Fluorescence Yield Mode Research Articles

Development of soft x-ray absorption spectroscopy technique with simultaneous measurement in electron- and fluorescence-yield modes from high vacuum to ambient pressure for observation of surface adsorbed species.

To understand the reactions of heterogeneous catalysts at the solid-gas interface under actual reaction conditions, it is important to develop a method to observe the surface-adsorbed species during the reaction, including the changes before and after the adsorption of light elements involved in the surface reaction. We developed a soft x-ray absorption spectroscopy (XAS) technique that allows simultaneous measurements in the electron- and fluorescence-yield modes in the pressure range of 10-4-1 × 105Pa. In the developed system, the reaction gas near the sample surface is separated from the beamline vacuum by a Si3N4 window and confined to a small area to suppress x-ray absorption by the gas. The electron-yield spectra were obtained by measuring the sample current while applying a bias potential to the Si3N4 window. XAS measurements were performed from high vacuum to ambient pressure by setting the bias potential to 600 and 39V below and above 100Pa, respectively. An anatase TiO2 nanoparticle-deposited film was prepared by spin coating, and soft XAS was performed to observe the photocatalytic oxidative decomposition reactions of isopropanol in the presence of water and oxygen. The obtained O K-edge spectra showed that it is possible to observe adsorbed oxygen on solid oxides even under ambient pressure conditions containing 0.1% of oxygen gas.

Deep understanding of LiCoO2 electrode degradation for optimized recycling strategies

Empowered by a synergistic combination approach of Scanning Transmission X-ray Microscopy (STXM) imaging and X-ray Absorption Near Edge Structure (XANES) spectroscopy, a quantitative analysis on the composition and spatial distribution of the cathode in a failed 18650 LiCoO2 (LCO) battery was conducted. Distinct compositional differences between the central and peripheral cathode regions in the failed LCO battery were discerned by utilizing bulk XANES in Total Electron Yield (TEY) and Fluorescence Yield (FY) modes. The central region shows more severe degradation in terms of LCO reduction and excess cathode-electrolyte interface (CEI) buildup. Meanwhile, the STXM technique precisely imaged a specific region of interest in the center of the cathode, covering C, O, and F K-edges, and Co L2,3-edge, to deeply investigate the degradation. Quantitative chemical mapping with spatially resolved XANES spectroscopy, facilitate an in-depth understating of the interfacial reactions on battery electrodes and battery failure fundamentals.

A versatile beamline for soft x-ray reflectivity, absorption, and fluorescence measurements at Indus-2 synchrotron source.

A versatile beamline for performing reflectivity, fluorescence, and absorption experiments in the soft x-ray region of 100-1500eV is commissioned on a bending magnet port of the Indus-2 synchrotron source. A high vacuum 2-axis reflectometer with x, y, and z sample scanning stages is installed. This reflectometer is used to measure the reflectivity of large samples up to 300mm in length and 5kg in weight. This feature is useful for characterizing x-ray optical elements, such as mirrors, gratings, and multilayers. A flange mounted silicon drift detector is installed in the downstream of the reflectometer for soft x-ray fluorescence measurements. The soft x-ray absorption measurements are carried out in the total electron yield and partial fluorescence yield modes. Integration of three different experimental techniques in the experimental station makes the beamline versatile for materials science applications as it provides structural, chemical, and electronic state information by performing the required experiments in an identical environment. The beamline uses a varied line spacing plane grating monochromator and gives a high flux (∼109 to 1011 photon/s) with a moderate resolution (λ/Δλ ~1000-5000). A three-mirror-based higher harmonic setup is incorporated to get rid of harmonics and to get a high spectral purity monochromatic beam with less than 0.1% harmonic content. In the present article, the beamline optical scheme, mechanical configuration, and details of the experimental setups are presented, along with a few representative results of each experimental mode.

