X-ray Absorption Fine Structure Spectra Research Articles

Reduction of Cofed Carbon Dioxide Modifies the Local Coordination Environment of Zeolite-Supported, Atomically Dispersed Chromium to Promote Ethane Dehydrogenation.

The reduction of CO2 is known to promote increased alkene yields from alkane dehydrogenations when the reactions are cocatalyzed. The mechanism of this promotion is not understood in the context of catalyst active-site environments because CO2 is amphoteric, and even general aspects of the chemistry, including the significance of competing side reactions, differ significantly across catalysts. Atomically dispersed chromium cations stabilized in highly siliceous MFI zeolite are shown here to enable the study of the role of parallel CO2 reduction during ethylene-selective ethane dehydrogenation. Based on infrared spectroscopy and X-ray absorption spectroscopy data interpreted through calculations using density functional theory (DFT), the synthesized catalyst contains atomically dispersed Cr cations stabilized by silanol nests in micropores. Reactor studies show that cofeeding CO2 increases stable ethylene-selective ethane dehydrogenation rates over a wide range of partial pressures. Operando X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine-structure (EXAFS) spectra indicate that during reaction at 650 °C the Cr cations maintain a nominal 2+ charge and a total Cr-O coordination number of approximately 2. However, CO2 reduction induces a change, correlated with the CO2 partial pressure, in the population of two distinct Cr-O scattering paths. This indicates that the promotional effect of parallel CO2 reduction can be attributed to a subtle change in Cr-O bond lengths in the local coordination environment of the active site. These insights are made possible by simultaneously fitting multiple EXAFS spectra recorded in different reaction conditions; this novel procedure is expected to be generally applicable for interpreting operando catalysis EXAFS data.

Influence of A‐Site Doping on the Local Structure and Magnetic Behavior of BiFeO3 Nanoparticles Investigated by X‐Ray Absorption Fine Structure Spectroscopy

Structural transformation and magnetic change of pure and 1% La/Nd‐doped multiferroic BiFeO3 (BFO) nanoparticles are presented to study the influence of introducing rare‐earth elements at the A‐site in BFO. All samples are successfully produced by the conventional hydrothermal method. Scanning electron microscopy picture demonstrates that doping has an impact on the surface and size of particles, and the magnetic properties of BFO also are improved, even significant ferromagnetism is observed in the Nd‐doped sample. Though the room‐temperature X‐ray diffraction patterns reveal the rhombohedral R3c symmetry for all samples, further identified by the Bi L3‐edge and Fe K‐edge X‐ray absorption fine structure (XAFS) spectra, the nanoscale secondary orthorhombic phase is only detected in Bi0.99Nd0.01FeO3 when analyzing the doped elements according to the La and Nd L3‐edge XAFS spectra. The different magnetic behavior is speculated to be in connection with the orthorhombic symmetry around the Nd atom.

Local Atomic Displacements and Sign of the Structural Transformation in Medium-Entropy Alloys Observed in Extended X-ray Absorption Fine Structure Spectra

Local Atomic Displacements and Sign of the Structural Transformation in Medium-Entropy Alloys Observed in Extended X-ray Absorption Fine Structure Spectra

Structural characterization of nitrogen-doped [formula omitted]-graphynes by computational X-ray spectroscopy

Heterogeneous atomic doping, especially with nitrogens, can improve the interfacial properties and electrical conductivity of carbon-based materials like γ-graphyne (γ-GY), thereby improving the energy storage capacity and expanding various electrical applications. Properties of doped structures are affected by the local structures around the impurity atoms, which can be sensitively determined by the X-ray photoelectron (XPS) and near-edge X-ray absorption fine-structure (NEXAFS) spectra. In the present work, we have simulated the XPS and NEXAFS spectra of pristine γ-GY as well as five nitrogen-doped γ-GYs in both the C and N K-edges with the density functional theory. The theoretical C and N 1s ionic potentials of the N-doped systems can be well fitted to the experimentally XPS spectra, which can accurately characterize the experimental peaks. These five N-doped γ-GYs can be well distinguished by NEXAFS. Our results provide clear local structure-spectroscopy relation and a theoretical reference for identifying different N-doped γ-GYs, which are also insightful for general two-dimensional carbon-based materials.

