EXAFS Research Articles

Isomorphic substitution behavior of Pt on synthetic vernadite (δ-MnO2):A model reaction to elucidate the mechanism of Pt enrichment in marine ferromanganese crust

Marine ferromanganese crusts are considered to be the potential Pt deposits, and recent studies report a strong association between Pt and vernadite in these crusts. However, there are still uncertainties regarding the molecular processes involved in the enrichment of Pt by vernadite. This study investigates the adsorption behavior and molecular-scale immobilization mechanisms of Pt at water/vernadite interfaces through a combination of batch adsorption and desorption experiments, X-ray absorption fine structure (XAFS) analyses, and atomic resolution chemical imaging. Our findings suggest that the Pt enrichment on vernadite is attributed to an isomorphic substitution of Mn by Pt. The extended X-ray absorption fine structure (EXAFS) analysis reveals that Pt undergoes ligand exchange reactions during the enrichment process, and provides a comprehensive explanation of how Pt penetrates into the vernadite structure starting from the surface. Scanning transmission electron microscopy high-angle annular dark field (STEM-HAADF) observations visually confirm the infiltration of Pt atoms into the crystal structure of vernadite. Additionally, we demonstrate that the Pt enrichment in vernadite is a multi-stage chemical process. X-ray absorption near edge structure (XANES) analysis shows that Pt is initially rapidly adsorbed onto the surface of vernadite and then slowly oxidized. Our study provides a detailed understanding of the molecular-scale mechanisms involved in the adsorption of Pt onto vernadite and its subsequent structural incorporation. This knowledge is valuable for gaining insights into the Pt enrichment in marine ferromanganese crusts.

Effects of Fe Ions, Ultraviolet Irradiation, and Heating on Microscopic Structures of Black Lacquer Films.

The chemical reaction between Fe and lacquer has been used to create the black color in lacquer coatings since ancient times. Here, the effects of Fe ion addition, UV irradiation, and heating on the microscopic structures of black lacquer films were investigated by using X-ray absorption near edge structure (XANES), extended X-ray absorption fine structure (EXAFS), Fourier transform-infrared spectroscopy (FT-IR), small-angle X-ray scattering (SAXS), and small angle neutron scattering (SANS). The EXAFS result indicated that heating and UV irradiation made the coordination structure of Fe3+ in the lacquer nonuniform, and that heating caused the greatest nonuniformity. The FT-IR, SAXS, and SANS results demonstrated that the microscopic structural changes in the black lacquer films were induced by both heating and UV irradiation, but the changes were different. Heating caused a substantial structural change on the nanoscale, and UV irradiation mainly caused changes in the molecular binding mode. The results provide important knowledge for analyzing archeological lacquer samples and for developing lacquer-based materials. This work also demonstrates the utility of the complementary use of XANES, EXAFS, FT-IR, SAXS, and SANS for nondestructive analysis of black lacquer in precious cultural relics.

Combined experimental and theoretical studies of some uranium(VI) complexes derived from imidazole-based carbenes

A series of bidentate, viz., 1,1'-(1,2-ethylene)-3,3'-dimethyldiimidazoline-2,2'-diylidene (L1), 1-methyl-3-(2-pyridylmethyl)-imidazoline-2-ylidene (L2) and tridentate, viz., 1,3-bis(2-pyridyl)-imidazoline-2-ylidene (L3) ligands have been obtained from their corresponding imidazolium salts through deprotonation reactions. Treatment of UO2Cl2(THF)3 with one equivalent L3 produces air-stable U(VI)-carbene complex 3, characterized by elemental analysis, FTIR, NMR spectroscopy as well as extended X-ray absorption fine structure (EXAFS) analysis. EXAFS result indicates that the ligand is bonded through one C and two N-atoms to the uranium atom. Attempts to synthesize uranyl complexes derived from L1 and L2 were also successful but these complexes are decomposed quickly within one hour at room temperature. 3 produces pure UO2 powder when heated under an argon atmosphere from room temperature to 600 °C with constant heating rate of 5 °C/min. The solid-state UV–Vis spectrum of the compound shows absorption peaks at 332 and 450 nm. The excitation spectrum of 3 (at λ em = 520 nm) exhibits two almost symmetrical peaks at 273 and 368 nm. Density functional theory-based quantum mechanical calculations indicate that a partial covalent interaction exists between the carbene C and U while a weak non-covalent interaction exists between carbene N and U atoms.

