EXAFS Research Articles

Biogeochemical interaction between thallium (Tl) and schwertmannite in acidic environment and the anti-dissolution mechanisms of Tl(I)-coprecipitated schwertmannite

Highly toxic thallium (Tl) can be released into the environment through acid mine drainage (AMD). However, our knowledge on the biogeochemical processes of Tl in such acidic, iron (Fe)-rich environments is limited. Here, we show that schwertmannite, a naturally formed Fe(III) mineral in AMD, can effectively immobilize Tl(I) through coprecipitation and adsorption. Tl(I) coprecipitation into schwertmannite removed a large portion of Tl(I) under a wide range of initial Tl(I) concentrations (0.01~1.0mg/L) and within a short duration (48h). The saturated adsorption capacities of the biosynthetic and chemically synthesized schwertmannite for Tl(I) (1.0mg/L) were 1.96 and 1.59mg/g, respectively, under acidic conditions (pH=3.0). The kinetic dissolution results indicated that biogenic Tl-coprecipitated schwertmannite exhibited greater stability, which was attributed mainly to the elevated extent of Tl oxidation and enhanced crystallinity of Tl-bearing schwertmannite. The extended X-ray absorption fine structure (EXAFS) analyses revealed that the incorporation of Tl into schwertmannite involves the heterovalent substitution of Fe(III) by the formation of double-corner sharing linkages between the Tl-O tetrahedra and Fe-O octahedra. These results suggested that coprecipitation combined with adsorption can achieve retention of Tl in acidic environment throughout the entire mineralization process of schwertmannite, which provides a comprehensive understanding of biogeochemical fate of Tl in AMD-affected areas.

Advanced EXAFS analysis techniques applied to the L-edges of the lanthanide oxides.

The unique properties of the lanthanide (Ln) elements make them critical components of modern technologies, such as lasers, anti-corrosive films and catalysts. Thus, there is significant interest in establishing structure-property relationships for Ln-containing materials to advance these technologies. Extended X-ray absorption fine structure (EXAFS) is an excellent technique for this task considering its ability to determine the average local structure around the Ln atoms for both crystalline and amorphous materials. However, the limited availability of EXAFS reference spectra of the Ln oxides and challenges in the EXAFS analysis have hindered the application of this technique to these elements. The challenges include the limited k-range available for the analysis due to the superposition of L-edges on the EXAFS, multielectron excitations (MEEs) creating erroneous peaks in the EXAFS and the presence of inequivalent absorption sites. Herein, we removed MEEs to model the local atomic environment more accurately for light Ln oxides. Further, we investigated the use of cubic and non-cubic lattice expansion to minimize the fitting parameters needed and connect the fitting parameters to physically meaningful crystal parameters. The cubic expansion reduced the number of fitting parameters but resulted in a statistically worse fit. The non-cubic expansion resulted in a similar quality fit and showed non-isotropic expansion in the crystal lattice of Nd2O3. In total, the EXAFS spectra and the fits for the entire set of Ln oxides (excluding promethium) are included. The knowledge developed here can assist in the structural determination of a wide variety of Ln compounds and can further studies on their structure-property relationships.

Pairing a Global Optimization Algorithm with EXAFS to Characterize Lanthanide Structure in Solution.

Ensemble-average sampling of structures from ab initio molecular dynamics (AIMD) simulations can be used to predict theoretical extended X-ray absorption fine structure (EXAFS) signals that closely match experimental spectra. However, AIMD simulations are time-consuming and resource-intensive, particularly for solvated lanthanide ions, which often form multiple nonrigid geometries with high coordination numbers. To accelerate the characterization of lanthanide structures in solution, we employed the Northwest Potential Energy Surface Search Engine (NWPEsSe), an adaptive-learning global optimization algorithm, to efficiently screen first-shell structures. As case studies, we examine two systems: Eu(NO3)3 dissolved in acetonitrile with a terpyridine ligand (terpyNO2), and Nd(NO3)3 dissolved in acetonitrile. The theoretical spectra for structures identified by NWPEsSe were compared to both experimental and AIMD-derived EXAFS spectra. The NWPEsSe algorithm successfully identified the proper solvation structure for both Eu(NO3)3(terpyNO2) and Nd(NO3)(acetonitrile)3, with the calculated EXAFS signals closely matching the experimental spectra for the Eu-ligand complex and showing good similarity for the Nd salt; the better agreement with the ligand-containing structure is attributed to a less dynamic coordination environment due to the rigid ligand. The key advantage of the global optimization algorithm lies in its ability to sample the coordination environment across the potential energy surface and reduce the time required to identify structures from generally a month to within a week. Additionally, this approach is versatile and can be adapted to characterize main-group metal complexes.

