Impact of Anion Vacancies on the Local and Electronic Structures of Iron-Based Oxyfluoride Electrodes

The properties of crystalline solids can be significantly modified by deliberately introducing point defects. Understanding these effects, however, requires understanding the changes in geometry and electronic structure of the host material. Here we report the effect of forming anion vacancies, via dehydroxylation, in a hexagonal-tungsten-bronze–structured iron oxyfluoride, which has potential use as a lithium-ion battery cathode. Our combined pairdistribution function and density-functional–theory analysis indicates that oxygen vacancy formation is accompanied by a spontaneous rearrangement of fluorine anions and vacancies, producing dual pyramidal (FeF4)–O–(FeF4) structural units containing five-fold–coordinated Fe atoms. The addition of lattice oxygen introduces new electronic states above the top of the valence band, with a corresponding reduction in the optical band gap from 4.05 eV to 2.05 eV. This band gap reduction relative to the FeF3 parent material is correlated with a significant improvement in lithium insertion capability relative to defect-free compound

Magnetic Ordering in Ultrasmall Potassium Ferrite Nanoparticles Grown on Graphene Nanoflakes

Magnetic nanoparticles are central to the development of efficient hyperthermia treatments, magnetic drug carriers, and multimodal contrast agents. While the magnetic properties of small crystalline iron oxide nanoparticles are well understood, the superparamagnetic size limit constitutes a significant barrier for further size reduction. Iron (oxy)hydroxide phases, albeit very common in the natural world, are far less studied, generally due to their poor crystallinity. Templating ultrasmall nanoparticles on substrates such as graphene is a promising method to prevent aggregation, typically an issue for both material characterization and applications. We generate ultrasmall nanoparticles, directly on the carbon framework by the reaction of a graphenide potassium solution, charged graphene flakes, with iron(II) salts. After mild water oxidation, the obtained composite material consists of ultrasmall potassium ferrite nanoparticles bound to the graphene nanoflakes. Magnetic properties as evidenced by magnetometry and X-ray magnetic circular dichroism, with open magnetic hysteresis loops near room temperature, are widely different from classical ultrasmall superparamagnetic iron oxide nanoparticles. The large value obtained for the effective magnetic anisotropy energy density Keff accounts for the presence of magnetic ordering at rather high temperatures. The synthesis of ultrasmall potassium ferrite nanoparticles under such mild conditions is remarkable given the harsh conditions used for the classical syntheses of bulk potassium ferrites. Moreover, the potassium incorporation in the crystal lattice occurs in the presence of potassium cations under mild conditions. A transfer of this method to related reactions would be of great interest, which underlines the synthetic value of this study. These findings also give another view on the previously reported electrocatalytic properties of these nanocomposite materials, especially for the sought-after oxygen reduction/evolution reaction. Finally, their longitudinal and transverse proton NMR relaxivities when dispersed in water were assessed at 37 °C under a magnetic field of 1.41 T, allowing potential applications in biological imaging.IdEx Bordeau

Tendances actuelles dans la caractérisation des obsidiennes pour les études de provenance

Les recherches de provenance des obsidiennes de sites paléolithiques et néolithiques connaissent actuellement une grande expansion, notamment par leur intégration dans les études de chaînes opératoires, qui nécessitent de déterminer l’origine de la matière première de séries importantes de pièces archéologiques. Il existe par ailleurs une forte demande de mesures peu à non destructives et éventuellement à effectuer in situ. D’où un certain nombre de développements méthodologiques récents. Le but de cette contribution est d’en présenter le statut actuel.Sourcing studies involving obsidians from Palaeolithic and Neolithic sites are presently undergoing a period of change, notably in terms of the average number of samples analyzed per project, due largely to their integration into the chaînes opératoires approach of lithic technologies. This can entail working with a large number of artifacts from an archaeological context. There is also an ever-increasing demand to employ quasi non-destructive characterization methods and completely non-destructive methods that eventually could be performed in situ. As such, recent methodological advances have taken several directions. The purpose of this short note is to give an overview of these approaches