A new satellite of manganese revealed by extended-range high-energy-resolution fluorescence detection.

The discovery of a new physical process in manganese metal is reported. This process will also be present for all manganese-containing materials in condensed matter. The process was discovered by applying our new technique of XR-HERFD (extended-range high-energy-resolution fluorescence detection), which was developed from the popular high-resolution RIXS (resonant inelastic X-ray scattering) and HERFD approaches. The acquired data are accurate to many hundreds of standard deviations beyond what is regarded as the criterion for `discovery'. Identification and characterization of many-body processes can shed light on the X-ray absorption fine-structure spectra and inform the scientist on how to interpret them, hence leading to the ability to measure the dynamical nanostructures which are observable using the XR-HERFD method. Although the many-body reduction factor has been used universally in X-ray absorption spectroscopy in analysis over the past 30 years (thousands of papers per year), this experimental result proves that many-body effects are not representable by any constant reduction factor parameter. This paradigm change will provide the foundation for many future studies and X-ray spectroscopy.

First-principles simulation of X-ray spectra of graphdiyne and graphdiyne oxides at the carbon K-edge.

The experimental C 1s near-edge X-ray absorption fine-structure (NEXAFS) spectra of graphdiyne (GDY) show an evident change at different exposure periods, which is explained by oxidation. Herein, to better understand the structure-spectra relationship and the influence of oxidization, we performed a first-principles simulation of the NEXAFS spectra and X-ray photoelectron spectra (XPS) of both pure GDY and its four typical graphdiyne oxides (GDO) at the carbon K-edge. Pure GDY contains one sp2-hybridized (C1) and two sp-hybridized (C2, C3) carbons, while oxidation introduces more nonequivalent carbons. The experimental NEXAFS spectrum exhibits the lowest peak at ca. 285.8 eV. It was found that the C 1s → π* excitation from the sp2-hybridized carbon atoms (C1) in pure GDY and the sp-hybridized atoms (C2, C3) in GDOs contributes to this peak. The two weak resonances at around 289.0 and 290.6 eV in the experiment are contributed by the carbon atoms bonded to the oxygen atoms. Meanwhile, we found that oxidization leads to an increase in the C 1s ionization potentials (IPs) by 0.3-2.7 eV, which is consistent with the XPS experiments. Our calculations provide a clear explanation of the structure-spectra relationships of GDY and GDOs, and the signatures are useful for estimating the degree of oxidation.

In situ gas cell for the analysis of adsorption behaviour on surfaces using X-ray spectroscopy

A gas cell for in-situ measurements of Volatile Organic Compounds (VOCs) and their adsorption behaviour on different surfaces by means of X-ray Fluorescence (XRF) and X-ray Absorption Fine-Structure (XAFS) spectroscopy has been developed. The cell is especially designed to allow for the efficient excitation and detection of low-Z elements such as carbon, oxygen or nitrogen as main components of VOCs. Two measurement modes are available. In the surface mode, adsorption on a surface can be studied using XAFS by fluorescence detection under shallow angles of incidence. The transmission mode enables the simultaneous investigation of gaseous samples via XAFS in transmittance and fluorescence detection modes. Proof-of-principle experiments were performed at the PTB plane grating monochromator beamline for soft X-ray radiation at the synchrotron radiation facility BESSY II. The flexible design and high versatility of the cell are demonstrated with the investigation of ethanol (EtOH) as one of the most abundant VOCs. The comparison of Near-Edge X-ray Absorption Fine-Structure (NEXAFS) spectra under transmission and fluorescence detection in the gas phase with measurements of adsorbed molecules on a Si-wafer surface both at the C and O-K absorption edges proves the applicability of the cell for the monitoring of adsorption processes.