Atomically Tailored Zn-ZIF-8 via RuNi Nanoalloy Replacement for Improved Photocatalytic H2 Evolution.

In this study, we developed a solid-state atomic replacement method for metal catalysts, enabling the exchange of metal atoms between single atoms and nanoalloys to create new combinations of nanoalloys and single atoms. We observed that partial metal interchange occurred between the RuNi nanoalloy and Zn from the zeolitic imidazolate framework-8 (ZIF-8) on a carbon-nitrogen framework (CNF) at a high temperature of 900 °C, leading to the creation of RuZn nanoparticles and single nickel atoms (Ni-CN). Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) analyses revealed that Ni is atomically dispersed within (RuZn)/Ni-CN. This finding confirms the migration of Zn and Ni during the pyrolysis of the RuNi@ZIF-8 precursor, providing definitive evidence of atomic replacement. Due to the synergistic influence of RuZn nanocrystals and Ni-CN, the resulting (RuZn)/Ni-CN multisite catalyst exhibited superior hydrogen evolution reaction (HER) ability compared to the conventional nanoalloy-based catalysts. Density functional theory calculations revealed that the integration of the (RuZn)n cluster on Ni surrounded with different N-coordinated carbon structures enhanced HER activity with the optimized (RuZn)n/NiN2C2 catalyst exhibiting a low ΔGH and improved electron charge redistribution, thereby promoting favorable hydrogen adsorption. Our findings provide valuable insights into the design and optimization of photocatalysts through atomic-level engineering, opening new avenues for efficient and sustainable energy conversion technologies.

Does Pb2+ Form Holodirected or Hemidirected Solvation Geometries in Water?

The hydration properties of the Pb2+ ion in aqueous solution have been investigated by using a synergic approach based on Classical and Car-Parrinello molecular dynamics (CPMD) simulations and extended X-ray absorption fine structure (EXAFS) spectroscopy. A definite answer has been given to the main question on the Pb2+ hydration structure, which concerns the formation of either holodirected or hemidirected solvation geometries, such terms referring to the arrangements of the ligands that can be directed either throughout the surface of an encompassing globe around Pb2+ or throughout only part of the globe, respectively. Our CPMD results show that the Pb2+ ion in water forms a hemidirected 4-fold cluster with a well-defined distorted pyramidal geometry. This cluster is directed throughout one side of the first shell globe, while the other side contains either two or three very mobile water molecules that do not form a well-defined geometry around the Pb2+ ion. The Pb2+ first shell structural arrangement determined from the CPMD simulation was confirmed by the EXAFS experimental results. A homodirected Pb2+ first shell complex like the 8-fold SAP structure obtained from the classical MD simulations cannot be reconciled with the EXAFS experimental data. These findings represent a significant step forward in the understanding of the solvation chemistry of the Pb2+ ion, which is fundamental to improving the efficiency of lead removal procedures that are crucial to the safety of water resources.

Visualizing the Structure and Dynamics of Transition Metal-Based Electrocatalysts Using Synchrotron X-Ray Absorption Spectroscopy.