Reassessing the role of magnetite during natural hydrogen generation

Interactions between water and ferrous rocks are known to generate natural H2 in oceanic and continental domains via the oxidation of iron. Such generation has been mainly investigated through the alteration of Fe2+-silicate and some Fe2+-carbonates. So far, magnetite (α-Fe3O4) has never been considered as a potential source mineral for natural H2 since it is considered as a by-product of every known chemical reaction leading to the formation of H2, despite it bears 1/3 of Fe2+ in its mineral lattice. This iron oxide is rather seen as a good catalyst for the formation of H2. Recently, hydrogen emissions were observed in the surroundings of banded iron formations (BIF) that are constituted of, among other minerals, magnetite. Thus, this work is an attempt to constrain the true potential of magnetite by means of batch reactor experiments and additional thermodynamic calculations. It explores theoretical and experimental reaction pathways of magnetite during water-rock interactions, focusing on low temperatures (T < 200°C). For the purpose of the experiments, gold capsules filled with magnetite powders were run at 80°C and 200°C. Gas products were analyzed using gas chromatography (GC) while solid products were characterized by X-ray diffraction (XRD), Mössbauer spectroscopy, and scanning electron microscopy (SEM). After experimental alteration, high amounts of H2 were quantified while mineralogical transitions were observed by SEM. It showed self-reorganization of the primary iron oxide resulting in sharp-edge and better crystalized secondary minerals. In parallel, XRD analyses showed tiny changes between the patterns of the initial powder and the solid products of reaction. Finally, Mössbauer spectroscopy revealed that the starting magnetite was partly converted to maghemite (γ-Fe2O3), a metastable Fe-oxide only containing Fe3+. Major implications arise from these results. Concerning H2 exploration, this work provides evidence that natural hydrogen can be generated at near-ambient temperature. It also infers that magnetite-rich lithologies such as BIF should be targeted while looking for H2 source rocks. In addition, these outcomes could be of major interest for mining companies as they provide key elements to understand the formation of BIF-hosted iron ores

Feasibility and Limitations of High-Voltage Lithium-Iron-Manganese Spinels

Positive electrodes with high energy densities for Lithium-ion batteries (LIB) almost exclusively rely on toxic and costly transition metals. Iron based high voltage spinels can be feasible alternatives, but the phase stabilities and optimal chemistries for LIB applications are not fully understood yet. In this study, LiFe$_{x}$Mn$_{2-x}$O$_{4}$ spinels with x = 0.2 to 0.9 were synthesized by solid-state reaction at 800 °C. High-resolution diffraction methods reveal gradual increasing partial spinel inversion as a function of x and early secondary phase formation. Mössbauer spectroscopy was used to identify the Fe valences, spin states and coordination. The unexpected increasing lattice parameters with Fe substitution for Mn was explained considering the anion-cation average bond lengths determined by Rietveld analysis and Mn$^{3+}$ overstoichiometries revealed by cyclic voltammetry. Finally, galvanostatic cycling of Li-Fe-Mn-spinels shows that the capacity fading is correlated to increased cell polarization for higher upper charging cut-off voltage, Fe-content and C-rate. The electrolyte may also contribute significantly to the cycling limitations

Revealing defects in crystalline lithium-ion battery electrodes by solid state NMR: applications to LiVPO4F