Identification of oxidation states in [formula omitted]-graphyne by computational XPS and NEXAFS spectra

Oxidation is an inevitable process for γ-graphyne (γ-GY), a two-dimensional (2D) material of carbon, both in its natural state and during large-scale processing. Understanding the oxidation states within γ-GY is of fundamental importance for further applications. In this work, with density functional theory (DFT) simulations of seven representative oxygen-doped γ-GY structures, we demonstrate that the X-ray photoelectron (XPS) and near-edge X-ray absorption fine-structure (NEXAFS) spectra at the oxygen and carbon K-edges are sensitive for different local structures of oxygen dopants. The theoretical calculation results of γ-GY and seven oxygen-doped γ-GY configurations fit well with the experiment, and all characteristic experimental XPS peaks are assigned. We found that the C 1s ionization potential (IP) of sp2-hybridized carbons is higher than that of sp-hybridized carbons for the pristine γ-GY. By complete and elaborate analysis of the NEXAFS spectrum of each structure at the carbon and oxygen K edges, the seven oxygen-doped γ-GY configurations have been successfully identified. The results can be considered as a benchmark for the determination of oxygen-doped γ-GY configurations, and provide a deep insight into the structure–spectroscopy relationships.

Low Temperature Epitaxy of In Situ GaDoped Si1-XGex: Dopant Incorporation, Structural and Electrical Properties

Over the last few decades, MOSFET architectures have evolved from planar to 3D structures (FinFET and, more recently, gate-all-around and Forksheet devices) to improve the channel control at very scaled dimensions. In this framework, highly doped conformal Si1-xGex epitaxial films are used for source/drain (S/D) regions in pMOS devices to form ohmic metal-semiconductor contacts with low specific contact resistivities (ρc). Boron is the main p-type dopant in Si1-xGex thanks to its high active concentration (~ 1×1021 at./cm3) for Ge concentrations of ~ 50 at.% [1]. For higher Ge/Si ratios, gallium is considered a promising doping element due to its higher solid solubility in Ge compared with boron. However, a strong Ga surface segregation is reported to occur during the epitaxy of in situ doped Si1-xGex, leading to a Ga-rich second phase at the epilayer surface [2]. In films grown at temperatures ≥ 450°C, the Ga surface segregation limits the bulk Ga concentration ([Ga]chem) to 1×1018 at./cm3 [3]. This low doping level results in inferior electrical properties for current MOSFET applications. Models in the literature [4] indicate that, at sufficiently low temperature, dopant segregation is limited by surface kinetic mechanisms. Thus, the segregation ratio could be controlled by changing the process temperature. Based on this approach, low temperature, Si2H6 + Ge2H6 based processes with improved Ga incorporation were developed [5].In this contribution, we report on novel in situ Ga doped Si1-xGex epitaxial layers grown at temperatures ≤ 400°C. Using a HCl surface clean, necessary to avoid