Structure determination from XAFS using high-accuracy measurements of x-ray mass attenuation coefficients of silver, 11 keV–28 keV, and development of an all-energies approach to local dynamical analysis of bond length, revealing variation of effective thermal contributions across the XAFS spectrum

Abstract

We use the x-ray extended range technique (XERT) to experimentally determine the mass attenuation coefficient of silver in the x-ray energy range 11 kev-28 kev including the silver K absorption edge. The results are accurate to better than 0.1%, permitting critical tests of atomic and solid state theory. This is one of the most accurate demonstrations of cross-platform accuracy in synchrotron studies thus far. We derive the mass absorption coefficients and the imaginary component of the form factor over this range. We apply conventional XAFS analytic techniques, extended to include error propagation and uncertainty, yielding bond lengths accurate to approximately 0.24% and thermal Debye-Waller parameters accurate to 30%. We then introduce the FDMX technique for accurate analysis of such data across the full XAFS spectrum, built on full-potential theory, yielding a bond length accuracy of order 0.1% and the demonstration that a single Debye parameter is inadequate and inconsistent across the XAFS range. Two effective Debye-Waller parameters are determined: a high-energy value based on the highly-correlated motion of bonded atoms (σ(DW) = 0.1413(21) Å), and an uncorrelated bulk value (σ(DW) = 0.1766(9) Å) in good agreement with that derived from (room-temperature) crystallography.

FDMX: extended X-ray absorption fine structure calculations using the finite difference method

“…The resulting XANES and XAFS spectra are in very strong agreement with the experimental data, as has previously been shown with prototype FDMX calculations for solid metals (Chantler & Bourke, 2014a;Tantau et al, 2015). The only significant difference appears in the height of the peak at 83 eV, most likely due to an overestimate of the electron IMFP in the default data (Bourke & Chantler, 2012).…”

“…The FDMX package is the first full-potential package capable of calculating robust and accurate XAFS spectra across all applicable energies within a single self-consistent computation. Over the course of development of the package, this capacity has been shown to enable new extractions of fundamental physical properties from experimental XAFS spectra (Bourke & Chantler, 2010b;Tantau et al, 2015), which in turn have led to new insights in fundamental theory (Chantler & Bourke, 2014b;.…”

“…This formalism enables a sensible estimate of the ITP 2 j of the dominant scattering path j, and hence a sufficiently accurate value for the effective ITP 2 eff in many applications. In the high-energy limit, for a mono-atomic cubic structure, it has been shown to approach the correct experimental value from crystallography (Tantau et al, 2015). The ITP is implemented within FDMX by a direct adjustment to the calculated absorption cross section following…”

“…A final contribution of background absorption is sometimes necessary in order to directly compare theoretical results with experiment spectra on an absolute scale. This is due to the edge-jump discrepancy, sometimes called the triangle effect (Tantau et al, 2015), that is currently common to all photoabsorption computations (Chantler & Bourke, 2014a). By default this contribution is not included in FDMX, but the user may add an additional background absorption using the Expntl or Victoreen keywords.…”

Structural investigation of mMNi(II) complex isomers using transmission XAFS: the significance of model development

“…Data on key experimental systematics including energy calibration, dark current, harmonic contamination and scattering were measured and the spectra corrected using published procedures (Islam et al, 2014;Glover & Chantler, 2009;Tantau et al, 2015;Tran et al, 2004;Barnea et al, 2011). Detailed analytical methodologies are addressed by Chantler et al (2015).…”