International audienceIdentifying and characterizing defects in crystalline solids is a challenging problem, particularly for lithium-ion intercalation materials, which often exhibit multiple stable oxidation and spin states as well as local ordering of lithium and charges. Here, we reveal the existence of characteristic lithium defect environments in the crystalline lithium-ion battery electrode LiVPO4F and establish the relative subnanometer-scale proximities between them. Well-crystallized LiVPO4F samples were synthesized with the expected tavorite-like structure, as established by X-ray diffraction (XRD) and scanning transmission electron microscopy (STEM) measurements. Solid-state 7Li nuclear magnetic resonance (NMR) spectra reveal unexpected paramagnetic 7Li environments that can account for up to 20% of the total lithium content. Multidimensional and site-selective solid-state 7Li NMR experiments using finite-pulse radio frequency-driven recoupling (fp-RFDR) establish unambiguously that the unexpected lithium environments are associated with defects within the LiVPO4F crystal structure, revealing the existence of dipole–dipole-coupled defect pairs. The lithium defects exhibit local electronic environments that are distinct from lithium ions in the crystallographic LiVPO4F site, which result from altered oxidation and/or spin states of nearby paramagnetic vanadium atoms. The results provide a general strategy for identifying and characterizing lithium defect environments in crystalline solids, including paramagnetic materials with short 7Li NMR relaxation times on the order of milliseconds

How to measure the ESR intensity of the Al centre in optically bleached coarse quartz grains for dating purpose?

The ESR intensity of the Al signal has been traditionally extracted from the measurement of the amplitude between the top of the first peak and the bottom of the last peak from g = 2.0185 to g = 1.9928. However, a recent study by Kabacin´ska and Timar-Gabor (2022) showed the limitations of this method. As a follow-up, we investigated and compared various Al signal intensity extraction methods to evaluate their impact on the De estimation of several coarse-grained quartz samples (100-200 μm) from Early Pleistocene to modern-age deposits. In particular, we tested the potential of using the area of the Al signal near g = 2.0603, as it is theoretically free of any major interfering signals. However, our results show that the extraction methods related to this area do not offer any substantial advantages over the traditional method in the case of coarse-grained samples. Instead, measurement of the ESR intensity is more time consuming, and resulting dose response curves are more scattered. Actually, most Al intensity extraction methods tested in this study return equivalent dose (De) estimates (as well as bleaching coefficient values) within error, suggesting that the interfering signals do not seem to strongly bias the ESR dose evaluation in those coarse-grained quartz samples. This outcome provides additional support in favor of the use of the traditional method. However, the significant inter-sample variability observed in our study nevertheless shows the necessity to extend this investigation to a larger number of samples of various grain sizes, origins and chronologies in order to identify more meaningful patterns on a larger scale

Recherche de provenance de quartz et d'obsidiennes préhistoriques en Europe occidentale

Le quartz et l'obsidienne, de part leurs qualités mécaniques et esthétiques, ont été utilisés par les hommes préhistoriques pour confectionner des outils, des armes, des éléments de parure, etc. L'étude de ces matériaux par différentes méthodes physiques offre aux archéologues les moyens d'appréhender la circulation des matières premières et les prémices d'échanges organisés. L'étude préliminaire de quartz de synthèse par cathodoluminescence (CL), thermoluminescence (TL) et résonance paramagnétique électronique (RPE) a mis en évidence l'influence des conditions de cristallogenèse, notamment de la pression et de la composition du fluide minéralisateur, sur l'insertion des impuretés (aluminium, germanium, ...) dans le réseau cristallin. L'étude par RPE et spectrométrie de CL de quartz hyalins naturels prélevés au sein de différents gîtes des Alpes occidentales et de pièces archéologiques (déchets de taille) mises au jour sur trois sites alpins (méso- à néolithiques) a montré les potentialités de ces méthodes pour la recherche de provenance. Les expériences de RPE menées sur des obsidiennes provenant des principales îles-sources de Méditerranée et sur des échantillons d'origine archéologique et de source géographique connue ont confirmé les possibilités discriminantes de cette technique et nous conduisent à la proposer pour rechercher la provenance d'obsidiennes archéologiques. En effet, comparée aux méthodes habituellement utilisées pour les études de recherche de provenance, la RPE apparaît suffisamment discriminante, rapide et économique (en matière et en moyens) pour permettre l'analyse d'un grand nombre d'échantillons et répondre aux attentes des archéologues.BORDEAUX1-BU Sciences-Talence (335222101) / SudocBORDEAUX3-BU Lettres-Pessac (335222103) / SudocSudocFranceF

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