Journal of Synchrotron Radiation

Volume 20, Part 4 (July 2013)

research papers

J. Synchrotron Rad. (2013). 20, 555-566    [ doi:10.1107/S0909049513010303 ]

Predicting XAFS scattering path cumulants and XAFS spectra for metals (Cu, Ni, Fe, Ti, Au) using molecular dynamics simulations

M. A. Karolewski, R. G. Cavell, R. A. Gordon, C. J. Glover, M. Cheah and M. C. Ridgway

Abstract: The ability of molecular dynamics (MD) simulations to support the analysis of X-ray absorption fine-structure (XAFS) data for metals is evaluated. The low-order cumulants ([Delta]R, [sigma]2, C3) for XAFS scattering paths are calculated for the metals Cu, Ni, Fe, Ti and Au at 300 K using 28 interatomic potentials of the embedded-atom method type. The MD cumulant predictions were evaluated within a cumulant expansion XAFS fitting model, using global (path-independent) scaling factors. Direct simulations of the corresponding XAFS spectra, [chi](R), are also performed using MD configurational data in combination with the FEFF ab initio code. The cumulant scaling parameters compensate for differences between the real and effective scattering path distributions, and for any errors that might exist in the MD predictions and in the experimental data. The fitted value of [Delta]R is susceptible to experimental errors and inadvertent lattice thermal expansion in the simulation crystallites. The unadjusted predictions of [sigma]2 vary in accuracy, but do not show a consistent bias for any metal except Au, for which all potentials overestimate [sigma]2. The unadjusted C3 predictions produced by different potentials display only order-of-magnitude consistency. The accuracy of direct simulations of [chi](R) for a given metal varies among the different potentials. For each of the metals Cu, Ni, Fe and Ti, one or more of the tested potentials was found to provide a reasonable simulation of [chi](R). However, none of the potentials tested for Au was sufficiently accurate for this purpose.

Keywords: XAFS; metals; molecular dynamics; potentials; cumulants.

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[ doi:10.1107/S0909049513010303/hf5227sup1.pdf ]
MD third cumulant predictions for Ni, Fe and Ti; XAFS data for Ni, Fe, Ti and Au fitted with scaled MD cumulant.


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