the SIMS Ga decay from the surface which can mask the real concentration profile [3], Ga box-shaped profiles with concentrations above 1×1020 at./cm3 are found (Fig. 1). Two growth temperatures are compared: 400°C and 350°C. For each temperature, variable Ga precursor (tri-tert-butyl gallium) flows (fTTBGa) were used. A reduction in the growth temperature enhances the Ga incorporation, as evidenced from the two SEM morphology inspections displayed in Fig. 1. Here, two samples with a similar bulk [Ga]chem of ~ 6 ×1019 at./cm3, as measured by SIMS, exhibit either a clustered or smooth surface depending on the deposition temperature applied.This improved Ga incorporation allows for systematic studies of in situ Ga doping on the structural and electrical properties of Si1-xGex. In both series of samples, the layer resistivities (ρ) follow a similar trend, characterized by a steep decrease at smaller fTTBGa and a plateau for higher flows (Fig. 2). The lowest ρ values are found for the layers grown at 350°C. This denotes that different saturation levels in the active Ga concentration ([Ga]act) exist for the process conditions used here. In a similar way to what is typically observed for B doping, an increase in the Ga doping level causes a boost in epilayer growth rate. This growth rate trend saturates simultaneously with ρ, suggesting that the phenomenon is linked to the level of incorporated substitutional Ga atoms. In addition, the resulting Ge concentrations of the Si1-xGex layers are found to increase with Ga concentration. From HR-XRD analysis, strain-relaxation is also found to progressively increase with the TTBGa flow and so the incorporated Ga may not be fully active. For this reason, X-ray absorption fine structure spectra (XAFS) were acquired at the Ga K-edge on Si0.4Ge0.6:Ga and Ge:Ga epilayers with the results obtained reported in Fig.3. Based on comparative analysis of the first coordination Ga shell with DFT simulation results, Ga dopants appear to be placed in substitutional positions for both materials. Furthermore, a shorter nearest-neighbors distance is measured for Ga in Si0.4Ge0.6 compared to Ge, indicating a difference in the local strain associated to the dopant for the two considered matrixes.[1] R. Loo et al., “Processing Technologies for Advanced Ge Devices,” ECS J. Solid State Sci. Technol., vol. 6, no. 1, pp. P14–P20, 2017.[2] J. Margetis et al., “Epitaxial Growth of Ga-doped SiGe for Reduction of Contact Resistance in finFET Source/Drain Materials,” ECS Trans., vol. 93, no. 1, pp. 7–10, 2019.[3] G. Rengo et al., “B and Ga Co-Doped Si1−xGex for p-Type Source/Drain Contacts,” ECS J. Solid State Sci. Technol., vol. 11, no. 2, 024008, 2022.[4] C. B. Arnold and M. J. Aziz, “Unified kinetic model of dopant segregation during vapor-phase growth,” Phys. Rev. B - Condens. Matter Mater. Phys., vol. 72, no. 19, 195419, 2005.[5] A. Y. Hikavyy et al., “Cutting-Edge Epitaxial Processes for Sub 3 Nm Technology Nodes: Application to Nanosheet Stacks and Epitaxial Wrap-Around Contacts,” ECS Trans., vol. 104, no. 4, pp. 139–146, Oct. 2021. Figure 1