Origin of microscopically coupled ferromagnetic Cu-ions in a distorted system of Cu-doped ZnO and their synchrotron-based electronic structures

Spintronics-based studies have produced significant attention in the last decade while claiming the observation of room temperature ferromagnetism (RTFM). Nevertheless, there is a lack of consensus on a mechanism responsible for this phenomenon. In this study, we focus on Cu-doped ZnO (ZCO) to understand the microscopic origin of RTFM and the role of different oxidation states of Cu in RTFM. We have performed different spectroscopic techniques using synchrotron facilities. The values of spin-moment obtained from x-ray magnetic circular dichroism sum-rule truly exhibit a ferromagnetic interaction in the nanocrystalline powder of ZCO with ∼0.58 μB for 5% of Cu concentration in the total fluorescence yield mode. Such an enhanced magnetization is attributed to the presence of Cu2+, which is mainly localized in the bulk region. Cu in ZCO is mostly dominated by the presence of Cu2+. This is clearly reflected by the profiles of x-ray photoemission spectroscopy. Consequently, the weakly magnetized total electron yield mode is attributed to a state of magnetic frustration as the majority of Cu3+ is found on the surface. Some of these Cu3+ when come in the vicinity of Cu2+ ions result in a highly correlated state of double exchange mechanism, which is the microscopic origin of RTFM in ZCO. The coupling between Cu2+-Cu3+ is mediated via oxygen vacancies (VO), the presence of which is confirmed through the features of electron energy loss spectroscopy over different edges. The confirmation of VO is also supported by the deconvolution of E2high-phonon in the Raman spectra. Moreover, the defects in the local electronic structures of ZCO are demonstrated by the deconvoluted spectra of Cu L3 x-ray absorption spectroscopy. The images obtained from high-resolution transmission electron microscopy confirm the incorporation of Cu into the wurtzite crystal of ZnO. A clear enhancement in magnetization upon an increase in carriers of Cu in ZCO indicates carrier-induced ferromagnetism. Cu2+ and VO are the two attributes of RTFM in ZCO.

Origin of Enhancement of Orbital Magnetic Moment in SiO2-Coated Fe3O4 Nanocomposites Studied by X-ray Magnetic Circular Dichroism.

In this study, magnetic Fe3O4 nanoparticles (NPs) were dispersed uniformly by varying the thickness of the SiO2 coating, and their electronic and magnetic properties were investigated. X-ray diffraction confirmed the structural configuration of monophase inverse-spinel Fe3O4 NPs in nanometer size. Scanning electron microscopy revealed the formation of proper nonporous crystallite particles with a clear core-shell structure with silica on the surface of Fe3O4 NPs. The absorption mechanism studied through the zeta potential indicates that SiO2-coated Fe3O4 nanocomposites (SiO2@Fe3O4 NCs) possess electrostatic interactions to control their agglomeration in stabilizing suspensions by providing a protective shield of amorphous SiO2 on the oxide surface. High-resolution transmission electron microscopy images demonstrate a spherical morphology having an average grain diameter of ∼11-17 nm with increasing thickness of SiO2 coating with the addition of a quantitative presence and proportion of elements determined through elemental mapping and electron energy loss spectroscopy studies. Synchrotron-based element-specific soft X-ray absorption spectroscopy and X-ray magnetic circular dichroism (XMCD) techniques have been involved in the bulk-sensitive total fluorescence yield mode to understand the origin of magnetization in SiO2@Fe3O4 NCs. The magnetization hysteresis of Fe3O4 was determined by XMCD. At room temperature, the magnetic coercivity (Hc) is as high as 1 T, which is about 2 times more than the value of the thin film and about 5 times more pronounced than that of NPs. For noninteracting single-domain NPs with the Hc spread from 1 to 3 T, the Stoner-Wohlfarth model provided an intriguing explanation for the hysteresis curve. These curves determine the different components of Fe oxides present in the samples that derive the remnant magnetization involved in each oxidation state of Fe and clarify which Fe component is responsible for the resultant magnetism and magnetocrystalline anisotropy based on noninteracting single-domain particles.

