Extended X-ray Absorption Fine Structure Analysis 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.

Speciation of Technetium Dibutylphosphate in the Third Phase Formed in the TBP/HNO3 Solvent Extraction System.

The speciation of technetium in the nitric acid/dibutylphosphoric acid (HDBP)-n-dodecane system was studied by extended X-ray absorption fine structure (EXAFS) spectroscopy and theoretical methods. Tetravalent technetium, produced by the hydrazine reduction of TcO4- in 3 M HNO3, was extracted by HDBP in n-dodecane (30% by volume). During extraction, the splitting of the organic phase into a heavy phase and a light phase was observed. EXAFS analysis is consistent with the presence of Tc(NO3)3(DBP)(HDBP)2 in the light phase and Tc(NO3)2(DBP)2(HDBP)2 in the heavy phase. Density functional theory calculations at the B3LYP/6-31G* level confirm the stability of the proposed species and indicate that stereoisomers -mer- and fac-Tc(NO3)3(DBP)(HDBP)2 for the light phase and cis- and trans-Tc(NO3)2(DBP)2(HDBP)2 for the heavy phase] could coexist in the system (in the n-dodecane solution). Mechanisms of formation of Tc(NO3)3(DBP)(HDBP)2 and Tc(NO3)2(DBP)2(HDBP)2 are proposed.

The structures of iron silicate glasses with varying iron redox ratios from molecular dynamics simulations and EXAFS analysis

Iron oxides are frequently found in natural and industrial glass compositions and can affect various physical and chemical properties of the glasses and their melts. Thus, a fundamental understanding of iron-bearing silicate melts and glasses is of both scientific and technological importance. This study investigates the structures of sodium iron silicate glasses with compositions of NaFeSiO4, NaFeSi2O6, NaFeSi3O8, and Na5FeSi4O12 using molecular dynamics simulations in combination with Extended X-ray Absorption Fine Structure (EXAFS) characterizations. Short and medium range structural features of these glasses support that ferrous (Fe2+) and ferric (Fe3+) ions play the roles of network modifier and network former, respectively, with the Fe oxidation states playing an important role in the polymerization of the glass network. These simulation results agree well with newly measured room temperature EXAFS spectra. The simulated glass structures were also compared to the melts structures with the same composition but different redox ratios. The average coordination numbers of the cations were found to be affected both by the melt temperature and iron redox ratio.

Elucidating the Magnetoelastic Coupling, Pressure-Dependent Magnetic Behavior, and Anomalous Hall Effect in FexTi2S4 Intercalation Sulfides.

Transition-metal chalcogenides with intercalated layered structures are interesting systems in material physics due to their attractive electronic and magnetic properties, with applications in the fields of magnetic refrigerators, catalysts, and thermoelectrics, among others. In this work, we studied in detail the structural, electronic, and magnetic properties of (Fe,Ti)-based sulfides with formula FexTi2S4 (x = 0.24, 0.32, and 0.42), prepared as polycrystalline materials under high-pressure conditions. They present a layered Heideite-type crystal structure, as assessed by synchrotron X-ray diffraction. A local structure analysis using Fe K-edge extended X-ray-absorption fine structure (EXAFS) data unveiled a conspicuous contraction of the main Fe-S bond in Fe0.24Ti2S4 at the vicinity of the magnetic transition 60-80 K. We suggest that this anomaly is related to magnetoelastic coupling effects. The EXAFS analysis allowed extraction of the Einstein temperatures (θE), i.e., the phonon contribution to the specific heat, for the two bond pairs Fe-S(1) [θE ≈318 K; 290 K (C/T)] and Fe-Ti(1) [θE ≈218 K; 190 K (C/T)]. In addition to the structural and local vibrational measurements, we probed the magnetic properties using magneto-calorimetry, magnetometry under applied pressure, magnetoresistance (MR), and Hall effect measurements. We observed the appearance of a broad peak in the specific heat around 120 K in the x = 0.42 compound that we associated with an antiferromagnetic ordering electronic transition. We found that the antiferromagnetic transition temperature is pressure and composition sensitive and reduces at 1.2 GPa by ∼12 and ∼3 K, for the members with x = 0.24 and x = 0.42, respectively. Similarly, the saturation magnetization in the ordered phase depends on both pressure and iron content, reducing its value by 50, 90, and 30% for x = 0.24, 0.32, and 0.42, respectively. We observed clear jumps in the magnetic hysteresis loops, MR, and anomalous Hall effect (AHE) below 2 K at fields around 2-4 T. We associated this observation with the metamagnetic transitions; from the Berry-curvature a decoupling parameter of SH = 0.12 V-1 is determined. Comparison of the results on the temperature-dependent magnetization, MR, and AHE elucidates a strong inelastic scattering contribution to the AHE at higher temperatures due to the cluster spin-glass phase.