As the global energy structure evolves and clean energy technologies advance, electrocatalysis has become a focal point as a critical conversion pathway in the new energy sector. Transitional metal electrocatalysts (TMEs) with their distinctive electronic structures and redox properties show great potential in electrocatalytic reactions. However, complex reaction mechanisms and kinetic limitations hinder the improvement of energy conversion efficiency, highlighting the necessity for comprehensive studies on structure and performance of electrocatalysts. X-ray Absorption Fine Structure (XAFS) spectra stand out as a robust tool for examining the electrocatalyst's structures and performance due to its atomic selectivity and sensitivity to local environments. This review delves into the application of XAFS technology in characterizing TMEs, providing in-depth analyses of X-ray Absorption Near-Edge Structure (XANES) spectra, and Extended XAFS (EXAFS) spectra in both R-space and k-space. These analyses reveal intrinsic structural information, electronic interactions, catalyst stability, and aggregation morphology. Furthermore, the paper examines advancements in in-situ XAFS techniques for real-time monitoring of active site changes, capturing critical intermediate and transitional states, and elucidating the evolution of active species during electrocatalytic reactions. These insights deepen our understanding on structure-activity relationship of electrocatalysts and offer valuable guidance for designing and developing highly active and stable electrocatalysts.

Formation of β-U3O8 from UCl3 Salt Compositions under Oxygen Exposure.

Complementary X-ray absorption fine structure (XAFS) and Raman spectroscopy studies were conducted on various UCl3 concentrations in alkali chloride salt compositions. The samples were 5 mol % UCl3 in LiCl (S1), 5 mol % UCl3 in KCl (S2), 5 mol % UCl3 in LiCl-KCl eutectic (S4), 50 mol % UCl3 in KCl (S5), and 20 mol % UCl3 in KCl (S6) molar concentrations. Samples were heated to 800 °C and allowed to cool to room temperature with measurements performed at selected temperatures; the highest temperatures showed the most stability and will be primarily referenced for conclusions. The processing and interpretation of the Raman and extended X-ray absorption fine structure (EXAFS) peaks revealed several uranium-oxygen bond lengths and symmetries in the samples before, during, and after heating. Based on published thermodynamic data of similar systems, X-ray absorption fine structure spectroscopy, and identification of Raman peaks, a β variation of α-U3O8, typical at room temperature, is the suspected dominant phase of all samples at high temperatures (800 °C). In the existing literature, this β structure of U3O8 was synthesized by slow cooling of uranium oxides from 1350 °C. This paper suggests the rapid formation of the compound due to the decomposition of the uranium chlorides or oxychlorides at increasing temperatures and O2 reaction kinetics.

Orientation-dependent structural properties during growth and growth mechanism of CoO films

The orientation-dependent local structural properties of CoO films on sapphire substrates during growth were investigated through linearly-polarized extended X-ray absorption fine structure (EXAFS) measurements. Specifically, CoO(111) and (100) crystals with a rock-salt structure (Fm3m) were epitaxially grown on α-Al2O3(0001) and (101¯2) substrates, respectively, at 700℃ using a radio-frequency sputtering system. The local structural properties of CoO films in the in-plane and out-of-plane orientations were quantitatively determined using linearly-polarized EXAFS at the Co K-edge during growth. The EXAFS analysis revealed that during the initial stages of growth, the local structural properties exhibit significant differences compared to thick films, with short atomic distances and large (small) Debye-Waller factors observed in the out-of-plane (in-plane) orientations. The local structural strain mostly diminished when approximately 20 CoO layers accumulated on the substrate. Density functional theory (DFT) calculations further supported these findings by confirming that cobalt atoms initially form stable bonds with the sapphire surface, leading to the simultaneous growth of oxygen and cobalt layers in a coordinated manner through layer-by-layer growth.