Magnetic properties of Cr-doped VO2 thin films in tetragonal phase of rutile and their synchrotron-based electronic structures

The present study is focused on the investigation at 400 K of the tetragonal-rutile phase of Cr-doped VO2 (CVO) thin films grown by pulsed laser deposition. Synchrotron-based x-ray measurements of both the surface-sensitive total electron yield (TEY) and bulk-sensitive total fluorescence yield (TFY) modes were used to investigate the pristine and Cr-doped VO2 (5%, 10%, 20%, and 30% of atomic weight). The structural analysis and purity of the crystalline phase of the as-deposited films are manifested via grazing incidence x-ray diffraction patterns, which confirm the tetragonal-rutile phase. The purity of the phase is also confirmed by the presence of Eg-mode phonons in the Raman spectra and its deconvolution reflects on the oxygen-mediated electronic/vibrational transitional effect. A clear hysteretic behavior obtained through vibrating sample magnetometry strongly suggests the ferromagnetic interaction in the thin films of CVO. The local-electronic property of the samples is examined using x-ray absorption spectroscopy (XAS) in TEY and TFY modes where the difference in the configured helicity photons resulted in the fine spectra of x-ray magnetic circular dichroism (XMCD). XAS and XMCD measurements performed at V L2,3 and Cr L2,3 edges explicitly demonstrate the ferromagnetism in the thin films of CVO. The strong hybridization between V 3d and Cr 3d states with O 2p states is evident from the spectra of the O K-edge, resulting in the onset of the cation-pair formation V5+–Cr3+, which is ferromagnetic by means of double-exchange interaction. The theoretical calculation of density functional theory made upon Vienna ab initio simulation package suggests that CVO is in a mixed state of a ferromagnetic-insulator and a half-metallic ferromagnet.

Synchrotron X-ray absorption spectroscopy and fluorescence spectroscopy studies of collagen type I-talc intercalated nanocomposites

We investigated the properties of collagen type I-talc intercalated nanocomposites using different techniques. Understanding the conformational changes of collagen induced by clay is noticeably lacking, which is pivotal for understanding the structure of collagen-clay hybrid nanocomposites. We aim to investigate the influence of talc clay minerals on the microstructure of collagen type I molecules. Our main goal is to understand the interaction between talc and collagen molecules and to elucidate the microstructure–property relation of intercalated hybrid nanocomposites. The FT-IR spectrum of collagen type I-talc confirmed the presence of collagen amide bands and talc silicate bonds. Also, the differential scanning calorimetry (DSC) result showed that the addition of talc to the matrix could increase the denaturation temperature (Td = 40.8 °C) compared to pure collagen (Td = 19 °C). The fluorescence intensity increased in collagen type I-talc nanocomposites, indicating that the interaction of talc with collagen chains in collagen type I-talc dispersion varies the micro-environment of the tyrosine amino acids in the collagen fibrils and its fluorescence. We also analyzed X-ray absorption spectroscopy (XAS) in total fluorescence yield (TFY) and total energy yield (TEY) modes of pure collagen and collagen type I-talc nanocomposites prepared at different concentrations of talc and at different temperatures (4 °C, 8 °C, and 14 °C). We have found that the synthesis conditions have a considerable effect on the local atomic environments of nitrogen, carbon, and oxygen atoms in collagen type I-talc nanocomposites. These results are of significance for developing new applications of talc and collagen-based biomaterials in different fields, such as cellular and molecular biology, and drug delivery that enhances the optical signals are needed.