Uranium Speciation in a Chalky Soil.

In this article, the speciation and behavior of anthropogenic metallic uranium deposited on natural soil are approached by combining EXAFS (extended X-ray absorption fine structure) and TRLFS (time-resolved laser-induced fluorescence spectroscopy). First, uranium (uranyl) speciation was determined along the vertical profile of the soil and bedrock by linear combination fitting of the EXAFS spectra. It shows that uranium migration is strongly limited by the sorption reaction onto soil and rock constituents, mainly mineral carbonates and organic matter. Second, uranium sorption isotherms were established for calcite, chalk, and chalky soil materials along with EXAFS and TRLFS analysis. The presence of at least two adsorption complexes of uranyl onto carbonate materials (calcite) could be inferred from TRLFS. The first uranyl tricarbonate complex has a liebigite-type structure and is dominant for low loads on the carbonate surface (<10 mgU/kg(rock)). The second uranyl complex is incorporated into the calcite for intermediate (∼10 to 100 mgU/kg(rock)) to high (high: >100 mgU/kg(rock)) loads. Finally, the presence of a uranium-humic substance complex in subsurface soil materials was underlined in the EXAFS analysis by the occurrence of both monodentate and bidentate carboxylate (or/and carbonate) functions and confirmed by sorption isotherms in the presence of humic acid. This observation is of particular interest since humic substances may be mobilized from soil, potentially enhancing uranium migration under colloidal form.

Constructing N-Coordinated Co and Cu Single-Atomic-Pair Sites toward Boosted CO2 Photoreduction

The visible-light-driven photocatalytic reduction reaction of carbon dioxide (CO2) (CO2RR) to value-added fuels presents a feasible approach to curb anthropogenic CO2 emissions and mitigate the increasing energy crisis. However, developing photocatalysts with excellent performance still remains a great challenge in this field. Herein Co,Cu,N-codoped carbon nanoparticles (Co1Cu1/NC) were fabricated through the pyrolysis of zeolitic imidazolate framework (ZIF-8) with Cu(NO3)2 adsorbed inside the cavities and CoTBPP decorated over the surface of ZIF. Spherical aberration-corrected high-angle annular dark-field scanning transmission electron microscopy and electron energy loss spectroscopy measurements disclose the dual-metal single-atomic nature of Co1Cu1/NC consisting of atomically dispersed Co–Cu pair sites on a nitrogen-doped carbon support. Extended X-ray absorption fine-structure analysis reveals the tetra-N-coordinated nature of each metal in Co1Cu1/NC (N2–Co–N2–Cu–N2). For the purpose of comparative study, Co,N- and Cu,N-codoped carbon nanoparticles (Co1/NC and Cu1/NC) also with single atomic site nature have been fabricated following the same route. The as-prepared Co1Cu1/NC exhibits highly effective photocatalytic CO2-to-CO reduction with a considerably high CO-generating yield of 22.46 mmol g–1 and a CO selectivity of 83.4% after 2 h of visible-light irradiation. Experimental characterizations and in particular theoretical calculations disclose the close association of the remarkable CO2RR catalytic activity of Co1Cu1/NC with the synergetic effect of the Co–Cu atomic-pair sites, which facilitate the conversion of CO2 to CO via lowering the energy barrier for the formation of the *COOH intermediate. This work paves a new avenue for the rational design and construction of atomic-pair photocatalysts with boosted performance.