Laser heating induced spatial homogenization of phase separated Na2O-B2O3-SiO2 glass plate with bearing NiO for heat center and structural probe

Na2O-B2O3-SiO2 glass plate phase separated in spinodal was partially remelted and homogenized by laser heating. As heat generation source, 0.8 mol% NiO was added to absorb continuous-wave laser with λ=1064 nm. After acid leaching, borate phase with NiO was remaining partially in laser irradiated area. Smooth, non-porous surface was found at cross section of laser irradiated area below surface, and nano-scale porous structure was found at non-irradiated area observed by scanning electron microscopy (SEM). X-ray Absorption Fine Structure (XAFS) spectra revealed that Ni2+ in laser irradiated area showed coordination change from 6 coordinated octahedral to 5 coordinated in square pyramid along with apparent color change from green to brown. Similar change was observed in water-quenched specimen from 1000°C. Ni2+ transition gave additional structural information in phase separate / homogeneous glass, which was difficult to be distinguished by spectroscopy which focuses on covalent bonding of glass network.

Coupling between γ-irradiation and synchrotron-radiation-based XAFS techniques for studying Mn-doped ZnO nanoparticles

γ-Irradiation and synchrotron-radiation-based X-ray absorption fine-structure (XAFS) spectroscopy have been used to induce structure disorder through the interaction of γ-rays (200 kGy) with fabricated Mn-doped ZnO nanoparticles (NPs) and then to examine thoroughly the resultant structural change. The extracted electronic/fine XAFS structural parameters reflect a compositional and γ-irradiation co-dependence. The average crystal structure of samples prepared by the sol-gel method was investigated by X-ray diffraction (XRD). A detailed structural XRD data analysis was carried out by applying a Rietveld refinement using the MAUD program. XAFS spectra were collected at the Zn K-edge (9659 eV) in transmission mode and at the Mn K-edge (6539 eV) in fluorescence mode. Direct evidence of the solubility of Mn ions in the ZnO structure was demonstrated by fitting the extended-XAFS (EXAFS) signal. Near-edge XAFS (XANES) analysis provided the oxidation states of Zn and Mn ions through fingerprint XANES spectra of the sample along with those of standard compounds. Linear combination fitting showed that the most fit chemical forms of Zn and Mn in the samples are ZnO and MnO, respectively. The oxidation states of both Zn and Mn XAFS absorbers were confirmed from pre-edge fitting. The results of the magnetic measurements were explained in light of the average and electronic/local structural information obtained from XRD, XANES and EXAFS techniques. The magnetic properties of the samples translate into an induced change in the average crystal and electronic/local structures upon Mn concentration change and γ-irradiation. XRD confirmed the successful preparation of hexagonal Mn-doped ZnO NPs with a crystallite size in the range 33-41 nm. Both XRD and EXAFS analysis detected a minor amount of Mn3O4 as a secondary phase. XANES and EXAFS provided information exploring the outstanding potential of the utilized protocol for detecting precisely the presence of the secondary phase of Mn3O4, which changes with Mn content (x). Mean-square relative displacement (σ2) values extracted from the EXAFS fitting were found to grow for Zn-Zn/Mn paths demonstrating the substitution of Mn/Zn into Zn crystal sites. The EXAFS analysis explains the reasons behind the enhancement in the magnetic properties and shows that the Mn doping content at x = 0.05 produces the most local atomic disorder in ZnO NPs. There is a strong harmony among the XRD, XANES, EXAFS and magnetization behavior of the Mn-doped ZnO NPs. Maximum magnetization was acquired at an Mn content of 0.05. γ-Ray-irradiated Zn1-xMnxO NPs are recommended as optimized candidates for showing the diversity of the applications.

The Valence Band Structure of the [Ni(Salen)] Complex: An Ultraviolet, Soft X-ray and Resonant Photoemission Spectroscopy Study

The valence band photoemission (VB PE) spectra of the [Ni(Salen)] molecular complex were measured by ultraviolet, soft X-ray and resonant photoemission (ResPE) using photons with energies ranging from 21.2 eV to 860 eV. It was found that the Ni 3d atomic orbitals’ (AOs) contributions are most significant for molecular orbitals (MOs), which are responsible for the low-energy PE band at a binding energy of 3.8 eV in the VB PE spectra. In turn, the PE bands in the binding energies range of 8–16 eV are due to the photoionization of the MOs of the [Ni(Salen)] complex with dominant contributions from C 2p AOs. A detailed consideration was made for the ResPE spectra obtained using photons with absorption resonance energies in the Ni 2p3/2, N 1s, and O 1s Near-Edge X-ray Absorption Fine Structure (NEXAFS) spectra. A strong increase in the intensity of the PE band ab was found when using photons with an energy 854.4 eV in the Ni 2p3/2 NEXAFS spectrum. This finding is due to the high probability of the participator-Auger decay of the Ni 2p3/2−13d9 excitation and confirms the relationship between the PE band ab with the Ni 3d-derived MOs.

Structure Determination, Mechanical Properties, Thermal Stability of Co2MoB4 and Fe2MoB4.

The precise determination of atomic position of materials is critical for understanding the relationship between structure and properties, especially for compounds with light elements of boron and single or multiple transition metals. In this work, the single crystal X-ray diffraction is employed to analyze the atomic positions of Co2MoB4 and Fe2MoB4 with a Ta3B4-type structure, and it is found that the lengths of B-B bonds connecting the two zig-zag boron chains are 1.86 Å and 1.87 Å, but previously unreported 1.4 Å. Co and Fe atoms occupy the same crystallographic position in lattice for the doped samples and the valence is close to the metal itself, and Co/Fe K-edge X-ray Absorption Fine Structure(XAFS) spectra of borides with different ratios of Co to Fe are collected to detect the local environment and chemical valence of Co and Fe. Vickers hardness and nano indentation measurements are performed, together with the Density Functional Theory (DFT) calculations. Finally, Co2MoB4 possess better thermal stability than Fe2MoB4 evaluated by Thermogravimetric Differential Thermal Analysis (TG-DTA) results.