Mechanistic Insights of Ethylene Polymerization on Phillips Chromium Catalysts.

Silica-supported chromium oxide catalysts, also named Phillips chromium catalysts (PCCs), provide more than half of the world's production of high- and medium-density polyethylenes. PCCs are usually prepared in the Cr(VI)/SiO2 form, which is subjected to reductive activation. It has been explicitly proven that CO reduces Cr(VI) to Cr(II) species that initiate ethylene polymerization; ethylene activates Cr(VI) sites as well, but the nature of the catalytic species is complicated by the presence of the ethylene oxidation products. It is widely accepted that the catalytic species are of a Cr(III)-alkyl nature, but this common assumption faces the challenge of "extra" hydrogen: the formation of similar species under the action of even-electron reducing agents requires an additional H atom. Relatively recently, it was found that saturated hydrocarbons can also activate CrOx/SiO2, and alkyl fragments turn out to be bonded with a polyethylene chain. In recent years, there have been numerous experimental and theoretical studies of the structure and chemistry of PCCs at the different stages of preparation and activation. The use of modern spectral methods (such as extended X-ray absorption fine structure (EXAFS), X-ray absorption near-edge structure (XANES), and others); operando IR, UV-vis, EPR, and XAS spectroscopies; and theoretical approaches (DFT modeling, machine learning) clarified many essential aspects of the mechanisms of CrOx/SiO2 activation and catalytic behavior. Overall, the Cosse-Arlman mechanism of polymerization on Cr(III)-alkyl centers is confirmed in many works, but its theoretical support required the development of nontrivial and contentious mechanistic concepts of Cr(VI)/SiO2 or Cr(II)/SiO2 activation. On the other hand, conflicting experimental data continue to be obtained, and certain mechanistic concepts are being developed with the use of outdated models. Strictly speaking, the main question of what type of catalytic species, Cr(II), Cr(III), or Cr(IV), comes into polymerization still has not received an unambiguous answer. The role of the chemical nature of the support-through the prism of the nature, geometry, and distribution of the active sites-is also not clear in depth. In the present review, we endeavored to summarize and discuss the recent studies in the field of the preparation, activation, and action of PCCs, with a focus on existing contradictions in the interpretation of the experimental and theoretical results.

Medium-range order structure analysis of metallic glasses based on reverse Monte Carlo modeling

The elucidation of amorphous alloy characteristics presents a formidable challenge due to the absence of crystalline arrangements and enigmatic imperfections. This study aimed to tackle this challenge through a comprehensive investigation utilizing synchrotron X-ray diffraction (XRD), extended X-ray absorption fine structure spectroscopy (EXAFS), ab initio molecular dynamics (AIMD) simulations, and reverse Monte Carlo (RMC) modeling. The primary objective was to delve into the medium range order (MRO) structure of metallic glasses (MGs) containing CuXZr100-X, wherein X represents values of 50, 56, 60, and 64. The results obtained from this research demonstrate that an increase in the Cu atom concentration within Cu-Zr MGs leads to the formation of complete icosahedral clusters and high-coordination polyhedral connections, thereby bolstering the stability and heterogeneity of local structures. Examination of the stiff full icosahedral bones shows that defects exist in the network connections on the MRO, resulting in a lack of connectivity in the positive icosahedron. Conversely, the density of defects decreases with an increase in Cu content. The observation suggests that the observed low MRO defects in Cu64Zr36 MGs may be accountable for their excellent glass-forming ability and kinetic stability.