Effect of atomic size mismatch and chemical complexity on the local lattice distortion of BCC solid solution alloys

The effects of atomic size mismatch and chemical complexity on the local lattice distortion of solid-solution alloys VNbTa, TiVNbMo, TiVNbMoTa and TiVNbMoTaWRe with body-centered cubic (BCC) structure are quantitatively studied with time-of-flight neutron total scattering, extended X-ray absorption fine structure (EXAFS) measurements and first principles calculations. Neutron atomic pair distribution function (PDF) measurements found that the local lattice distortion in the ternary solid-solution alloy VNbTa is 1.1 %, obviously larger than that of the most complicated solid-solution alloy (0.27% for TiVNbMoTaWRe). Our results suggested that atomic size mismatch in high entropy alloys is more critical to the local lattice distortion than chemical complexity. Both EXAFS analysis and theoretical calculations indicate that there is a maximum of ∼4 % difference between the bonding length of different atomic pairs in VNbTa.

Influence of catalase encapsulation on Cobalt@Nanoporous carbon with multiwall shell for supercapacitor and polyurethane synthesis using carbon dioxide

Enzyme encapsulated ZIFs exhibit significant host–guest synergism and protect the relative activity of the enzymes. Elucidation of metallic active sites including the nature of electronics and valance state has been forefront in energy conversion studies supported by high-resolution transmission electron microscopy and synchrotron X-ray absorption fine structure (XAFS) spectrogram. Here, we access metallic cobalt (Co) sites that were set by pyrolysis of catalase encapsulated ZIF-67 (cat@ZIF-67) at a high temperature (700 °C) to obtain cat@ZIF-67-NC. The local environment of Co-sites from X-ray photoelectron spectroscopic (XPS) and extended X-ray absorption fine structure (EXAFS) analysis suggests the presence of Co sites in a metallic valance state. HR-TEM and STEM analysis further revealed the Co particles encapsulated with multiwall shell morphology as support. An activity reversal is witnessed, where cat@ZIF-67-NC exhibited higher activity in CO2-cycloaddition-polymerization but was found weaker in capacitance than ZIF-67-NC for 3-electrode supercapacitor applications. The ZIF-67-NC has exhibited maximum Cs of 223 F/g at a scan rate of 5 mV/s, which retained about 87 F/g on increasing scan rate to 200 mV/s, in contrast to cat@ZIF-67-NC with a Cs value of 133.4 F/g at 5 mV/s with improved retention (58 % at 200 mV/s). The contrasting nature of cat-ZIF-67-NC and ZIF-67-NC in supercapacitor and CO2 cycloaddition-polymerization originates from encapsulated catalase in ZIF-67 based on XPS and EXAFS analysis. The electronic configuration and surface composition analysis using EXAFS and XPS techniques revealed the presence of metallic Co-sites responsible for supercapacitor and CO2-cycloaddition-polymerization. An influence of encapsulated catalase is evident in a decrease in the capacitance for cat@ZIF-67-NC due to metallic Co-sites. Co-sites in cat-ZIF-67-NC are more active in producing polyurethane by a non-isocyanate route via CO2-cycloaddition.

Local structure of Fe2+ and Fe3+ in organic solvents

We systematically investigated the local structure of Fe2+ and Fe3+ in 11 solvents by extended X-ray absorption fine structure (EXAFS) analysis and X-ray absorption near-edge structure (XANES). The EXAFS analyses with a one-shell model indicate that Fe2+ and Fe3+ are coordinated by six oxygen of the ligand molecules (L) and form a FeL 6-type complex in all solvents. The XANES suggests that the [FeL 6]3+ complex shows noncentrosymmetric deformation in several solvents. In protic solvents, the EXAFS analyses with a two-shell model revealed that the coordination number (n C) of the second nearest carbon around Fe3+ significantly depends on L. We interpreted the observation in terms of the distribution of the Fe-C distance.

Short‐Range Order Investigation of Cu x Ge 50− x Te 50 Phase‐Change Materials

The stability against crystallization of the amorphous phase-change materials Cu x Ge 50 − x Te 50 (CGT) exhibits an unusual behavior at low Cu concentrations with x ≈ 15 being the most stable composition. Three CGT compositions with x = 5 , 15, and 25 are analyzed using extended X-ray absorption fine structure (EXAFS) at the Ge-K, Cu-K, and Te-K edge at 10 K. EXAFS analysis shows that the average coordination number (CN) increases with the Cu concentration from 2.8 at x = 5 to 4.0 at x = 25 . Cu-EXAFS spectra indicate the formation of Cu clusters for low Cu concentrations ( x = 5 and x = 15 ). The existence of these clusters is further analyzed by X-ray absorption near-edge structure calculations, supporting small polyhedral and/or disk-like Cu clusters which are linked to the surrounding amorphous GeTe matrix via CuTe bonds. Hence, the GeTe network is Te enriched, leading to a maximum of GeTe bonds at x = 15 . As a result, this establishes an amorphous network which is stabilized against crystallization.

