Exploring the Limits of Fe-Rich Chemistries in Na-Based CaFe2O4-Type Postspinel Oxides.

Structural trends, physical properties, and electrochemical performances of the NaFexRu2-xO4 system have been investigated. Synthesis attempts using both conventional solid-state routes and high-pressure methods were explored for the compositional range 1.0 ≤ x ≤ 1.67. Based on Rietveld refinements against powder X-ray diffraction data, analyses of 57Fe Mössbauer spectroscopy data, and elemental analysis by electron microprobe, the existence of a confined compositional solid solution (1 ≤ x ≤ 1.3) adopting the CaFe2O4-type postspinel structure is demonstrated. This is contrasted with the NaFexTi2-xO4 system, for which no evidence of a solid solution is observed. However, for all explored synthetic routes of NaFexRu2-xO4 compositions, a trivalent iron oxidation state is maintained. Structural analysis and qualitative bond valence energy landscape models reveal that the progressive integration of iron into the postspinel framework results in narrowed sodium ion diffusion channels, restricting electrochemical deintercalation of sodium. Consequently, the CaFe2O4-type iron-rich compounds explored in this study demonstrate limited potential as positive electrode materials for sodium batteries. It is expected that this fundamental insight will help guide the exploration of alternative NaM2O4-based positive electrode materials with similar structure types.

Spray-dried ternary bioactive glass microspheres: Direct and indirect structural effects of copper-doping on acellular degradation behavior.

Silicate-based bioactive glass nano/microspheres hold significant promise for bone substitution by facilitating osteointegration through the release of biologically active ions and the formation of a biomimetic apatite layer. Cu-doping enhances properties such as pro-angiogenic and antibacterial behavior. While sol-gel methods usually yield homogeneous spherical particles for pure silica or binary glasses, synthesizing poorly aggregated Cu-doped ternary glass nano/microparticles without a secondary CuO crystalline phase remains challenging. This article introduces an alternative method for fabricating Cu-doped ternary microparticles using sol-gel chemistry combined with spray-drying. The resulting microspheres exhibit well-defined, poorly aggregated particles with spherical shapes and diameters of a few microns. Copper primarily integrates into the microspheres as Cu0 nanoparticles and as Cu2+ within the amorphous network. This doping affects silica network connectivity, as calcium and phosphorus are preferentially distributed in the glass network (respectively as network modifiers and formers) or involved in amorphous calcium phosphate nano-domains depending on the doping rate. These differences affect the interaction with simulated body fluid. Network depolymerization, ion release (SiO44-, Ca2+, PO43-, Cu2+), and apatite nanocrystal layer formation are impacted, as well as copper release. The latter is mainly provided by the copper involved in the silica network and not from metal nanoparticles, most of which remain in the microspheres after interaction. This understanding holds promising implications for potential therapeutic applications, offering possibilities for both short-term and long-term delivery of a tunable copper dose. STATEMENT OF SIGNIFICANCE: A novel methodology, scalable to industrial levels, enables the synthesis of copper-doped ternary bioactive glass microparticles by combining spray-drying and sol-gel chemistry. It provides precise control over the copper percentage in microspheres. This study explores the influence of synthesis conditions on the copper environment, notably Cu0 and Cu2+ ratios, characterized by EPR spectroscopy, an aspect poorly described for copper-doped bioactive glass. Additionally, copper indirectly affects silica network connectivity and calcium/phosphorus distribution, as revealed by SSNMR. Multiscale characterization illustrates how these features impact acellular degradation in simulated body fluid, highlighting the therapeutic potential for customizable copper dosing to address short- and long-term needs.

Investigation of the Photochromism of WO3, TiO2, and Composite WO3-TiO2 Nanoparticles.