Mild nitridation of perovskite manganite: Synthesis, structure and magnetism

The nitrogen lightly doped perovskite manganite La0.5Ba0.5MnO(3-δ)-xNx was synthesized following a mild nitridation strategy by optimizing annealing temperature, annealing time and NH3/Ar flow rate ratio. Neutron powder diffraction (NPD) characterization evidences an N-doping triggered structural phase transition from pseudo-cubic Pm-3m to tetragonal P4/mmm and further confirms a preferable substitution at O1 and O2 sites closing to the Ba-rich layers. N 1s K-edge X-ray absorption near edge spectrum (XANES) with total fluorescence yield (TFY) mode, X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FT-IR) spectroscopy collectively convince the Mn–N bonded state. The O/N atomic ratio is determined to be 2.26/0.22 by XPS analysis closing to the value of 2.25/0.18 from NPD refinement. Magnetic property characterization reveals that both long-range Mn3+-O-Mn4+ ferromagnetic coupling and high Curie temperature (TC) are maintained upon N-doping. Besides, significant spin frozen with robust domain pinning is identified below TC which can be ascribed to the short-range Mn–N–Mn clusters and/or oxygen defects. This work expands the connotation of transition-metal based perovskite oxynitride and may inspire the development of photosensitive semiconductor device with magnetic response.

Pt-Mediated Interface Engineering Boosts the Oxygen Reduction Reaction Performance of Ni Hydroxide-Supported Pd Nanoparticles

Fuel cells are considered potential energy conversion devices for utopia; nevertheless, finding a highly efficacious and economical electrocatalyst for the oxygen reduction reaction (ORR) is of great interest. By keeping this in view, we have proposed a novel design of a trimetallic nanocatalyst (NC) comprising atomic Pt clusters at the heterogeneous Ni(OH)2-to-Pd interface (denoted NPP-70). The as-prepared material surpasses the commercial J.M.-Pt/C (20 wt %) catalyst by ∼ 166 and ∼19 times with exceptionally high specific and mass activities of 16.11 mA cm-2 and 484.8 mA mgPt-1 at 0.90 V versus reversible hydrogen electrode (RHE) in alkaline ORR (0.1 M KOH), respectively. On top of that, NPP-70 NC retains nearly 100% performance after 10k accelerated durability test (ADT) cycles. The results of physical characterization and electrochemical analysis confirm that atomic-scale Pt clusters induce strong lattice strain (compressive) at the Ni(OH)2-to-Pd interface, which triggers the electron relocation from Ni to Pt atoms. Such charge localization is vital for O2 splitting on surface Pt atoms, followed by the relocation of OH- ions from the Pd surface. Besides, a sharp fall down in ORR performance (mass activity is 37 mA mgPt-1 at 0.90 V versus RHE) is observed when the Pt clusters are decorated on the surface of NiOx and Pd (denoted NPP-RT). In situ partial fluorescence yield mode X-ray absorption spectroscopy (PFY-XAS) was employed to reveal the ORR pathways on both configurations. The obtained results demonstrate that interface engineering can be a potential approach to boost the electrocatalytic activity of metal hydroxide/oxide-supported Pd nanoparticles and in turn allow Pd to be a promising alternative for commercial Pt catalysts.

Origin and Quantitative Description of the NESSIAS Effect at Si Nanostructures

AbstractThe electronic structure of SiO2‐ versus Si3N4‐coated low nanoscale intrinsic silicon (Si) shifts away from versus toward the vacuum level Evac, originating from the Nanoscale Electronic Structure Shift Induced by Anions at Surfaces (NESSIAS). Using the quantum chemical properties of the elements involved to explain NESSIAS, an analytic parameter Λ is derived to predict the highest occupied energy level of Si nanocrystals (NCs) as verified by various hybrid‐density functional calculations and NC sizes. First experimental data of Si nanowells (NWells) embedded in SiO2 versus Si3N4 were measured by X‐ray absorption spectroscopy in total fluorescence yield mode (XAS‐TFY), complemented by ultraviolet photoelectron spectroscopy (UPS), characterizing their conduction band and valence band edge energies EC and EV, respectively. Scanning the valence band sub‐structure over NWell thickness yields an accurate estimate of EV shifted purely by spatial confinement, and thus the actual EV shift due to NESSIAS. Offsets of ΔEC = 0.56 eV and ΔEV = 0.89 eV were obtained for 1.9 nm thick NWells in SiO2 versus Si3N4, demonstrating an intrinsic Si type II homojunction. This p/n junction generated by NESSIAS eliminates any deteriorating impact of impurity dopants, offering undoped ultrasmall Si electronic devices with much reduced physical gate lengths and CMOS‐compatible materials.