Antimony speciation, phytochelatin stimulation and toxicity in plants

Antimony (Sb) is a toxic metalloid that has been listed as a priority pollutant. The environmental impacts of Sb have recently attracted attention, but its phytotoxicity and biological transformation remain poorly understood. In this study, Sb speciation and transformation in plant roots was quantified by Sb K-edge X-ray absorption spectroscopy. In addition, the phytotoxicity of antimonate (SbV) on six plant species was assessed by measuring plant photosynthesis, growth, and phytochelatin production induced by SbV. Linear combination fitting of the Sb K-edge X-ray absorption near-edge structure (XANES) spectra indicated reduction of SbV was limited to ∼5–33% of Sb. The data confirmed that Sb-polygalacturonic acid was the predominant chemical form in all plant species (up to 95%), indicating Sb was primarily bound to the cell walls of plant roots. Shell fitting of Sb K-edge X-ray absorption fine-structure (EXAFS) spectra confirmed Sb–O and Sb–C were the dominant scattering paths. The fitting indicated that SbV was bound to hydroxyl functional groups of cell walls, via development of a local coordination environment analogous to Sb-polygalacturonic acid. This is the first study to demonstrate the key role of plant cell walls in Sb metabolism.

A polycrystalline diamond micro-detector for X-ray absorption fine-structure measurements.

The microminiaturization of detectors used to record the intensity of X-ray beams is very favorable for combined X-ray experimental techniques. In this paper, chemical-vapor-deposited (CVD) polycrystalline diamond film was used to fabricate a micro-detector owing to its well controlled size, good thermostability, and appropriate conductivity. The preparation process and the main components of the CVD diamond micro-detector are described. The external dimensions of the packaged CVD diamond micro-detector are 15 mm × 7.8 mm × 5.8 mm. To demonstrate the performance of the detector, K-edge X-ray absorption fine-structure (XAFS) spectra of Cr, Fe, Cu, and Se foils were collected using the CVD diamond micro-detector and routine ion chamber. These XAFS measurements were performed at beamline 1W2B of Beijing Synchrotron Radiation Facility, covering an energy range from 5.5 to 13.5 keV. By comparison, it can be seen that the CVD diamond micro-detector shows a more excellent performance than the routine ion-chamber in recording these XAFS spectra. The successful application of the CVD diamond micro-detector in XAFS measurements shows its feasibility in recording X-ray intensity.

Influence of Chemical Substitution and Light Element Insertion on the Magnetic Properties of Nanocrystalline Pr2Co7 Compound

