Abstract: Reverse Monte Carlo (RMC) refinements of local structure using a simultaneous fit of X-ray/neutron total scattering and extended X-ray absorption fine structure (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 SrAl1/2Nb1/2O3. 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 overlapping peaks of the constituent partial PDFs (e.g. Al—O and Nb—O or Sr—O and O—O). The results obtained here suggest that the combined use of the total scattering and EXAFS data provides sufficient constraints for RMC refinements to recover fine details of local structure in complex perovskites. Among other results, it was found that the probability density distribution for Sr in SrAl1/2Nb1/2O3 adopts Td point-group symmetry for the Sr sites, determined by the ordered arrangement of Al and Nb, as opposed to a spherical distribution commonly assumed in traditional Rietveld refinements.
TL;DR: The combination of AET and atom-tracing algorithms has enabled the determination of the coordinates of individual atoms and point defects in materials with a 3D precision, allowing direct measurements of 3D atomic displacements and the full strain tensor.
Abstract: Crystallography has been fundamental to the development of many fields of science over the last century. However, much of our modern science and technology relies on materials with defects and disorders, and their three-dimensional (3D) atomic structures are not accessible to crystallography. One method capable of addressing this major challenge is atomic electron tomography. By combining advanced electron microscopes and detectors with powerful data analysis and tomographic reconstruction algorithms, it is now possible to determine the 3D atomic structure of crystal defects such as grain boundaries, stacking faults, dislocations, and point defects, as well as to precisely localize the 3D coordinates of individual atoms in materials without assuming crystallinity. Here we review the recent advances and the interdisciplinary science enabled by this methodology. We also outline further research needed for atomic electron tomography to address long-standing unresolved problems in the physical sciences.
Cites background from "A Combined Fit of Total Scattering ..."
...Combining detailed structures from AET with total scattering approaches holds the transformational promise of giving us truly robust models of complex, heterogeneous materials in action; helping us to understand why high-performance materials work so well; and providing insights into how to design better ones (130, 131)....
TL;DR: The limits and possibilities of X-ray absorption near-edge spectroscopy in determining several effects associated with the nanocrystalline nature of materials are discussed in connection with the development of ZnO-based dilute magnetic semiconductors and iron oxide nanoparticles.
Abstract: Worldwide research activity at the nanoscale is triggering the appearance of new, and frequently surprising, materials properties in which the increasing importance of surface and interface effects plays a fundamental role. This opens further possibilities in the development of new multifunctional materials with tuned physical properties that do not arise together at the bulk scale. Unfortunately, the standard methods currently available for solving the atomic structure of bulk crystals fail for nanomaterials due to nanoscale effects (very small crystallite sizes, large surface-to-volume ratio, near-surface relaxation, local lattice distortions etc.). As a consequence, a critical reexamination of the available local-structure characterization methods is needed. This work discusses the real possibilities and limits of X-ray absorption spectroscopy (XAS) analysis at the nanoscale. To this end, the present state of the art for the interpretation of extended X-ray absorption fine structure (EXAFS) is described, including an advanced approach based on the use of classical molecular dynamics and its application to nickel oxide nanoparticles. The limits and possibilities of X-ray absorption near-edge spectroscopy (XANES) to determine several effects associated with the nanocrystalline nature of materials are discussed in connection with the development of ZnO-based dilute magnetic semiconductors (DMSs) and iron oxide nanoparticles.
Cites background from "A Combined Fit of Total Scattering ..."
...…(Metropolis et al., 1953) or reverse Monte Carlo (RMC) simulations (Winterer, 2000; McGreevy, 2001; Di Cicco & Trapananti, 2005; Gereben et al., 2007; Krayzman et al., 2009; Krayzman & Levin, 2010; Levin et al., 2014; Timoshenko, Anspoks et al., 2014a,b; Timoshenko, Kuzmin & Purans, 2014)....
Abstract: While the lattice volume in the solid-solution Ba1−xCaxTiO3 decreases with increasing x, the Curie temperature remains unaffected, in contrast to Ba1−xSrxTiO3. We have determined the origin of this phenomenon by comparing the local structures in (Ba,Ca)TiO3 and (Ba,Sr)TiO3. Reverse Monte Carlo refinements of instantaneous atomic positions using simultaneous fitting of multiple types of experimental data (neutron total scattering, X-ray absorption fine structure, patterns of diffuse scattering in electron diffraction) reveal both ferroelectric Ca displacements and their amplification of the Ti off-centering, which mitigate the lattice-volume effects. The activity of Ca is triggered by the anomalously strained Ca-O bonds.
TL;DR: The design and performance of an improved XAFS and XES spectrometer based on the general conceptual design of Seidler et al. are reported, which enables a new class of routine applications that are incompatible with the mission and access model of the synchrotron light sources.
Abstract: X-ray absorption fine structure (XAFS) and x-ray emission spectroscopy (XES) are advanced x-ray spectroscopies that impact a wide range of disciplines. However, unlike the majority of other spectroscopic methods, XAFS and XES are accompanied by an unusual access model, wherein the dominant use of the technique is for premier research studies at world-class facilities, i.e., synchrotron x-ray light sources. In this paper, we report the design and performance