X-ray absorption spectroscopy without the self-absorption effect by detecting l-line fluorescence at the K-edge

A novel measurement method that addresses the self-absorption effect, which is a significant disadvantage in X-ray absorption spectroscopy in fluorescence yield mode for surface analysis, is proposed. The validity and features were experimentally verified using copper. This method uses the l-line fluorescence that cascades from the KLL Auger process caused by K-shell absorption. The obtained spectrum is unaffected by the self-absorption effect and is not distorted. The confirmed features are that the spectral oscillation with a wide energy range is obtained, and the analysis depth is comparable to that of the l-edge measurement. In addition, the proposed method theoretically applies to samples with uneven surfaces, that is, various actual materials.

Calcium Solvation Dynamics Probed By in-Situ/Operando Soft X-Ray Absorption Spectroscopy

Calcium is considered a promising candidate in multivalent battery technologies due to its safe, economic, and nontoxic nature. It has received considerable attention since the recent work showing that calcium can be plated and stripped at room temperature with good stabilities and capacities. It offers the promise of a more than two-fold increase in the volumetric capacity compared to the monovalent lithium-ion batteries. Additionally, because of the higher mobility of Ca2+ compared to that of Mg2+ because of the lower charge density of Ca2+, the interest in calcium batteries has been growing rapidly. The understanding of the solvation and charge transfer mechanism at the electrolyte/electrode interface and how different types of cations and anions will affect the calcium solvation at this interface is the key to develop novel calcium batteries. To probe the Ca2+ coordination at the electrolyte/electrode interface as a function of the type of anions, total electron yield (TEY) mode soft X-ray absorption spectroscopy (XAS) sensitive to the interfacial speciation has been used under operando electrochemical conditions. Meanwhile, total fluorescence yield (TFY) mode XAS is sensitive to bulk speciation. By comparing the operando TEY and TFY mode XAS from 0.5 M calcium bis(trifluoromethanesulfonyl)imide (CaTFSI2) in THF, the Ca L3,2-edge TEY-XAS intensity increased at positive potentials while the Ca L3,2-edge TFY-XAS intensity decreases at these positive potentials. Such a comparison clearly shows the solvation and desolvation dynamics of calcium at the electrolyte/electrode interface, which will benefit the future development of next-generation multivalent batteries.

Sulfur in humin as a redox-active element for extracellular electron transfer

Humin (HM), an insoluble humic substance that occurs as an organo-mineral complex, mediates extracellular electron transfer (EET) among microorganisms in the geosphere. Although sulfur (S)-containing functional groups have been posited as possible redox-active groups in HM, there has been no direct evidence in this regard. Here, we characterized the redox-active S in HM from different natural sources using synchrotron radiation-based X-ray absorption spectroscopy in partial fluorescence yield mode. The S content in HM is less than 0.9%, with the oxidation states ranged widely from −2 to +6. About 9% to 19% of total S in HM was redox variable, corresponding to 1.2 × 10−5 to 17.1 × 10−5 Eq/g-HM redox capacity. The redox capacity of HM driven by S is comparable to or larger than the electron donating capacity of HM observed in anaerobic microbial dechlorination of pentachlorophenol, suggesting that S is one of the significant redox-active elements in HM. These findings provide new insights into the role of S cycling in biogeochemical processes and microbial energy networks.

Turning Low-Nanoscale Intrinsic Silicon Highly Electron-Conductive by SiO2 Coating.