Uranium (VI) Adsorbate Structures on Portlandite [Ca(OH)2] Type Surfaces Determined by Computational Modelling and X-ray Absorption Spectroscopy

Portlandite [Ca(OH)2] is a potentially dominant solid phase in the high pH fluids expected within the cementitious engineered barriers of Geological Disposal Facilities (GDF). This study combined X-ray Absorption Spectroscopy with computational modelling in order to provide atomic-scale data which improves our understanding of how a critically important radionuclide (U) will be adsorbed onto this phase under conditions relevant to a GDF environment. Such data are fundamental for predicting radionuclide mass transfer. Surface coordination chemistry and speciation of uranium with portlandite [Ca(OH)2] under alkaline groundwater conditions (ca. pH 12) were determined by both in situ and ex situ grazing incidence extended X-ray absorption fine structure analysis (EXAFS) and by computational modelling at the atomic level. Free energies of sorption of aqueous uranyl hydroxides, [UO2(OH)n]2–n (n = 0–5) with the (001), (100) and (203) or (101) surfaces of portlandite are predicted from the potential of mean force using classical molecular umbrella sampling simulation methods and the structural interactions are further explored using fully periodic density functional theory computations. Although uranyl is predicted to only weakly adsorb to the (001) and (100) clean surfaces, there should be significantly stronger interactions with the (203/101) surface or at hydroxyl vacancies, both prevalent under groundwater conditions. The uranyl surface complex is typically found to include four equatorially coordinated hydroxyl ligands, forming an inner-sphere sorbate by direct interaction of a uranyl oxygen with surface calcium ions in both the (001) and (203/101) cases. In contrast, on the (100) surface, uranyl is sorbed with its axis more parallel to the surface plane. The EXAFS data are largely consistent with a surface structural layer or film similar to calcium uranate, but also show distinct uranyl characteristics, with the uranyl ion exhibiting the classic dioxygenyl oxygens at 1.8 Å and between four and five equatorial oxygen atoms at distances between 2.28 and 2.35 Å from the central U absorber. These experimental data are wholly consistent with the adsorbate configuration predicted by the computational models. These findings suggest that, under the strongly alkaline conditions of a cementitious backfill engineered barrier, there would be significant uptake of uranyl by portlandite to inhibit the mobility of U(VI) from the near field of a geological disposal facility.

Precise Electrochemical Testing in Half-Cells: Controlling Catalyst Layer Formation and the Three-Phase Boundary in Gas Diffusion Electrodes

Novel and tailored electrocatalysts need solid benchmarking for evaluation of e.g. the oxygen reduction reaction activity (ORR), and the performance in a model approach should strictly reflect their eventual performance in the fuel cell devices. It is strongly believed that the routinely used testing methods, like rotating disk electrode (RDE) method, provide erroneous results, as they do not mimic the original fuel cell conditions in terms of water management, presence of ionomer, relevant current densities, etc. Half-cell tests of gas diffusion electrodes (GDEs) have been shown to be a good compromise as these tests exhibit comparable oxygen mass transport to the electrode as a fuel cell device, in contrast to the limited mass transport in RDEs. Realistic operational conditions, such as potential range and higher current densities in the GDE set-ups, are more akin to membrane electrolyte assemblies (MEA) testing than RDEs without the hassle and cost of full-cell tests [1-4]. In this presentation, half-cell test results will be reported for gas diffusion electrodes prepared using various methods, namely: 1) Drop-casting; 2) Spray coating with sonication; 3) Air brushing with spray gun; and 4) Hand painting. Commercially available Pt/C HiSpec 4000 was used as electrocatalyst and results of parallel RDE and MEA tests with similar Pt loadings will be discussed. Detailed electrochemically active surface area (ECSA) analysis using Hupd and CO stripping studies of various loadings from 5μg cm-2 Pt loading to 50 μg cm-2 will be reported. Drop-casting method on GDE was found to be the most efficient process to study the ECSA using Hupd (38 m2 gPt -1) and CO-stripping (61 m2 gPt -1) which were achieved at a loading of 20 μgPt cm- 2. Spray-coating with ultrasonication with a diluted catalyst ink was the ideal procedure to fabricate a uniform thin layer of catalyst with multiple deposition cycles. The current densities achieved during ORR evaluation in half-cell set-up were almost 2 orders of magnitude higher than those achieved using the RDE method. Different strategies of implementing the Nafion ionomer were utilized and compared, like: 1) Nafion 117 membrane hot-pressed on the GDE; 2) Nafion solution sprayed on the surface of the GDE reaching a loading of 2 mg cm-2 and; 3) GDE without a membrane. This report will also aim to assess how the presence of phosphoric acid in the perchloric acid electrolyte and its purity affects the catalyst’s performance for the ORR reaction. Scanning electron microscopy (SEM) was used to study the catalyst layer formation on the gas diffusion layers (GDLs). The GDEs were characterized using X-Ray Absorption Spectroscopy (XAS) [5, 6], ex-situ and in-situ, allowing us to extract structural information through the extended X-ray absorption fine structure analysis (EXAFS) (catalyst particle size, composition, morphology, and oxidation state); furthermore, by using a subtractive-normalization technique in the near-edge region (Δμ XANES) it is possible to obtain adsorbate coverage (H, O, PO4 species adsorbed on Pt surface) and binding-site information for a detailed comparison between the above samples.

