A Combined Fit of Total Scattering and Extended X-ray Absorption Fine Structure Data for Local-Structure Determination in Crystalline Materials
Igor Levin, Victor L. Krayzman, Joseph C. Woicik, Terrell A. Vanderah, M. G. Tucker, Thomas Proffen
Reverse Monte Carlo (RMC) refinements of local structure using a simultaneous fit of x-ray/neutron total scattering and EXAFS data were developed to incorporate an explicit treatment of both single- and multiple-scattering contributions to EXAFS. The refinement algorithm, implemented as an extension to the public-domain computer software RMCProfile, enables accurate modeling of EXAFS over distances encompassing several coordination shells around the absorbing species. The approach was first tested on Ni, which exhibits extensive multiple scattering in EXAFS, and then applied to perovskite-like Sr(Al Nb )O3. This compound crystallizes with a cubic double-perovskite structure but presents a challenge for local-structure determination using a total pair-distribution function (PDF) alone because of a close overlap between the peaks of constituent partial PDFs (e.g. Al-O and Nb-O or Sr-O and O-O). Our results suggest that a combined use of the total scattering and EXAFS provides sufficient constraints for RMC refinements to recover fine details of local structure in complex perovskites. Among other results, we found that the probability density distribution for Sr in Sr(Al Nb )O3 adopts Td point-group symmetry of the Sr sites, determined by the ordered arrangement of Al and Nb, as opposed to a spherical distribution commonly assumed in traditional Rietveld refinements.