Demonstration of Sensitivity of the Total-Electron-Yield Extended X-ray Absorption Fine Structure Method on Plastic Deformation of the Surface Layer

X-ray Absorption Fine Structure Spectroscopy (XAFS) has proven instrumental for the study of atomic-scale structures across diverse materials. This study conducts a meticulous comparative analysis between total electron yield (TEY) and absorption coefficients at the K absorption edge of polycrystalline Fe and Zr60Cu20Fe20 alloy. Our findings not only highlight differences between TEY and transmission XAFS measurements but also demonstrate the capabilities and limitations inherent in these measurement modes within the context of XAFS. This article provides an experimental exploration of widely used X-ray absorption spectroscopy methods, shedding light on the nuances of TEY and transmission XAFS. Through presenting experimental results, we aim to offer insights crucial to the material science community, guiding experimentalists in optimizing measurements while raising awareness about potential misinterpretations.

Anomalies in the short-range local environment and atomic diffusion in single crystalline equiatomic CrMnFeCoNi high-entropy alloy

Multi-edge extended X-ray absorption fine structure (EXAFS) spectroscopy combined with reverse Monte Carlo (RMC) simulations was used to probe the details of element-specific local coordinations and component-dependent structure relaxations in single crystalline equiatomic CrMnFeCoNi high-entropy alloy as a function of the annealing temperature. Two representative states, namely a high-temperature state, created by annealing at 1373 K, and a low-temperature state, produced by long-term annealing at 993 K, were compared in detail. Specific features identified in atomic configurations of particular principal components indicate variations in the local environment distortions connected to different degrees of compositional disorder at the chosen representative temperatures. The detected changes provide new atomistic insights and correlate with the existence of kinks previously observed in the Arrhenius dependencies of component diffusion rates in the CrMnFeCoNi high-entropy alloy.

Acetylene ligands stabilize atomically dispersed supported rhodium complexes under harsh conditions

Facile sintering of atomically dispersed supported noble metal catalysts at catalytically relevant temperatures, particularly under reducing conditions, poses a challenge for their practical applications. Some ligands, such as carbonyls, aid in improving the stability at the expense of severely suppressing the catalytic activity. Here, we demonstrate that substitution of the carbonyl ligands with reactive acetylene ligands can maintain the atomic dispersion of the supported mononuclear rhodium complex under harsh reducing conditions (>573 K), as confirmed by in-situ X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopies. In contrast, the supported rhodium carbonyl complex aggregates into nanoclusters under identical conditions. Furthermore, our results indicate that the acetylene ligands provide this anti-sintering ability while retaining the hydrogenation activity.

Incorporation of palladium into pyrite: Insights from X-ray absorption spectroscopy analysis and modelling

Pyrite (FeS2) often accommodates elevated concentrations of platinum-group elements in ores of magmatic and hydrothermal origin. In order to elucidate the role of pyrite in concentrating Pd, Pd-doped synthetic crystals were studied via X-ray absorption spectroscopy (XAS). Crystals were obtained by salt-flux method in the system saturated with respect to Pd at the temperature of 580 °C and sulphur fugacity of log f (S2) = −0.4. Scanning electron microscopy, electron probe microanalysis, and laser ablation inductively coupled plasma mass spectrometry studies demonstrated a uniform distribution of Pd within the pyrite crystals. The median and average values of Pd content of ∼0.7 ± 0.1 wt% were measured. Comparison of the Pd K-edge X-ray absorption near edge structure (XANES) spectra with the spectra of standards revealed that the formal oxidation state of Pd was close to +2. Fitting of the extended X-ray absorption fine structure (EXAFS) and Finite Difference Method for Near-Edge Structure (FDMNES) theoretical simulations of XANES spectra showed that Pd substituted for Fe in the crystal structure of pyrite. The isomorphous Pd in pyrite was octahedrally coordinated by S atoms at ∼2.385 Å. The PdS interatomic distance was 5.6 % larger than that of FeS due to the difference in their covalent radii of ∼5.3 %. The expansion caused by the incorporation of Pd into the pyrite structure disappeared at the distance of R > 3 Å. The information on the state of Pd in pyrite, including the local atomic environment and formal oxidation state, is essential for scientific and industrial purposes, e.g. physical-chemical modelling and improvement of leaching and extraction processing respectively.