Cadmium Isotope Fractionation during Adsorption and Substitution with Iron (Oxyhydr)oxides.

Cadmium (Cd) isotopes have great potential for understanding Cd geochemical cycling in soil and aquatic systems. Iron (oxyhydr)oxides can sequester Cd via adsorption and isomorphous substitution, but how these interactions affect Cd isotope fractionation remains unknown. Here, we show that adsorption preferentially enriches lighter Cd isotopes on iron (oxyhydr)oxide surfaces through equilibrium fractionation, with a similar fractionation magnitude (Δ114/110Cdsolid-solution) for goethite (Goe) (-0.51 ± 0.04‰), hematite (Hem) (-0.54 ± 0.10‰), and ferrihydrite (Fh) (-0.55 ± 0.03‰). Neither the initial Cd2+ concentration or ionic strength nor the pH influence the fractionation magnitude. The enrichment of the light isotope is attributed to the adsorption of highly distorted [CdO6] on solids, as indicated by Cd K-edge extended X-ray absorption fine-structure analysis. In contrast, Cd incorporation into Goe by substitution for lattice Fe at a Cd/Fe molar ratio of 0.05 preferentially sequesters heavy Cd isotopes, with a Δ114/110Cdsolid-solution of 0.22 ± 0.01‰. The fractionation probably occurs during the transformation of Fh into Goe via dissolution and reprecipitation. These results improve the understanding of the Cd isotope fractionation behavior being affected by iron (oxyhydr)oxides in Earth's critical zone and demonstrate that interactions with minerals can obscure anthropogenic and natural Cd isotope characteristics, which should be carefully considered when applying Cd isotopes as environmental tracers.

W[sbnd]O bond shortening by doping of first-row transition metal ions that enhances its catalytic potency

Here, we report thorough spectroscopic characterization of tungsten oxide (WO3) nanoparticles (NPs) and WO3 NPs surface doped with transition metal (TM) ions (TM@WO3; TM = Cr3+, Mn2+, Fe3+, Co2+, and Ni2+) by using various surface analysis techniques such as transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), high-resolution X-ray photoelectron spectroscopy (HRXPS), and extended X-ray absorption fine structure (EXAFS). At first glance, the morphologically distinct features were not perceivable across TM dopants. However, the detailed XPS analysis confirmed that the WO3 NPs doped with Cr and Fe ions formed a relatively large number of surface defects. Moreover, we found through EXAFS analysis that the chromium and iron ions doping on WO3 NPs promoted the dramatic shortening of the WO bond length as compared to pristine WO3 and other metal ion doped WO3 NPs. Much enhanced photocatalytic and electrocatalytic activities of Cr@WO3 and Fe@WO3, which relied on the different density of surface defects created by the charge and size of the TM dopants, were demonstrated in the photocatalytic degradation reactions of 4-chlorophenol and the electrocatalytic hydrogen evolution reaction. The present result can serve as a practical guideline for atomically precise control of oxygen vacancy toward the development of high-performance metal oxide photocatalysts.

