Extended X-ray Absorption Fine Structure Fitting Research Articles

Local structure and phase composition of Neodymium-Zircon solid solution (NdxZr1-xSiO4-x/2)

To address the structural stability of host materials for immobilizing radioactive actinides in nuclear waste management applications, the present study is focused on the effect of various concentrations of Nd substitution on structural properties of synthetic zircon (NdxZr1-xSiO4-x/2). To achieve this, both pure and Nd-substituted zircon powders were prepared using the solid-state synthesis route. Rietveld refinement of X-ray Diffraction (XRD) data was employed to investigate the impact of various Nd substitutions on the phase composition of the zircon (ZrSiO4) crystal structure. This refined data was then utilized as input for the analysis of X-ray absorption spectroscopy (XAS) data. The results were further supported by observations from Raman spectroscopy, which indicated the substitution of Zr by Nd. In this work, we present an analysis of both regimes of XAS i.e. X-ray Absorption Near Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. Qualitative analysis of XANES data suggests that disorder in the samples increases with higher levels of Nd doping. Additionally, EXAFS analysis around Zr absorption edge also confirms an increased disorder in the system with higher Nd doping levels. However, EXAFS fitting around Nd edge indicated that disorder induced changes are more prominent and Nd–O bond is elongated as compared to Zr–O bond. This disorder in the system escalates at a Nd concentration of 10 atomic % (at %). An important finding of this study is the phase composition analysis using Rietveld refinement of XRD data. While XAS provided an insight into the changes in the local structure, the overall crystal structure remained unaffected even at a high doping of 10 at %. This work provides a comprehensive local structure analysis on this system. Such analysis would help in understanding the structural stability of the system and to determining the maximum loading of actinides in the host zircon for potential nuclear waste management applications.

Local structure and structural properties of vanadium oxide thin films prepared by radio-frequency reactive magnetron sputtering at various oxygen flow rate

In this work, vanadium oxide thin films were prepared by using radio frequency reactive magnetron sputtering with vanadium metal target. The effect of O2 flow rate was investigated in terms of vanadium oxide thin films by varying O2/Ar flow rate at 1/20, 2/20, 3/20, 4/20, 5/20, 6/20 (in sccm unit) with fixed RF power at 200 W and sputtering time at 80 minutes. The surface morphology and crystal structure of V2O5 thin films were investigated by scanning electron microscope (SEM), atomic force microscope (AFM), and X-Ray Diffractometer (XRD) respectively. The influence of the O2/Ar ratio on a local structure of vanadium oxide thin films was studied by the Synchrotron x-ray absorption spectroscopy (XAS) technique. The oxidation states of vanadium oxide thin films were measured at V K-edge by the x-ray absorption near-edge structure (XANES). The results of XANES spectra at the V K-edge showed that the prepared films under the O2/Ar flow rate at 1/20 sccm contained the vanadium with the oxidation states of V3+ and V5+ while the others contained only V5+. This is because the higher O2/Ar flow rate possibly causes an increase in energy flux at the substrate and the 1s→3d transition peak’s intensity, which could lead to a higher deposition rate. To achieve the desired V2O5 structure, the gas flow rate could be adjusted properly. The evolution of the local structure around V atoms was studied using the extended x-ray absorption fine structure (EXAFS) which supported the XANES results indicating that the structural environment around V atoms is in good agreement with the V2O5 orthorhombic structure. Finally, XANES and EXAFS fitting results confirmed that the O2 flow rate affected the local structure of vanadium oxide thin films.

EXAFS wavelet transform analysis of Cu-MOR zeolites for the direct methane to methanol conversion.