Effect of B-site cationic substitution on the structural, spectroscopic, and conductivity behaviour of Ho2(Hf1-xZrx)2O7 (x=0 and 1)

Ideally, a solid electrolyte which is a central component of SOFC should exhibit high anionic or cationic ionic conductivity at the proposed operating temperatures. In most cases, the performance is compromised when operating above 1000 °C due to poor mechanical, thermal and chemical stability of selected functional and non-functional materials. In this context, pyrochlores are one of the potential candidates due to their high conductivity, flexibility to accommodate large cations, and high mechanical, thermal and chemical stability. In this study, we report the synthesis of nano-powders of Ho2Hf2O7 (HH) and Ho2Zr2O7 (HZ) pyrochlore ceramics by eco-friendly alginate mediated ion-exchange process also known as Leeds Alginate Process (LAP) and provide further insight into the structure - conductivity relationship of HH and HZ compounds by EXAFS studies. Both the compositions were sintered at temperatures, ranging from 1100 °C-1500 °C at 2 h dwell time to achieve the desired high-density ceramic with stable pyrochlore structure. X-ray diffraction and Extended X-ray absorption fine structural (EXAFS) analysis showed superlattice reflections and complex coordination geometry of these oxides. The coordination number and disorder factor in the case of HZ were found to be more stable than the HH sample, as evident from the EXAFS. Impedance spectroscopy and dc-conductivity analysis showed a better charge transport behavior in HZ ceramics than in HH making HZ as a preferred solid electrolyte for SOFC. The conductivity of Ho2Zr2O7 is comparable with the best-known fluorite oxide-ion conductor such as Sc2O3-stabilized ZrO2 at 500 °C.

Interactions of Perrhenate (Re(VII)O4−) with Fe(II)-Bearing Minerals

Rhenium (Re) is an extremely rare element, with a crustal abundance of approximately 0.4 parts per billion (ppb) and a sea water concentration of 8.3 parts per trillion (ppt). However, Re concentrations in anoxic marine sediments range from 2 to 184 ppb, which is attributed to reduction of the highly soluble perrhenate ion (Re(VII)O4−) to insoluble Re(IV) species. Anoxic sediments typically contain Fe(II) and sulfide species, which could potentially reduce Re(VII) to Re(IV). In this study, we examined the interactions of KReO4 with magnetite (Fe3O4), siderite (FeCO3), vivianite (Fe3(PO4)2•8H2O), green rust (mixed Fe(II)/Fe(III) layered double hydroxide), mackinawite (FeS), and chemically reduced nontronite (NAu-1) using X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy to determine the valence state and speciation of Re. Uptake of Re by green rust was rapid, with ~50% associated with the solids within 2 days. In contrast, there was <10% uptake by the other Fe(II) phases over 48 days. Reduction of Re(VII) to Re(IV) was only observed in the presence of green rust, producing clusters of bidentate-coordinated Re(IV)O6 octahedra.. These results suggest that except for green rust, the potential for other Fe(II)-bearing minerals to act as reductants for ReO4− in sedimentary environments requires further investigation.

Using XAS to monitor radiation damage in real time and post-analysis, and investigation of systematic errors of fluorescence XAS for Cu-bound amyloid-β.