The uptake of selenite in calcite revealed by X-ray absorption spectroscopy and quantum chemical calculations

Selenium (Se) is an important trace element in the environment, but the interaction of Se with calcite that may control the fate and geochemical behavior of Se is not fully understood. In this study, the molecular-scale mechanism for the uptake of selenite in calcite was investigated by a combination of laboratory experiments, extended X-ray absorption fine structure (EXAFS) spectroscopy, and quantum chemical calculations. Results showed that selenite can be largely distributed to calcite at circumneutral pH. The local structure of Se in calcite obtained from EXAFS analyses, in combination with quantum chemical calculations, demonstrated that selenite can be incorporated into calcite by substituting for the carbonate, and that the geometric incompatibility of selenite could be accommodated by a slight expansion of crystal volume. The findings from this study suggest that calcite could be a potential Se sink, providing an important insight into the understanding of the mobility and geochemical behavior of Se in the subsurface environments particularly in the groundwater system.

Structural Effects of Anomalous Current Densities on Manganese Hexacyanoferrate for Li-Ion Batteries

A battery management system (BMS) plays a pivotal role in providing optimal performance of lithium-ion batteries (LIBs). However, the eventual malfunction of the BMS may lead to safety hazards or reduce the remaining useful life of LIBs. Manganese hexacyanoferrate (MnHCF) was employed as the positive electrode material in a Li-ion half-cell and subjected to five cycles at high current densities (10 A gMnHCF−1) and to discharge at 0.1 A gMnHCF−1, instead of classical charge/discharge cycling with initial positive polarization at 0.01 A gMnHCF−1, to simulate a current sensor malfunctioning and to evaluate the electrochemical and structural effects on MnHCF. The operando set of spectra at the Mn and Fe K-edges was further analyzed through multivariate curve resolution analysis with an alternating least squares algorithm (MCR–ALS) and extended X-ray absorption fine structure (EXAFS) spectroscopy to investigate the structural modifications arising during cycling after the applied electrochemical protocol. The coulombic efficiency in the first cycle was dramatically affected; however, the local structural environment around each photo absorber recovered during charging. The identification of an additional spectral contribution in the electrochemical process was achieved through MCR-ALS analysis, and the Mn-local asymmetry was thoroughly explored via EXAFS analysis.

The Crystallization Process of Vaterite Microdisc Mesocrystals via Proto-Vaterite Amorphous Calcium Carbonate Characterized by Cryo-X-ray Absorption Spectroscopy

Investigation on the formation mechanism of crystals via amorphous precursors has attracted a lot of interests in the last years. The formation mechanism of thermodynamically meta-stable vaterite in pure alcohols in the absence of any additive is less known. Herein, the crystallization process of vaterite microdisc mesocrystals via proto-vaterite amorphous calcium carbonate (ACC) in isopropanol was tracked by using Ca K-edge X-ray absorption spectroscopy (XAS) characterization under cryo-condition. Ca K-edge X-ray absorption near edge structure (XANES) spectra show that the absorption edges of the Ca ions of the vaterite samples with different crystallization times shift to lower photoelectron energy while increasing the crystallization times from 0.5 to 20 d, indicating the increase of crystallinity degree of calcium carbonate. Ca K-edge extended X-ray absorption fine structure (EXAFS) spectra exhibit that the coordination number of the nearest neighbor atom O around Ca increases slowly with the increase of crystallization time and tends to be stable as 4.3 (±1.4). Crystallization time dependent XANES and EXAFS analyses indicate that short-range ordered structure in proto-vaterite ACC gradually transform to long-range ordered structure in vaterite microdisc mesocrystals via a non-classical crystallization mechanism.