It is well recognized that intermetallics based on rare-earth (R) and transition metals (T) display numerous interesting magnetic properties, leading to potential applications in different fields. The latest progress regarding magnetic properties and the magnetocaloric effect (MCE) in the nanostructured Pr2Co7 compound, as well as its carbides and hydrides, is reviewed in this paper. Some of this progress reveals remarkable MCE performance, which makes it attractive in the field of magnetic refrigeration at high temperatures. With the purpose of understanding the magnetic and magnetocaloric characteristics of these compounds, the crystal structure, microstructure, and magnetism are also brought into focus. The Pr2Co7 compound has interesting magnetic properties, such as a high Curie temperature TC and uniaxial magnetocrystalline anisotropy. It crystallizes in a hexagonal structure (2:7 H) of the Ce2Ni7 type and is stable at relatively low temperatures (Ta ≤ 1023 K), or it has a rhombohedral structure (2:7 R) of the Gd2Co7 type and is stable at high temperatures (Ta ≥ 1223 K). Studies of the magnetocaloric properties of the nanocrystalline Pr2Co7 compound have shown the existence of a large reversible magnetic entropy change (ΔSM) with a second-order magnetic transition. After its substitution, we showed that nanocrystalline Pr2Co7−xFex compounds that were annealed at Ta = 973 K crystallized in the 2:7 H structure similarly to the parent compound. The extended X-ray absorption fine-structure (EXAFS) spectra adjustments showed that Fe atoms preferably occupy the 12k site for x ≤ 1. The study of the magnetic properties of nanocrystalline Pr2Co7−xFex compounds revealed an increase in TC of about 26% for x = 0.5, as well as an improvement in the coercivity, Hc (12 kOe), and the (BH)max (9 MGOe) product. On the other hand, the insertion of C atoms into the Pr2Co7 cell led to a marked improvement in the TC value of 21.6%. The best magnetic properties were found for the Pr2Co7C0.25 compound annealed at 973 K, Hc = 10.3 kOe, and (BH)max = 11.5 MGOe. We studied the microstructure, hydrogenation, and magnetic properties of nanocrystalline Pr2Co7Hx hydrides. The crystal structure of the Pr2Co7 compound transformed from a hexagonal (P63/mmc) into an orthorhombic (Pbcn) and monoclinic (C2/c) structure during hydrogenation. The absorption of H by the Pr2Co7 compound led to an increase in the TC value from 600 K at x = 0 to 691 K at x = 3.75. The Pr2Co7H0.25 hydride had optimal magnetic properties: Hc = 6.1 KOe, (BH)max = 5.8 MGOe, and TC = 607 K. We tailored the mean field theory (MFT) and random magnetic anisotropy (RMA) methods to investigate the magnetic moments, exchange interactions, and magnetic anisotropy properties. The relationship between the microstructure and magnetic properties is discussed. The obtained results provide a fundamental reference for adapting the magnetic properties of the Pr2Co7, Pr2Co6.5Fe0.5, Pr2Co7C0.25, and Pr2Co7H0.25 compounds for potential permanent nanomagnets, high-density magnetic recording, and magnetic refrigeration applications.

Crystal structure of U1-yLnyO2-x (Ln = Gd, Er) solid solution

The crystal structure was evaluated for U 1-y Ln y O 2-x (Ln = Gd, Er; y = 0−0.4) samples sintered at 1973 K for 8 h under Ar and Ar-10% H 2 atmospheres. The effect of LnO 1.5 doping on the crystal structure was investigated by X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS). The structures were refined by Rietveld analysis of the XRD patterns. LnO 1.5 doping into UO 2 reduced the lattice parameter of UO 2 −LnO 1.5 solid solutions up to 40 mol % LnO 1.5 . The lattice parameters of these samples were comparable to those of stoichiometric (U,Ln)O 2.00 solid solutions, that is, the O/M ratios were close to 2.00. XAFS spectra in the X-ray absorption near edge structure region showed that higher U oxidation states of +5 or +6 formed, in addition to +4. Extended XAFS indicated that the interatomic distances of U−O and Gd−O decreased with increasing y , whereas those of Er−O may not decrease monotonically.

Insight into the structure and metabolic function of iron-rich nanoparticles in anammox bacteria

Anaerobic ammonium-oxidizing (anammox) bacteria are iron abundant and depend heavily on iron-binding proteins. The iron demand of anammox bacteria is relatively large. However, it still remains some doubts where these large quantities of available iron come from and how they are regulated in anammox bacteria. Herein, iron-rich nanoparticles in anammoxosomes were detected by synchrotron soft X-ray tomography coupled with scanning transmission X-ray microscopy (STXM). The iron-rich nanoparticles were identified as ferric oxide (α-Fe2O3) mineral cores, and the local atomic structure of iron-rich nanoparticles was obtained by X-ray absorption fine-structure (XAFS) spectra. The bacterioferritin of Q1Q315 and Q1Q5F8 were detected by proteomics analysis. On this basis, the metabolic pathway centered on iron-rich nanoparticles was proposed.