X-ray absorption spectroscopy (XAS) is a promising technique for determining structural information from sensitive biological samples, but high-accuracy X-ray absorption fine structure (XAFS) requires corrections of systematic errors in experimental data. Low-temperature XAS and room-temperature X-ray absorption spectro-electrochemical (XAS-EC) measurements of N-truncated amyloid-β samples were collected and corrected for systematic effects such as dead time, detector efficiencies, monochromator glitches, self-absorption, radiation damage and noise at higher wavenumber (k). A new protocol was developed using extended X-ray absorption fine structure (EXAFS) data analysis for monitoring radiation damage in real time and post-analysis. The reliability of the structural determinations and consistency were validated using the XAS measurement experimental uncertainty. The correction of detector pixel efficiencies improved the fitting χ2 by 12%. An improvement of about 2.5% of the structural fitting was obtained after dead-time corrections. Normalization allowed the elimination of 90% of the monochromator glitches. The remaining glitches were manually removed. The dispersion of spectra due to self-absorption was corrected. Standard errors of experimental measurements were propagated from pointwise variance of the spectra after systematic corrections. Calculated uncertainties were used in structural refinements for obtaining precise and reliable values of structural parameters including atomic bond lengths and thermal parameters. This has permitted hypothesis testing.

Ultra‐Low 4.3 wt% Silicon Thermal Reducing Doped Porous Si@MoC as Highly Capable and Stable Li‐Ion Battery Anode

AbstractSilicon is a promising anode material in lithium‐ion batteries (LIBs) for its ultra‐high theoretical capacity; however, its large volume expansion and low electrical conductivity trigger capacity degradation and poor stability. Herein, an ultra‐low 4.3 wt% Si‐doped porous MoC (p‐Si@MoC) is constructed by a facile thermal reduction on a core‐shelled precursor of ZnMo‐hybrid zeolitic imidazole framework (HZIF‐ZnMo) coated by tetraethyl orthosilicate (TEOS), delivering a high capacity and superior cycling stability (976.6 mAh g−1 after 250 cycles at 0.2 A g−1) in LIBs. The homogeneous distribution in the porous MoC matrix contributes to its maximum capacity utilization. Meanwhile, the porous substrate enhances Li ion transport kinetics and reduced the volume expansion of Si. The excellent electronic conductivity of p‐Si@MoC is revealed by the density functional theory (DFT) calculations. The Mo─Si bonds formed by Si‐doped in the MoC matrix are verified by X‐ray absorption near‐edge structure (XANES) and extended X‐ray absorption fine structure (EXAFS). Moreover, the in situ X‐ray diffraction (in situ XRD) reveals the lithium storage mechanism. This work presents an excellent structural design and synthesis strategy for high‐performance silicon‐based anode materials.

Local Environment of Sc and Y Dopant Ions in Aluminum Nitride Thin Films.

The local environments of Sc and Y in predominantly ⟨002⟩ textured, Al1-xDoxN (Do = Sc, x = 0.25, 0.30 or Y, x = 0.25) sputtered thin films with wurtzite symmetry were investigated using X-ray absorption (XAS) and photoelectron (XPS) spectroscopies. We present evidence from the X-ray absorption fine structure (XAFS) spectra that, when x = 0.25, both Sc3+ and Y3+ ions are able to substitute for Al3+, thereby acquiring four tetrahedrally coordinated nitrogen ligands, i.e., coordination number (CN) of 4. On this basis, the crystal radius of the dopant species in the wurtzite lattice, not available heretofore, could be calculated. By modeling the scandium local environment, extended XAFS (EXAFS) analysis suggests that when x increases from 0.25 to 0.30, CN for a fraction of the Sc ions increases from 4 to 6, signaling octahedral coordination. This change occurs at a dopant concentration significantly lower than the reported maximum concentration of Sc (42 mol % Sc) in wurtzite (Al, Sc)N. XPS spectra provide support for our observation that the local environment of Sc in (Al, Sc)N may include more than one type of coordination.