Cu-exchanged zeolites have been shown to possess Cu-oxo species active towards the direct methane to methanol (DMTM) conversion, carried out through a chemical-looping approach. Different Cu-zeolites have been investigated for the DMTM process, with Cu-mordenite (Cu-MOR) being among the most active. In this context, an accurate determination of the local structure and nuclearity of selective Cu-oxo species responsible for an efficient DMTM conversion still represents an ongoing challenge for characterization methods, including synchrotron-based X-ray absorption spectroscopy (XAS). Herein, we explore the potential of an alternative analysis of Extended X-ray Absorption Fine Structure (EXAFS) data using wavelet transform (WT) to enhance the technique sensitivity to multimeric Cu species hosted in the MOR framework. Combining ex situ XAS measurements under model red-ox conditions with in situ data collected after the key steps of the DMTM process, we demonstrate how EXAFS-WT enables unambiguous detection of Cu-Cu scattering contributions from multimeric Cu-species. As also confirmed by complementary in situ IR spectroscopy results, these are observed to dynamically respond to the chemical environment over the different conditions probed. We finally report a proof-of-concept EXAFS fit using the WT representation, applied to the structural refinement of O2-activated Cu-MOR. The fitting results reveal a Cu local coordination environment consistent with mono-(μ-oxo) di-copper cores, with Cu-Cu separation of ∼3.1 Å, paving the way to future applications and developments of the method in the field of Cu-zeolite research and beyond.

Insights into the Coordination and Extraction of Yttrium(III) Ions with a Phenoxyacetic Acid Ionic‐Liquid Extractant

Phenoxyacetic acid type extractants such as sec‐octylphenoxyacetic acid (CA‐12), sec‐nonylphenoxyacetic acid (CA‐100), and the resulting bifunctional ionic‐liquid extractants were developed to separate yttrium from heavy rare‐earth elements (REEs). To obtain information on the coordination chemistry of this type of bifunctional ionic‐liquid extractant, the CA‐12 analogue n‐octylphenoxyacetic acid (HOCTPOA) was synthesized, and its structure was characterized by single‐crystal X‐ray diffraction analysis. Then, a bifunctional ionic‐liquid extractant based on HOCTPOA, namely, [N1888][OCTPOA] (N1888 is methyltrioctylammonium) was synthesized. The study of the extraction of yttrium with [N1888][OCTPOA] in chloride media was conducted by IR spectroscopy, log D versus log C slope analysis, ESI‐MS, extended X‐ray absorption fine structure (EXAFS), and DFT studies. The EXAFS data indicate that a YIII extraction complex with phenoxyacetic acid formed. Three possible YIII coordination cations were calculated by the DFT method, and the obtained Y–O bond lengths were comparable to the results from EXAFS fitting. A possible structure for the overall extracted complex was proposed, and this knowledge should contribute to a better understanding of the extraction process with phenoxyacetate‐type bifunctional ionic liquids.

Trace Uranium Partitioning in a Multiphase Nano-FeOOH System.

The characterization of trace elements in minerals using extended X-ray absorption fine structure (EXAFS) spectroscopy constitutes a first step toward understanding how impurities and contaminants interact with the host phase and the environment. However, limitations to EXAFS interpretation complicate the analysis of trace concentrations of impurities that are distributed across multiple phases in a heterogeneous system. Ab initio molecular dynamics (AIMD)-informed EXAFS analysis was employed to investigate the immobilization of trace uranium associated with nanophase iron (oxyhydr)oxides, a model system for the geochemical sequestration of radiotoxic actinides. The reductive transformation of ferrihydrite [Fe(OH)3] to nanoparticulate iron oxyhydroxide minerals in the presence of uranyl (UO2)2+(aq) resulted in the preferential incorporation of U into goethite (α-FeOOH) over lepidocrocite (γ-FeOOH), even though reaction conditions favored the formation of excess lepidocrocite. This unexpected result is supported by atomically resolved transmission electron microscopy. We demonstrate how AIMD-informed EXAFS analysis lifts the strict statistical limitations and uncertainty of traditional shell-by-shell EXAFS fitting, enabling the detailed characterization of the local bonding environment, charge compensation mechanisms, and oxidation states of polyvalent impurities in complex multiphase mineral systems.