Effective photocatalytic hydrogen generation and degradation for single cobalt and tungsten metal atom oxide anchored on titanium dioxide-bismuth molybdate-reduced graphene oxide

A simple hydrothermal method was used to prepare the single-metal atom oxide (SMAO) catalyst. The prepared dual Cobalt and tungsten (CoW) single metal atom oxide anchored on the TiO2-Bi2MoO6-reduced graphene oxide (rGO), an interstitial atomic line of tungsten and cobalt. The prepared photocatalyst showed good photocatalytic hydrogen (H2) evolution and organic contaminant degradation. As co-catalysts, surface-dispersed CoW sites were created using a Co mono-substituted hetero-polyacid (HPA-Co). A larger quantity of single atoms were uniformly decorated on the TiO2-Bi2MoO6-Ethylenediamine (ED)-rGO catalyst. The presence of the CoW species was verified employing extended X-ray absorption near edge structure (XANES), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and extended X-ray absorption fine structure (EXAFS) analyses after a TiO2-Bi2MoO6-CoW-ED-rGO composite synthesized. The findings demonstrate that TiO2-Bi2MoO6-CoW-ED-rGO catalysts were more capable of producing hydrogen and degrading Ciprofloxacin (CIP) (21.46 mmol/g/h, 97.2%) than other bare and binary catalysts. Furthermore, it showed remarkable stability and reusability after five consecutive CIP photodegradation and H2 production cycles. Gas chromatography/mass spectroscopy (GC/MS) techniques were used to identify the intermediates in the photodegradation process. The prepared photocatalyst was significantly increased, and the separation of charge carriers was further boosted, thanks to the CoW material and the synergistic effect. A possible Z-scheme mechanism was proposed for the photocatalytic H2 production activity. More electrons can contribute to the reduction of H2 evolution because of the processability of the Z-scheme. This work created a recyclable, inexpensive, highly effective, and non-toxic catalytic material for H2 generation and CIP degradation.

Silicate coprecipitation reduces green rust crystal size and limits dissolution-precipitation during air oxidation

Green rusts (GR) are mixed-valence iron (Fe) hydroxides which form in reducing redox environments like riparian and wetland soils and shallow groundwater. In these environments, silicon (Si) can influence Fe oxides’ chemical and physical properties but its role in GR formation and subsequent oxidative transformation have not been studied starting at initial nucleation. Green rust sulfate [GR(SO4)] and green rust carbonate [GR(CO3)] were both coprecipitated from salts by base titration in increasing % mol Si (0, 1, 10, and 50). The minerals were characterized before and after rapid (24 h) aqueous air-oxidation by x-ray diffraction (XRD), scanning electron microscopy (SEM), Fe extended x-ray absorption fine structure spectroscopy (EXAFS), and N2-BET surface area. Results showed that only GR(SO4) or GR(CO3) was formed at every tested Si concentration. Increasing % mol Si caused decreased plate size and increased surface area in GR(CO3) but not GR(SO4). GR plate basal thickness was not changed at any condition indicating a lack of Si interlayering. Air oxidation of GR(SO4) at all % mol Si contents transformed by dissolution and reprecipitation into lepidocrocite and goethite, favoring ferrihydrite with higher % Si content. Air oxidation of GR(CO3) transformed into magnetite and goethite but increasing Si caused GR to oxidize while retaining its hexagonal plate structure via solid-state oxidation. Our results indicate that Si has the potential to cause GR to form in smaller particles and upon air oxidation, Si can either stabilize the plate structure or alter transformation to ferrihydrite.Graphical

Mechanism Insight into Direct Amidation Catalyzed by Zr Salts: Evidence of Zr Oxo Clusters as Active Species.

The capricious reactivity and speciation of earth-abundant metals (EAM) hinder the mechanistic understanding essential to boost their efficiency and versatility in catalysis. Moreover, metal's solution chemistry and reactivity are conventionally controlled using organic ligands, while their fundamental chemistry in operando conditions is often overlooked. However, in this study, we showcase how a better understanding of in operando conditions may result in improved catalytic reactions. By assessing the composition and structure of active species for Zr-catalyzed direct amide bond formations under operating conditions, we discovered zirconium oxo clusters form quickly and are likely active species in the reactions. Formation of these clusters dismisses the use of additional organic ligands, inert atmosphere, anhydrous solvents, or even water scavenging to provide amides in good to excellent yields. More specifically, dodeca- and hexazirconium oxo clusters (Zr12 and Zr6, respectively) rapidly form from commercial, readily available Zr salts under reaction conditions known to afford amides directly from nonactivated carboxylic acid and amine substrates. Extended X-ray absorption fine structure (EXAFS) experiments confirm the presence of oxo clusters in solution throughout the reaction, while their key role in the mechanism is supported by an in-depth computational study employing density functional theory (DFT) and molecular dynamics (MD) methods. These results underline the value of (earth-abundant) metals' intrinsic solution chemistry to transformative mechanistic understanding and to enhance the sustainability of organic transformations.