Novel Ni-Doped Bismuth-Magnesium Tantalate Pyrochlores: Structural and Electrical Properties, Thermal Expansion, X-ray Photoelectron Spectroscopy, and Near-Edge X-ray Absorption Fine Structure Spectra.

The samples of Ni-doped bismuth magnesium tantalate pyrochlores with the general formula Bi1.4(Mg1–xNix)0.7Ta1.4O6.3 (x = 0.3, 0.5, 0.7) were obtained by solid-phase synthesis. The crystal structure of the pyrochlore type (sp. gr. Fd3̅m:2) was clarified by the Rietveld method on the basis of X-ray powder diffraction data. The unit cell parameters increase with the decreasing nickel content in the range from 10.5319(1) to 10.5391(1) Å. The electronic state of atoms is established by the XPS method. According to XPS analysis, bismuth atoms have an effective charge of +3, nickel atoms +(2 + δ), and tantalum ions +(5 – δ). The coefficient of thermal expansion of the lattice of the samples was calculated from high-temperature X-ray structural measurements in the range of −180 to 1050 °C. The average values of linear TECs α in the temperature ranges of 30–570 and 600–1050 °C are 5.1 × 10–6 and 8.1 × 10–6 °C–1, respectively. The monotonicity of the change in the thermal expansion coefficient in the temperature range from −100 to 1050 °C indicates the absence of phase transformations. All samples are dielectric and exhibit high activation energies ∼2.0 eV, moderately high dielectric constants ∼24–28, and tangent dielectric losses ∼0.002 at 1 MHz and 21 °C. The electrical properties of the samples are described by a simple parallel equivalent scheme. The chemical composition of the materials has little effect on the polarizability of the medium or on the value of the activation energy of the conductivity. Ionic processes in investigated materials at frequencies 200–106 Hz and at temperatures 100–450 °C were not detected.

Hydration of Heavy Alkaline-Earth Cations Studied by Molecular Dynamics Simulations and X-ray Absorption Spectroscopy.

The physicochemical properties of the three heaviest alkaline-earth cations, Sr2+, Ba2+, and Ra2+ in water have been studied by means of classical molecular dynamics (MD) simulations. A specific set of cation–water intermolecular potentials based on ab initio potential energy surfaces has been built on the basis of the hydrated ion concept. The polarizable and flexible model of water MCDHO2 was adopted. The theoretical–experimental comparison of structural, dynamical, energetic, and spectroscopical properties of Sr2+ and Ba2+ aqueous solutions is satisfactory, which supports the methodology developed. This good behavior allows a reasonable reliability for the predicted Ra2+ physicochemical data not experimentally determined yet. Simulated extended X-ray absorption fine-structure (EXAFS) and X-ray absorption near-edge spectroscopy spectra have been computed from the snapshots of the MD simulations and compared with the experimental information available for Sr2+ and Ba2+. For the Ra2+ case, the Ra L3-edge EXAFS spectrum is proposed. Structural and dynamical properties of the aqua ions for the three cations have been obtained and analyzed. Along the [M(H2O)n]m+ series, the M–O distance for the first-hydration shell is 2.57, 2.81, and 2.93 Å for Sr2+, Ba2+, and Ra2+, respectively. The hydration number also increases when one is going down along the group: 8.1, 9.4, and 9.8 for Sr2+, Ba2+, and Ra2+, respectively. Whereas [Sr(H2O)8]2+ is a typical aqua ion with a well-defined structure, the Ba2+ and Ra2+ hydration provides a picture exhibiting an average between the ennea- and the deca-hydration. These results show a similar chemical behavior of Ba2+ and Ra2+ aqueous solutions and support experimental studies on the removal of Ra-226 of aquifers by different techniques, where Ra2+ is replaced by Ba2+. A comparison of the heavy alkaline ions, Rb+ and Cs+, with the heavy alkaline-earth ions is made.