Multishell EXAFS Fitting Analysis of a Compositionally Precise Thiolate-Gold Nanocluster

ABSTRACTThiolate-gold nanoclusters exhibit unique optical, magnetic and chiral properties, which are attractive for novel applications in nanotechnology. A fundamental challenge facing these nanomaterials is being able to study and understand their physical properties in various experimental conditions. To overcome this, extended X-ray absorption fine structure (EXAFS) spectroscopy can be employed to probe the Au local structure of thiolate-gold nanoclusters in a variety of conditions, providing valuable structural information from multiple bonding environments (i.e. metal-metal and metal-ligand interactions). This study discusses a methodology for conducting a multishell EXAFS fitting analysis that can be implemented for thiolate-gold nanocluster systems. Specifically, experimental and simulated EXAFS data for Au36(SR)24 nanoclusters are examined with a total of 5 scattering paths fitted to the experimental data.

An EXAFS study on the adsorption structure of phenyl-substituted organoarsenic compounds on ferrihydrite

Adsorption structures of mono- and diphenyl substituted organoarsenic compounds (PAA and DPAA, respectively) on ferrihydrite were analyzed by extended X-ray absorption fine structure (EXAFS), which suggested that PAA and DPAA form inner-sphere complexes with ferrihydrite regardless of their bulky functional groups. In addition, coexistence of two types of inner-sphere complex modes, i.e. bidentate–binuclear and monodentate surface complexes was suggested by EXAFS fitting with two type of the AsFe shell.

Probing local structure of pyrochlore lead zinc niobate with synchrotron x-ray absorption spectroscopy technique

Local structure of lead zinc niobate (PZN) ceramic, synthesized via B-site oxide precursor route in atmospheric pressure, was investigated using synchrotron x-ray absorption spectroscopy (XAS) technique. The x-ray absorption near-edge structure (XANES) simulation was first carried out. The XANES simulation results indicate that the PZN ceramic is in pyrochlore phase having Zn2+ substituted on Nb5+ site. Afterwards, the extended x-ray absorption fine structure (EXAFS) analysis was performed to extract the bond length information between Zn2+ and its neighboring atoms. From the EXAFS fitting, the bond length between Zn2+ and Pb2+ in the pyrochlore phase was found to be longer than the previously reported bond length in the perovskite phase. Further, with the radial distribution information of Zn2+'s neighboring atoms, the formation energies along the precursor-to-pyrochlore and precursor-to-perovskite reaction paths were calculated using the density functional theory (DFT). The calculated results show that the formation energy of the perovskite phase is noticeably higher than that of the pyrochlore phase, which is influenced by the presence of energetic Pb2+ lone pair, as the perovskite phase has shorter Zn2+ to Pb2+ bonding. This therefore suggests the steric hindrance of Pb2+ lone pair and the mutual interactions between Pb2+ lone pair and Zn2+ are main causes of the instability of lead zinc niobate in the perovskite structure and confirm the efficacy of XAS and DFT analysis in revealing local structural details of complex pyrochlore materials.

Quantitative Analysis of Modulated Excitation X-ray Absorption Spectra: Enhanced Precision of EXAFS Fitting

The sensitivity of X-ray absorption spectroscopy (XAS) can be increased by using the modulated excitation approach, where the sample is excited with a periodic external stimulation (e.g., changes in temperature, gas/liquid concentration, etc.) and the corresponding spectra are filtered with the frequency of the excitation. The resulting demodulated spectra contain similar spectral features as difference spectra but to much higher k values because of the isolation of the structural change that occurs with the same frequency as that of the excitation. Furthermore, multiple intermediate species that occur with a phase delay can uniquely be differentiated. We present a robust approach for the quantitative analysis of demodulated extended X-ray absorption fine structure (EXAFS) spectra. Simulations show that the proposed fitting approach resolves small changes in the sample’s structure with greatly enhanced precision. Experimentally, this is demonstrated by the formation of ruthenium oxide species upon partial oxidation of a Ru metal particle, which cannot be detected in a standard EXAFS experiment.