Elucidating Polyphosphate Anion Binding to Lanthanide Complexes Using EXAFS and Pulsed EPR Spectroscopy.

Reversible anion binding to lanthanide complexes in aqueous solution has emerged as an effective method for anion sensing. Through careful design of the organic ligand, luminescent lanthanide complexes capable of binding biologically relevant anions in a bidentate or monodentate manner can be realized. While single-crystal X-ray diffraction analyses and NMR spectroscopy have revealed the structural geometry of several host-guest complexes, the challenge remains in designing preorganized lanthanide receptors with enhanced anion selectivity for broader applications in diagnostics and bioimaging. To address this challenge, innovative and complementary methods to investigate host-anion binding geometry are becoming increasingly important. Herein, we demonstrate the combined use of Eu L3-edge extended X-ray absorption fine structure (EXAFS) and electron paramagnetic resonance (EPR) spectroscopy to elucidate the binding of nucleoside phosphates (ATP, ADP, and AMP) to a cationic lanthanide complex. We establish that ATP unequivocally binds the lanthanide center in a bidentate manner in water, while ADP adopts both bidentate and monodentate modes, and AMP binds in a monodentate manner. This interdisciplinary approach provides deeper insight into lanthanide host-guest chemistry in solution, laying the groundwork for designing emissive probes that undergo specific anion-induced structural changes and elicit desired optical responses upon binding.

Anomalous lattice contraction and emerging topological phases in Bi-substituted Sm2Ir2O7

We demonstrate that substituting Bi for Sm in the pyrochlore Sm$_2$Ir$_2$O$_7$ induces an anomalous lattice contraction, with $\Delta a \sim -0.012$~\AA~observed at 10\% Bi substitution, where 'a' denotes the lattice constant. Beyond 10\% Bi substitution, the lattice expands according to Vegard's law. Within this anomalous substitution range, the resistivity shows a 1/T behavior up to 2\% Bi-substitution, while near 10\% substitution a -lnT dependence is observed. These resistivity behaviors suggest the possibility of a Weyl phase up to 2\% Bi substitution, which transforms to a semimetallic quadratic band touching (QBT) topological phase near 10\%. For the intermediate composition (Sm$_{0.95}$Bi$_{0.05}$)$_2$Ir$_2$O$_7$, the resistivity scales as $\rm 1/T^{1/4}$, possibly due to its proximity to a proposed quantum critical point at the Weyl-QBT phase boundary [Phys. Rev. X 4, 041027 (2014)]. The samples were characterized using synchrotron powder X-ray diffraction, X-ray near-edge fine structure (XANES), and Extended X-ray absorption fine structure (EXAFS) probes. Additionally, magnetic susceptibility and heat capacity measurements were conducted to provide further support.

Displacive Jahn-Teller Transition in NaNiO2.

Below its Jahn-Teller transition temperature, TJT, NaNiO2 has a monoclinic layered structure consisting of alternating layers of edge-sharing NaO6 and Jahn-Teller-distorted NiO6 octahedra. Above TJT where NaNiO2 is rhombohedral, diffraction measurements show the absence of a cooperative Jahn-Teller distortion, accompanied by an increase in the unit cell volume. Using neutron total scattering, solid-state Nuclear Magnetic Resonance (NMR), and extended X-ray absorption fine structure (EXAFS) experiments as local probes of the structure we find direct evidence for a displacive, as opposed to order-disorder, Jahn-Teller transition at TJT. This is supported by ab initio molecular dynamics (AIMD) simulations. To our knowledge this study is the first to show a displacive Jahn-Teller transition in any material using direct observations with local probe techniques.