We report the synthesis of WO3, TiO2, and TiO2-WO3 nanoparticles by a polyol route, with the objective of studying the influence of the preparation method on their photochromic properties. By combining transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and diffuse reflectance experiments, we show that low W6+ concentrations and high ripening temperatures allow the preparation of WO3 nanoparticles with high photochromic efficiency. WO3-TiO2 nanocomposites (NCs) prepared by the introduction of a TiO2+ solution in a WO3 nanoparticle suspension exhibit a strong coloring photochromism, which is attributed to the TiO2 coating of the WO3 nanoparticles as it involves the formation of W-O-Ti oxo-bonds in place of W5+-νO defects. Especially, after an oxidative treatment in order to obtain an initial pale-yellow material, such WO3-TiO2 NCs exhibit a fully reversible photochromism with a large contrast between the colored and bleached state. They could therefore be incorporated in hybrid smart films for solar control on building window glasses. On the other hand, while the WO3-TiO2 NCs are functionalized with DPA (n-dodecyl phosphonic acid), the as-prepared nanocomposites exhibit exacerbated coloring contrast but with a nearly nonreversible photochromism (very limited bleaching), which makes them good candidates for the fabrication of smart UV-sensor devices that can indicate the cumulative UV dose which is received.

Dynamic Lone Pairs and Fluoride-Ion Disorder in Cubic-BaSnF4.

Introducing compositional or structural disorder within crystalline solid electrolytes is a common strategy for increasing their ionic conductivity. (M,Sn)F2 fluorites have previously been proposed to exhibit two forms of disorder within their cationic host frameworks: occupational disorder from randomly distributed M and Sn cations and orientational disorder from Sn(II) stereoactive lone pairs. Here, we characterize the structure and fluoride-ion dynamics of cubic BaSnF4, using a combination of experimental and computational techniques. Rietveld refinement of the X-ray diffraction (XRD) data confirms an average fluorite structure with {Ba,Sn} cation disorder, and the 119Sn Mössbauer spectrum demonstrates the presence of stereoactive Sn(II) lone pairs. X-ray total-scattering PDF analysis and ab initio molecular dynamics simulations reveal a complex local structure with a high degree of intrinsic fluoride-ion disorder, where 1/3 of fluoride ions occupy octahedral "interstitial" sites: this fluoride-ion disorder is a consequence of repulsion between Sn lone pairs and fluoride ions that destabilizes Sn-coordinated tetrahedral fluoride-ion sites. Variable-temperature 19F NMR experiments and analysis of our molecular dynamics simulations reveal highly inhomogeneous fluoride-ion dynamics, with fluoride ions in Sn-rich local environments significantly more mobile than those in Ba-rich environments. Our simulations also reveal dynamical reorientation of the Sn lone pairs that is biased by the local cation configuration and coupled to the local fluoride-ion dynamics. We end by discussing the effect of host-framework disorder on long-range diffusion pathways in cubic BaSnF4.

Impact of Anionic Ordering on the Iron Site Distribution and Valence States in Oxyfluoride Sr2FeO3+xF1-x (x = 0.08, 0.2) with a Layered Perovskite Network.

Sr2FeO3F, an oxyfluoride compound with an n = 1 Ruddlesden-Popper structure, was identified as a potential interesting mixed ionic and electronic conductor (MIEC). The phase can be synthesized under a range of different pO2 atmospheres, leading to various degrees of fluorine for oxygen substitution and Fe4+ content. A structural investigation and thorough comparison of both argon- and air-synthesized compounds were performed by combining high-resolution X-ray and electron diffraction, high-resolution scanning transmission electron microscopy, Mössbauer spectroscopy, and DFT calculations. While the argon-synthesized phase shows a well-behaved O/F ordered structure, this study revealed that oxidation leads to averaged large-scale anionic disorder on the apical site. In the more oxidized Sr2FeO3.2F0.8 oxyfluoride, containing 20% of Fe4+, two different Fe positions can be identified with a 32%/68% occupancy (P4/nmm space group). This originates due to the presence of antiphase boundaries between ordered domains within the grains. Relations between site distortion and valence states as well as stability of apical anionic sites (O vs F) are discussed. This study paves the way for further studies on both ionic and electronic transport properties of Sr2FeO3.2F0.8 and its use in MIEC-based devices, such as solid oxide fuel cells.

From the Irreversible Transformation of VO2 to V2O5 Electrochromic Films.