Single atom vacancy engineering with highly reversible N4 sites enable ultra-low overpotential for durable zinc-ion supercapacitors

Zn metal has delivered great promise as anode material for grid-level energy storage yet is challenged by dendrite growth. However, the overpotential, another important factor for energy conversion efficiency has been often overlooked, making the design of zincophilic substrates to guide the uniform deposition of Zn2+ with low overpotential are full of challenges. Here, inspired by density functional theory (DFT) that the N4 sites created by single atom vacancy with large binding energy with Zn2+ can induce the Zn2+ to form Zn-N4 bonds, according well with the electrochemical results that the Zn2+ can be reintroduced back into the electrolyte, demonstrating that the process is highly reversible. Extended X-ray absorption fine structure (EXAFS) combined with electron paramagnetic resonance (EPR) analysis certified the coexistence of both stable Zn-N4 sites and cation vacancies with high density in single atom Zn supported on nitrogen doped carbon materials (SA-Zn/CN-1). Most impressively, the SA-Zn/CN-1@Zn anode exhibits remarkable cycling stability at 1 mA cm−2 for over 3300 h with the ultra-low overpotential of about 22 mV in symmetric cells and a considerable capacity retention of 82% after 10000 cycles in capacitors assembled with active carbon. In a word, this work suggests a new approach to inhibit the growth of zinc dendrites with low overpotential through high reversible N4 sites provided by single atom vacancy engineering, which may be extended to inhibit other metal dendrites in the future.

Strain-induced specific orbital control in a Heusler alloy-based interfacial multiferroics

For the development of spintronic devices, the control of magnetization by a low electric field is necessary. The microscopic origin of manipulating spins relies on the control of orbital magnetic moments (morb) by strain; this is essential for the high performance magnetoelectric (ME) effect. Herein, electric-field induced X-ray magnetic circular dichroism (XMCD) is used to determine the changes in morb by piezoelectric strain and clarify the relationship between the strain and morb in an interfacial multiferroics system with a significant ME effect; the system consists of the Heusler alloy Co2FeSi on a ferroelectric Pb(Mg1/3Nb2/3)O3-PbTiO3 substrate. Element-specific investigations of the orbital states by operando XMCD and the local environment via extended X-ray absorption fine structure (EXAFS) analysis show that the modulation of only the Fe sites in Co2FeSi primarily contributes to the giant ME effect. The density functional theory calculations corroborate this finding, and the growth of the high index (422) plane in Co2FeSi results in a giant ME effect. These findings elucidate the element-specific orbital control using reversible strain, called the ‘orbital elastic effect,’ and can provide guidelines for material designs with a giant ME effect.

Atomic-scale environment of niobium in ore minerals as revealed by XANES and EXAFS at the Nb K-edge

Abstract. The mineralogy of niobium (Nb) is characterized by multicomponent oxides such as AB2O6, A2B2O7, ABO4, and ABO3 in which Nb is incorporated in the B site. Such complex crystal-chemistry prevents their unambiguous identification in ore deposits such as hydrothermal rocks and laterites which exhibit complex and fine-grained textures. The understanding of the processes controlling Nb ore deposit formation in various geological settings is therefore limited, although Nb is a critical element. In this study, we use X-ray absorption spectroscopy (XAS) at the Nb K-edge to investigate the local atomic-scale structure around Nb in a large set of natural and synthetic minerals of geological and technological importance. Our X-ray absorption near-edge structure (XANES) data at the Nb K-edge show three major features of variable position and intensity and then can be related to the local distortion and coordination number of the Nb site. Shell-by-shell fits of the extended X-ray absorption fine structure (EXAFS) data reveal that the NbO6 octahedra are distorted in a variety of pyrochlore species. At least two distinct first shells of O atoms are present while reported crystallographic data yield regular octahedra in the same minerals. Next-nearest Nb–Nb distances in pyrochlore and Nb-bearing perovskite mirror a corner-sharing NbO6 network, whereas the two Nb–Nb distances in columbite are typical of edge- and corner-sharing NbO6 octahedra. Such a resolution on the Nb site geometry and the intersite relationships between the next-nearest NbO6 octahedra is made possible by collecting EXAFS data under optimal conditions at 20 K and up to 16 Å−1. The local structure around substituted Nb5+ in Fe3+, Ti4+, and Ce4+ oxides suffers major changes relative to the unsubstituted structures. The substitution of Nb5+ for Ti4+ in anatase leads to the increase in the interatomic distances between Nb and its first and second Ti4+ neighbors. The substitution of Nb5+ for Ce4+ in cerianite reduces the coordination number of the cation from eight to four, and the Nb–O bonds are shortened compared to Ce–O ones. In hematite, Nb5+ occupies a regular site, whereas the Fe3+ site is strongly distorted suggesting major site relaxation due to charge mismatch. The sensitivity of XANES and EXAFS spectroscopies at the Nb K-edge to the local site geometry and next-nearest neighbors demonstrated in this study would help decipher Nb speciation and investigate mineralogical reactions of Nb minerals in deposit-related contexts such as hydrothermal and lateritic deposits.