Characteristics and mechanisms of Pb(II) sorption onto Fe-rich waste water treatment residue (WTR): A potential sustainable Pb immobilisation technology for soils

Pb contamination of soils is a global problem. This paper discusses the ability of an Fe-rich waste, water treatment residual (WTR), to adsorb Pb(II). This was investigated using batch sorption experiments, X-ray diffraction, electron microprobe microanalysis, PHREEQC modeling and Extended X-ray Absorption Fine Structure (EXAFS) analysis. The WTR is composed of approximately 23 wt. % natural organic matter (NOM), 70 wt. % ferrihydrite and <10 wt. % silicate material. Pb(II) sorption to WTR was dependent on initial Pb(II) load, particle size, time and pH, but not on ionic strength. EXAFS analysis at the Pb LIII-edge confirmed that Pb(II) sorbed to WTR by co-existing bidentate edge-sharing and monodentate or corner-sharing complexes, with 2 O at ∼2.31–2.34 Å, 1 Fe at ∼3.32–3.34 Å, 2 Fe at ∼3.97–3.99 Å and 1 Pb at ∼3.82–3.85 Å. Linear combination showed that the Pb(II)-sorbed spectra were best fit with a ∼0.9 ± 0.1 and 0.1 ± 0.1 contribution from Pb(II)-sorbed ferrihydrite and Pb(II)-sorbed humic acid end members, respectively. Overall, we show that Pb(II) sorbs via strong inner-sphere complexation of Pb(II) to the ferrihydrite component of the WTR, which itself is stable over a wide pH range. Therefore, we suggest that Fe-rich WTR wastes could be used as effective adsorbents in Pb(II)-contaminated soils to help ensure sustainable terrestrial ecosystems.

XAS analysis of copper binding in soils

Ecological distribution of copper from soil is depended on its bioavailability and mobility. Copper binding on heterogeneous soil composition is a key information for understanding the long-term fate benefiting suitable management practice for copper in the environment. X-ray absorption spectroscopy (XAS) techniques were used to provide microscopic scale information of copper-binding on different soils. Soil samples containing 14.9-56.6% clay and 0.5-3.4% organic matter were studied. Dry-powder soil samples containing approximated 0.3% copper from batch equilibrated equilibrium experiments were analyzed. Copper K-edge spectrum were collected at the Synchrotron light research institute (SLRI), Thailand in X-ray absorption near edge structure (XANES) and Extended x-ray absorption fine structure (EXAFS) spectrum region. Linear combination fit (LCF) analysis of XANES spectrum resulted the phase fraction of copper were copper acetate (55-79%) and copper sulfate (9-46%) in all soils, while, copper hydroxide (24%) was exhibited in siliceous coarse-texture soil. EXAFS analysis using copper acetated model structure found four oxygen surrounded copper atom at 1.91-1.974 Ã… in the first shell which positioned in the equatorial plane of a Jahn-Teller distorted octahedral without the existing of weak binding of two axial oxygen atom. Carbon atoms presented in the second shell at 2.73-2.91 Ã… and its coordination number related to soil organic matter content. XANES and EXAFS studied on copper binding in soils result that the predominant copper in soil is in organic matter complex via carboxyl functional group and sulfated ligand. Copper-C binding occurred in local structure which corresponded to soil organic matter content.

Cationic disorder: A pathway for demonstrating inverse exchange bias in Gd2CoRuO6

We report here the observation of the inverse exchange bias phenomenon in the ferrimagnetic double-perovskite ${\mathrm{Gd}}_{2}{\mathrm{CoRuO}}_{6}$. The compound crystallizes in a monoclinic structure (space group $P{2}_{1}/n$), and it undergoes long-range magnetic ordering below ${T}_{C}=60$ K. The ferrimagnetic state of the sample is manifested through magnetization reversal at low temperature. In order to corroborate the role of local structure on the magnetic properties of ${\mathrm{Gd}}_{2}{\mathrm{CoRuO}}_{6}$, we have performed a combined magnetization and x-ray absorption spectroscopy study on two different samples prepared through different cooling protocols (slow cooling and furnace cooling). Extended x-ray absorption fine-structure analysis indicates the presence of strong antisite disorder between Co and Ru in both samples. However, the slow-cooled sample shows completely random antisite disorder, whereas, small Co- and Ru-rich patches appear in the furnace-cooled sample. In both samples, a significant amount of mixed valency in Co and Ru ions is evident from our x-ray absorption near-edge structure. The antiparallel ordering along with different temperature dependence of the three magnetic sublattices, Gd, Co, and Ru, can be responsible for negative magnetization, whereas the inverse exchange bias effect can be attributed to the mixed interfacial coupling between adjacent magnetic nanoclusters. The difference in local structure can nicely be accounted for by the observed variations of the magnetic properties between two samples of ${\mathrm{Gd}}_{2}{\mathrm{CoRuO}}_{6}$.