Tuning the crystallinity of Cu-based electrocatalysts: Synthesis, structure, and activity towards the CO2 reduction reaction

The rising levels of CO2 have spurred growing concerns for our environment, and curbing CO2 emissions may not be practically viable with the expanding human population. One attractive strategy is the electrochemical CO2 reduction (CO2RR) into value added chemicals but because of the chemical inertness of the CO2 molecule, the electrochemical reduction requires a suitable catalyst. Cu-based catalysts have been largely investigated for CO2RR, however, the difficulty achieving a high selectivity and faradaic efficiency towards specific products, especially hydrocarbons, is still a challenge, alongside the concern over cost, stability and scarcity of the metal catalyst. The present research focuses on tuning the crystallinity of Cu nanoparticles via a green, cost-friendly, and facile method, called the urea glass route. Remarkably, the incorporation of a selected nitrogen-carbon rich source (namely, 4,5 dicyanoimidazole) at low temperatures allow the formation of an oxidized derived amorphous Cu system, whilst a second thermal treatment enables the transformation to crystalline Cu0. We found that the combination of surface Cu0 and Cu1+ (observed via XPS studies) present in our amorphous and crystalline Cu nanoparticles leads to interesting differences in the final catalytic activity when tested under CO2 reaction conditions. The combination of extended X-ray absorption fine structure (EXAFS) experiments and molecular dynamics simulations provides compelling evidence for the amorphous and metallic nature of Cu nanoparticles.

Measurements of K-edge and L-edge extended x-ray absorption fine structure at the national ignition facility (invited).

High-energy-density laser facilities and advances in dynamic compression techniques have expanded access to material states in the Terapascal regime relevant to inertial confinement fusion, planetary science, and geophysics. However, experimentally determining the material temperature in these extreme conditions has remained a difficult challenge. Extended X-ray Absorption Fine Structure (EXAFS), referring to the modulations in x-ray absorption above an absorption edge from photoelectrons' interactions with neighboring atoms, has proven to be a versatile and robust technique for probing material temperature and density for mid-to-high Z elements under dynamic compression. The current platform at the National Ignition Facility has developed six configurations for EXAFS measurements between 7 and 18keV for different absorption edges (Fe K, Co K, Cu K, Ta L3, Pb L3, and Zr K) using a curved-crystal spectrometer and a bright, continuum foil x-ray source. In this work, we describe the platform geometry, x-ray source performance, spectrometer resolution and throughput, design considerations, and data in ambient and dynamic-compression conditions.

Using synchrotron based ATR-FTIR, EXAFS, and XRF to characterize the chemical compositions of TSP in industrial estate area

In Thailand, the Map Ta Phut Industrial Estate (MTPIE), a prominent industrial hub, has substantial environmental and health issues caused by industrial pollution. This study uses advanced synchrotron-based techniques, such as Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR), Extended X-ray Absorption Fine Structure (EXAFS), and X-ray Fluorescence (XRF), to fully examine the chemical make-up of total suspended particulate (TSP) in the given area. Notable findings include the detection of remarkably high enrichment factors for magnesium and sulfur, indicating the presence of industrial operations. Additionally, we found that magnetite, which accounts for an average of 40 % of the total iron oxides in the samples, is the main iron oxide. The study also highlights about how calcium carbonate and different organic functional groups are found in large amounts, which shows that industrial emissions and natural sources are connected in a complex way. The findings underscore the susceptibility of children to TSP exposure, revealing increased rates of inhalation and significant health hazards. In order to safeguard public health in industrial areas such as MTPIE, it is imperative to implement more sophisticated pollution control techniques and maintain ongoing environmental monitoring.

Enhancing Europium Adsorption Effect of Fe on Several Geological Materials by Applying XANES, EXAFS, and Wavelet Transform Techniques.