Study of threshold energy [formula omitted] in EXAFS

This work demonstrates that the threshold energy E0 can be obtained from a simple tentative rule and the bond length R can be obtained correctly by running the FIT program while fixing E0. The extended X-ray absorption fine structure (EXAFS) spectra for Cu, W, and Au metal foils are tested and for the anharmonic system, the experimental Cu spectra measured at temperatures of 50, 150, 295, 400, 500, 600 and 700K. From the tests, it is confirmed that a more precise bond length R comparable to the crystallographic data can be obtained for both cases. Moreover, it is deduced that the new running mode of the conventional EXAFS FIT program yields satisfactory results for the anharmonic system.

Comprehensive understanding of the crystal structure of perovskite-type Ba3Y4O9 with Zr substitution: a theoretical and experimental study.

We previously reported that Zr substitution improves the chemical stability of Ba3Y4O9 and nominally 20 mol% Zr-substituted Ba3Y4O9 is an oxide-ion conductor at intermediate temperatures (500-700 °C). However, the influence of Zr substitution on the structural properties of Ba3Y4O9 was poorly understood. This paper aims to comprehensively understand the crystal structure of Ba3Y4O9 with Zr substitution by powder X-ray diffraction (XRD), extended X-ray absorption fine structure (EXAFS) measurements, and first-principles calculations. From the results, firstly we found that the hexagonal unit cell of Ba3Y4O9 reported in the database should be revised as doubled along the c-axis in terms of the periodicity of oxide-ion positions. The revised unit cell of Ba3Y4O9 consists of 18 layers of BaO3 and 24 layers of Y which periodically stack along the c-axis. In this work, we focused on the cationic lattice and noticed that the periodical stacking of Ba and Y layers comprises a similar sequence to that in the body-centered cubic (BCC) structure. There are two regions in the Ba3Y4O9 structure: one is a hetero-stacking region of Ba and Y layers (Ba-Y-Ba-Y-Ba) and the other is a homo-stacking region (Ba-Y-Y-Ba). It is noteworthy that the former region is similar to a cubic perovskite. In Zr-substituted Ba3Y4O9, Zr ions preferentially substitute for Y ions in the hetero-stacking region, and therefore the local environment of Zr ions in Ba3Y4O9 is quite similar to that in BaZrO3. Besides, the Zr substitution for Y in Ba3Y4O9 increases the fraction of the cubic-perovskite-like region in the stacking sequences. The structural change in the long-range order strongly affects the other material properties such as chemical stability and the ionic-conduction mechanism. Our adopted description of perovskite-related compounds based on the stacking sequence of the BCC structure should help in understanding the complex structure and developing new perovskite-related materials.

Hollow spherical nano-traps using pillararene-based polymer for efficient uranium extraction from seawater.

Herein, a hollow spherical pillar[5]arene-based polymer (P5-AO) adsorbent was synthesized. The P5-AO adsorbent was capable of effectively capturing uranium from simulated seawater (139.5 mg g-1) and real seawater (8.1 mg g-1). We also elucidated the uranium adsorption mechanism of P5-AOvia extended X-ray absorption fine structure (EXAFS). This study provides a novel direction for the development of uranium capture adsorbents.