Impurity doping in silicon (Si) ultra-large-scale integration is one of the key challenges which prevent further device miniaturization. Using ultraviolet photoelectron spectroscopy and X-ray absorption spectroscopy in the total fluorescence yield mode, we show that the lowest unoccupied and highest occupied electronic states of ≤3 nm thick SiO2-coated Si nanowells shift by up to 0.2 eV below the conduction band and ca. 0.7 eV below the valence band edge of bulk silicon, respectively. This nanoscale electronic structure shift induced by anions at surfaces (NESSIAS) provides the means for low-nanoscale intrinsic Si (i-Si) to be flooded by electrons from an external (bigger, metallic) reservoir, thereby getting highly electron- (n-) conductive. While our findings deviate from the behavior commonly believed to govern the properties of silicon nanowells, they are further confirmed by the fundamental energy gap as per nanowell thickness when compared against published experimental data. Supporting our findings further with hybrid density functional theory calculations, we show that other group IV semiconductors (diamond, Ge) do respond to the NESSIAS effect in accord with Si. We predict adequate nanowire cross-sections (X-sections) from experimental nanowell data with a recently established crystallographic analysis, paving the way to undoped ultrasmall silicon electronic devices with significantly reduced gate lengths, using complementary metal-oxide-semiconductor-compatible materials.

Identification of structural phases in ferroelectric hafnium zirconium oxide by density-functional-theory-assisted EXAFS analysis

Crystalline phase identification for hafnium-based ferroelectrics by diffraction techniques has been elusive. We use density-functional-theory (DFT)-assisted extended X-ray absorption fine-structure spectroscopy (EXAFS) to determine the crystal symmetry of thin hafnium zirconium oxide (Hf0.46Zr0.54O2) films grown by atomic layer deposition. Ferroelectric switching in TiN/Hf0.46Zr0.54O2/TiN metal–insulator–metal capacitors is verified. Grazing-incidence fluorescence-yield mode Hf L3 and Zr K absorption edge EXAFS data are compared with reference data calculated from DFT-based atomic coordinates for various structural phases of Hf0.5Zr0.5O2. Via EXAFS multiphase fitting, we confirm that the frequently invoked polar orthorhombic Pca21 phase is present in ferroelectric hafnium zirconium oxide, along with an equal amount of the nonpolar monoclinic P21/c phase. For comparison, we verify that paraelectric HfO2 films exhibit the P21/c phase.

Photon-in/photon-out endstation for studies of energy materials at beamline 02B02 of Shanghai Synchrotron Radiation Facility*Project supported by the National Natural Science Foundation of China (Grant No. 11227902) as part of NSFC ME2 beamline project, Science and Technology Commission of Shanghai Municipality, China (Grant No. 14520722100), and the National Natural Science Foundation of China (Grant Nos. 11905283 and U1632269).

A new photon-in/photon-out endstation at beamline 02B02 of the Shanghai Synchrotron Radiation Facility for studying the electronic structure of energy materials has been constructed and fully opened to users. The endstation has the capability to perform soft x-ray absorption spectroscopy in total electron yield and total fluorescence yield modes simultaneously. The photon energy ranges from 40 eV to 2000 eV covering the K-edge of most low Z-elements and the L-edge of 3d transition-metals. The new self-designed channeltron detector allows us to achieve good fluorescence signals at the low photon flux. In addition, we synchronously collect the signals of a standard reference sample and a gold mesh on the upstream to calibrate the photon energy and monitor the beam fluctuation, respectively. In order to cross the pressure gap, in situ gas and liquid cells for soft x-ray absorption spectroscopy are developed to study the samples under realistic working conditions.

Hydroxyl-Group Identification Using O K-Edge XAFS in Porous Glass Fabricated by Hydrothermal Reaction and Low-Temperature Foaming.

Porous glass was prepared by the hydrothermal reaction of sodium borosilicate glass, and oxygen-ion characterization was used to identify the hydroxyl groups in its surface area. A substantial amount of “water” was introduced into the ionic structure as either OH− groups or H2O molecules through the hydrothermal reaction. When the hydrothermally treated glass was reheated at normal pressures, a porous structure was formed due to the low-temperature foaming resulting from the evaporation of H2O molecules and softening of the glass. Although it was expected that the OH− groups would remain in the porous glass, their distribution required clarification. Oxygen K-edge X-ray absorption fine structure (XAFS) spectroscopy enables the bonding states of oxygen ions in the surface area and interior to be characterized using the electron yield (EY) and fluorescence yield (FY) mode, respectively. The presence of OH− groups was detected in the O K-edge XAFS spectrum of the porous glass prepared by hydrothermal reaction with a corresponding pre-edge peak energy of 533.1 eV. In addition, comparison of the XAFS spectra obtained in the EY and FY modes revealed that the OH− groups were mainly distributed in the surface area (depths of several tens of nanometers).