This study conducted adsorption experiments using Europium (Eu(III)) on geological materials collected from Taiwan. Batch tests on argillite, basalt, granite, and biotite showed that argillite and basalt exhibited strong adsorption reactions with Eu. X-ray diffraction (XRD) analysis also clearly indicated differences before and after adsorption. By combining X-ray absorption near-edge structure (XANES), extended X-ray absorption fine structure (EXAFS), and wavelet transform (WT) analyses, we observed that the Fe2O3 content significantly affects the Eu-Fe distance in the inner-sphere layer during the Eu adsorption process. The wavelet transform analysis for two-dimensional information helps differentiate two distances of Eu-O, which are difficult to analyze, with hydrated outer-sphere Eu-O distances ranging from 2.42 to 2.52 Å and inner-sphere Eu-O distances from 2.27 to 2.32 Å. The EXAFS results for Fe2O3 and SiO2 in argillite and basalt reveal different adsorption mechanisms. Fe2O3 exhibits inner-sphere surface complexation in the order of basalt, argillite, and granite, while SiO2 forms outer-sphere ion exchange with basalt and argillite. Wavelet transform analysis also highlights the differences among these materials.

Initiation of Medial Calcification: Revisiting Calcium Ion Binding to Elastin.

Pathological calcification of elastin, a key connective tissue protein in the medial layers of blood vessels, starts with the binding of calcium ions. This Mini-Review focuses on understanding how calcium ions interact with elastin to initiate calcification at a molecular level, and emphasizes water's critical role in mediating this interaction. In the past decade, great strides have been made in understanding and modeling ion-specific hydration and its effects on biomolecule interactions. However, these advances have been largely absent from our understanding of elastin calcification. Historically, charge-neutral backbone carbonyls and negatively charged carboxyl groups have been proposed as elastin's calcium binding sites. Recently, tropoelastin's only four carboxyl groups have been identified as binding sites from classical molecular dynamics (MD). While carboxyl groups have a much higher affinity for binding calcium ions than backbone carbonyls, conflicting evidence persists for both functional group's importance in elastin calcification. This can be attributed to the fact that divalent ions strongly polarize water, leading to a hydration shell that shields electrostatic forces. The hydration shell surrounding both a calcium ion and either of the proposed binding sites must be displaced to enable binding. Providing our own extended X-ray absorption fine structure (EXAFS) data and complementary simulations, we discuss the potential structures of calcium binding in elastin and review prior knowledge regarding the relative importance of the two proposed binding sites.

Mechanism Behind the Enhanced Ferromagnetic Contributions Upon Ru Substitution in Ca3Mn2O7 from X-Ray Absorption Spectroscopies.

We have studied the local structure and electronic and magnetic properties of hybrid improper ferroelectric Ca3Mn2O7 upon Ru substitution at the Mn site by a combination of atomic-selective X-ray absorption spectroscopies in the soft and hard X-ray energy regimes. Ru substitution enhances the macroscopic ferromagnetic contributions, whose origin is here elucidated. In particular, soft X-ray magnetic circular dichroism (XMCD) data indicate that the spin moments of Mn and Ru are aligned in opposite directions, with the effective magnetic moments of Ru being about 1 order of magnitude smaller than for Mn. The XMCD results also demonstrate the presence of an intrinsic ferromagnetic component in the Mn sublattice for Ru contents x ≥ 0.3, although the values of the net Mn magnetic moment are much smaller than expected for a fully saturated Mn4+ magnetic sublattice. Soft and hard X-ray absorption spectroscopy measurements show that Mn keeps a +4 valence state independently of the Ru content, whereas Ru is in an intermediate valence state, ranging between +4.7 (x ≤ 0.1) and +4.4 (x ≥ 0.7). Analysis of the extended X-ray absorption fine structure (EXAFS) signals at the Mn and Ru K-edges suggests the lack of a complete solid solution in the local structure across the entire range of compositions. These findings disclose the existence of charge transfer between Mn and Ru atoms, so the weak ferromagnetic component is attributed to a canting of the antiferromagnetically ordered Mn4+ spins caused by the oxygen-mediated hybridization between localized Mn4+ 3d and itinerant intermediate valence Ru 4d bands, in analogy to the mechanism proposed for ferromagnetism in ordered doubled perovskites with 3d and 4d/5d transition-metal ions. In the present case, disorder prevents the formation of long-range ferromagnetism.