A software package for the analysis of X-ray absorption spectroscopy (XAS) data is presented. This package is based on the IFEFFIT library of numerical and XAS algorithms and is written in the Perl programming language using the Perl/Tk graphics toolkit. The programs described here are: (i) ATHENA, a program for XAS data processing, (ii) ARTEMIS, a program for EXAFS data analysis using theoretical standards from FEFF and (iii) HEPHAESTUS, a collection of beamline utilities based on tables of atomic absorption data. These programs enable high-quality data analysis that is accessible to novices while still powerful enough to meet the demands of an expert practitioner. The programs run on all major computer platforms and are freely available under the terms of a free software license.

http://cars9.uchicago.edu/xafs_school/Material/Articles/Horae%20JSR2005.pdf

ATHENA, ARTEMIS, HEPHAESTUS: data analysis for X-ray absorption spectroscopy using IFEFFIT.

Highly active catalysts for the oxygen evolution reaction (OER) are required for the development of photoelectrochemical devices that generate hydrogen efficiently from water using solar energy. Here, we identify the origin of a 500-fold OER activity enhancement that can be achieved with mixed (Ni,Fe)oxyhydroxides (Ni(1-x)Fe(x)OOH) over their pure Ni and Fe parent compounds, resulting in one of the most active currently known OER catalysts in alkaline electrolyte. Operando X-ray absorption spectroscopy (XAS) using high energy resolution fluorescence detection (HERFD) reveals that Fe(3+) in Ni(1-x)Fe(x)OOH occupies octahedral sites with unusually short Fe-O bond distances, induced by edge-sharing with surrounding [NiO6] octahedra. Using computational methods, we establish that this structural motif results in near optimal adsorption energies of OER intermediates and low overpotentials at Fe sites. By contrast, Ni sites in Ni(1-x)Fe(x)OOH are not active sites for the oxidation of water.

/pdf/identification-of-highly-active-fe-sites-in-ni-fe-ooh-for-1kxniexq7l.pdf

Identification of Highly Active Fe Sites in (Ni,Fe)OOH for Electrocatalytic Water Splitting

Methanol oxidation and direct methanol fuel cells: a selective review

SIXPack (Sam's Interface for XAS analysis Package), a graphical user interface that allows users simple manipulation and analysis of data, is presented. The modules of SIXPack allow users to: (1) load, calibrate, and average raw data files; (2) perform background subtractions; (3) perform principal component analysis and target transforms; (4) perform least squares fitting of data to standards and functions; (5) perform EXAFS fitting to FEFF phase and amplitude files; (6) create single scattering FEFF phase and amplitude files using a periodic table interface. Novel features of the program allow for the fitted correction of XANES spectra due to self-absorption effects in unknown matrices, which is particularly useful for analysis of geochemical and environmental systems. The core of the XAS analysis routine uses IFEFFIT. SIXPack is developed in Python, is installable across many operating systems and platforms, and is freely available with an Open Source license.

https://iopscience.iop.org/article/10.1238/Physica.Topical.115a01011/pdf

SIXpack: A Graphical user interface for XAS analysis using IFEFFIT

Steam Reforming and Graphite Formation on Ni Catalysts

A high-order multiple-scattering (MS) approach to the calculation of polarized x-ray-absorption spectra, which includes both x-ray-absorption fine structure and x-ray-absorption near-edge structure, is presented. Efficient calculations in arbitrary systems are carried out by using a curved-wave MS path formalism that ignores negligible paths, and has an energy-dependent self-energy and MS Debye-Waller factors. Embedded-atom background absorption calculations on an absolute energy scale are included. The theory is illustrated for metallic Cu, Cd, and Pt. For these cases the MS expansion is found to converge to within typical experimental accuracy, both to experiment and to full MS theories (e.g., band structure), by using only a few dozen important paths, which are primarily single-scattering, focusing, linear, and triangular.

Multiple-scattering calculations of x-ray-absorption spectra

Theoretical X-ray Absorption Fine Structure Standards

High-order scattering is found to be essential for the convergence of the multiple-scattering (MS) theory of x-ray-absorption fine structure, both in the near-edge and the extended regimes. These contributions are calculated using an ab initio curved-wave scattering-matrix formalism. Convergence to full MS accuracy is demonstrated for fcc Cu, as well as for molecular ${\mathrm{O}}_{2}$ and ${\mathrm{N}}_{2}$, where our approach provides a high-order MS interpretation of the ${\mathrm{\ensuremath{\sigma}}}^{\mathrm{*}}$ shape resonances.

High-order multiple-scattering calculations of x-ray-absorption fine structure.

The most important elements of {ital ab} {ital initio} calculations of x-ray-absorption fine structure (XAFS) are studied. To obtain accurate results without {ital ad} {ital hoc} adjustable parameters, we find it essential to include (i) curved-wave effects, (ii) a complex, energy-dependent self-energy, (iii) an approximate molecular potential, and (iv) a fixed energy reference for the photoelectron wave number. Based on these findings, an automated code has been developed for {ital ab} {ital initio} calculations of single-scattering XAFS, in which curved-wave effects are treated exactly in terms of effective backscattering amplitudes, inelastic losses and self-energy shifts are incorporated with use of a Hedin-Lundqvist self-energy, an automated relativistic overlapping-atom muffin-tin potential is used, and the energy threshold is estimated from electron-gas theory. The efficiency of the code is made possible by analytic expressions for the Hedin-Lundqvist self-energy. This code replaces existing tables of XAFS phases and scattering amplitudes and yields reliable theoretical XAFS standards for arbitrary pairs of atoms throughout the Periodic Table ({ital Z}{le}94). These results are comparable to those from self-consistent calculations and are valid to within about 20 eV of the absorption edge. Comparisons with experiment are presented for Cu, Ge, Pt, Br{sub 2}, and GeCl{sub 4}. The calculatedmore » XAFS amplitudes are found to be accurate to within 15%; XAFS phases are accurate to within 0.2 rad; and nearest-neighbor distances are typically accurate to within 0.02 A.« less

Ab initio curved-wave x-ray-absorption fine structure.

Quantitative determination of local atomic structure in complex materials using extended X-ray absorption fine structure (EXAFS) analysis was tested on eight inorganic compounds of known structure, including natural and synthetic crystalline solids, at ambient conditions. Our aim was to test the accuracy of experimental and theoretical EXAFS standard functions in determining the number of backscattering atoms (N) at a distance (R) beyond the ligating shell of the central absorber atom where effects from disorder, multiple-scattering, and overlapping shells of atoms may significantly influence the EXAFS spectra. These compounds have complicated structures compared to metals and contain Fe, Co, or Ni as the central absorbing atom and mixtures of second-row (C,O,F), third-row (Si, Cl), and fourth-row (Ca, Fe, Co, Ni) atoms as backscatters. Comparison of results using both experimental phase shift and amplitude functions (derived from the EXAFS spectra of the compounds) and those calculated from ab initio theory (using the computer code FEFF 5) shows that interatomic distances for single-scattering paths among metal atoms can be determined to within 0.02 A of values determined independently by X-ray diffraction up to a distance of 4 A from the central absorber by either method. 25 refs., 7 figs., 6 tabs.

S. I. Zabinsky

Papers

Multiple-scattering calculations of x-ray-absorption spectra

Theoretical X-ray Absorption Fine Structure Standards

High-order multiple-scattering calculations of x-ray-absorption fine structure.

Ab initio curved-wave x-ray-absorption fine structure.

Extended x-ray absorption fine structure (exafs) analysis of disorder and multiple-scattering in complex crystalline solids