, where the edge maximum decreases in amplitude, while a pre-edge feature appears and increases in amplitude from Zn to Cu to Fe. The pre-edge feature was previously assigned to a 1s-to-3d atomic transition for Cu and Fe in the chalcopyrite structure. XANES calculations were performed for all three edges. The multiple- and single-scattering contributions to the calculated edge spectra were scaled down to better fit the experimental data. The conclusions from the calculations indicate that the pre-edge feature in the experimental Cu- and Fe-edge data for chalcopyrite can be due to interference effects from the atomic structure surrounding the absorber, but cannot exclude the possibility that the pre-edge feature is due to atomic bound-state transitions of the absorber.

The electron energy loss near edge structure of MGO: Theory and experiment

A similiarity between theoretical calculations of the differential scattering cross section for electron energy-loss near edge structure (ELNES) and the cross section for x-ray absorption near edge structure (XANES) has been demonstrated by several workers (see, e.g. Leapman et al). Since theoretical XANES calculations using band structure methods have often given impressive agreement with experimental results, it is expected hat these same methods can be used under certain experimental conditions to fit ELNES theory to experimental data. The interpretation of ELNES may however, be subject to additional complications involving the multiple scattering of both the beam electron and the ejected core electron. Band structure calculations and multiple scattering techniques have also been used to interpret the ELNES of the carbon K-edge in Be2C. MgO has also been studied using the K.K.R. or Green's function method by Lindner et al. Here we report a further comparison of theoretical and experimental ELNES for MgO.