Friction-condition-dependent sulfide and sulfate evolution on dialkylpentasulfide tribofilm studied by XANES

The effects of friction conditions, such as rotational speed, frictional time, and applied load, on the evolution mechanism of sulfide and sulfate on the top and bottom layers of tribofilm were investigated by total electron yield (TEY) and fluorescence yield (FY) mode X-ray absorption near-edge structure (XANES) spectra in the same beam line (4B7A). The results demonstrated that the top and bottom layers of tribofilms were covered by sulfide and sulfate. The addition of dialkylpentasulfide (DPS) could form complex nonuniform tribofilm. In addition, the friction condition (speed, load, or time) has its unique role in the generation of sulfide and sulfate at a specific depth on the tribofilm surface. The enhancement of friction conditions could promote the sulfur tribochemical reaction in a comparatively large range and alter the relative intensity of sulfurization and the sulfur-oxidizing process.

Cu 2p-1s x-ray emission spectroscopy of mineral tetrahedrite Cu12Sb4S13

The electronic structure of mineral tetrahedrite Cu12Sb4S13 showing a metal-semiconductor transition (MST) at TMST = 85 K has been investigated by means of Cu 1s x-ray absorption spectroscopy with the partial fluorescence yield mode (PFY-XAS) and Cu 2p-1s x-ray emission spectroscopy (XES). Remarkable changes are detected in both the PFY-XAS and XES spectra on cooling across TMST. The changes in the spectra indicate that the unoccupied Cu 3d and 4p densities of states (DOS) shift away from the Fermi energy (EF). We conclude that the MST of Cu12Sb4S13 is associated with the decrease of the Cu 3d DOS at EF.

Zirconium local environment in simplified nuclear glasses altered in basic, neutral or acidic conditions: Evidence of a double-layered gel

Borosilicate glasses containing 1 to 8 mol% ZrO2 were leached at pH 9, 7 and 1 until complete alteration. The Zr coordination in the bulk and at the gel surface is quantified using Zr L2- XANES spectra in fluorescence and total electron yield mode, respectively. This provides evidence of a single layer at pH 9 whereas a second alteration layer develops at the gel surface at pH 7 and at pH 1. The surface layer, a witness of the first stages of glass alteration, exhibits larger structural variations than the bulk of the gel. This is related with the alteration conditions, from incipient alteration of pristine glasses to chemical diffusion controlled by the gels formed at a later stage. In all gels investigated, a majority of Zr occurs in [6]Zr sites. However, at neutral and acidic pH, the gels contain a high fraction of 7- and 8-coordinated Zr ([7]Zr and [8]Zr, respectively) sites, and only 6-coordinated ([6]Zr) sites at pH 9. The Zr local environment in the gels, determined by Zr K-edge EXAFS, is similar to that in the pristine glasses, with some additional contribution of Zr second neighbors. Both Zr-site geometry and medium-range structure of the alteration gel bring direct evidence that, under the experimental conditions used, glass dissolution is driven by an in situ hydrolysis/condensation mechanism. This allows the gel structure to mimic that of the pristine glass, including the [6]Zr site distortion or the linkage to the silicate gel framework. The pH has a greater influence on the structure of gels than does their Zr content. The structural evolution around Zr is mainly governed by the pH during alteration, while the [6]Zr site symmetry is controlled by the Zr content of the gel. Certain gels contain more octahedral sites than expected from the concentration of charge compensating cations (Ca, Na), a situation similar to that observed in SiO2ZrO2 xerogels.