Since its discovery, electrochromism, known as the modulation of optical properties under an applied voltage, has attracted strong interest from the scientific community and has proved to be of significant utility in various applications. Although vanadium dioxide (VO2) has been a candidate for extensive research for its thermochromic properties, its intrinsic electrochromism has scarcely been reported so far. In this study, multi-electrochromism is described for VO2 thick films. Indeed, a VO2 opaque film, doctor bladed from homemade monoclinic VO2 powder, shows a pronounced color modulation from orange to green and blue associated with an amorphization-recrystallization phenomenon upon cycling in a lithium-based electrolyte. The strong memory effect allows us to follow the coloration mechanism by combining various ex situ and in situ characterizations addressing both structural and electronic aspects. Upon cycling, the multichromism of VO2 finds its origin in the transformation of VO2 into orange V2O5 upon oxidation, while in reduction, the blue lithiated state illustrates a mixed vanadium oxidation state.

Exotic FeII/FeIII Local Environments in the Hexagonal Channels of Hydroxyapatite.

In this fundamental solid-state chemistry study, two sample series were investigated in depth: iron(III)-doped hydroxyapatite (HA) compounds obtained from a co-sintering process of hematite and pure HA under air and iron(III)-doped HA compounds obtained from a co-sintering process from iron(II) acetate and pure HA under an argon atmosphere. X-ray diffraction, UV-visible, Fourier transform infrared, 1H and 31P NMR, electron paramagnetic resonance (EPR,) and Mössbauer spectroscopy methods were coupled to unravel the Fe valence states, the interactions with other anionic species (OH- and PO43-), and finally the complex local environments in hexagonal channels in both the series. In particular, we highlighted the associated mechanism to ensure electroneutrality with a focus on deprotonation versus calcium substitution. By diverging mechanisms, Fe3+ and Fe2+ ions were found to be located in different coordinated sites: 4(+1) coordinated site for Fe3+ and 2(+3) coordinated site for Fe2+ and clearly associated with very different Mössbauer and EPR signatures as various absorption bands (leading to different sample colors).

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.

Impact of Synthesis Conditions in Na-Rich Prussian Blue Analogues.

Sodium-rich iron hexacyanoferrates were prepared by coprecipitation, hydrothermal route, and under reflux, with or without dehydration. They were obtained with different structures described in cubic, orthorhombic, or rhombohedral symmetry, with variable compositions in sodium, water, and cationic vacancies and with a variety of morphologies. This series of sodium-rich Prussian blue analogues allowed addressing the relationship between synthesis conditions, composition, structure, morphology, and electrochemical properties in Na-ion batteries. A new orthorhombic phase with the Na1.8Fe2(CN)6·0.7H2O composition synthesized by an hydrothermal route at 140 °C is reported for the first time, whereas a phase of Na2Fe2(CN)6·2H2O composition obtained under reflux, previously described with a monoclinic structure, shows in fact a rhombohedral structure.

A Nano-Emulsion Platform Functionalized with a Fully Human scFv-Fc Antibody for Atheroma Targeting: Towards a Theranostic Approach to Atherosclerosis.

Atherosclerosis is at the onset of the cardiovascular diseases that are among the leading causes of death worldwide. Currently, high-risk plaques, also called vulnerable atheromatous plaques, remain often undiagnosed until the occurrence of severe complications, such as stroke or myocardial infarction. Molecular imaging agents that target high-risk atheromatous lesions could greatly improve the diagnosis of atherosclerosis by identifying sites of high disease activity. Moreover, a "theranostic approach" that combines molecular imaging agents (for diagnosis) and therapeutic molecules would be of great value for the local management of atheromatous plaques. The aim of this study was to develop and characterize an innovative theranostic tool for atherosclerosis. We engineered oil-in-water nano-emulsions (NEs) loaded with superparamagnetic iron oxide (SPIO) nanoparticles for magnetic resonance imaging (MRI) purposes. Dynamic MRI showed that NE-SPIO nanoparticles decorated with a polyethylene glycol (PEG) layer reduced their liver uptake and extended their half-life. Next, the NE-SPIO-PEG formulation was functionalized with a fully human scFv-Fc antibody (P3) recognizing galectin 3, an atherosclerosis biomarker. The P3-functionalized formulation targeted atheromatous plaques, as demonstrated in an immunohistochemistry analyses of mouse aorta and human artery sections and in an Apoe-/- mouse model of atherosclerosis. Moreover, the formulation was loaded with SPIO nanoparticles and/or alpha-tocopherol to be used as a theranostic tool for atherosclerosis imaging (SPIO) and for delivery of drugs that reduce oxidation (here, alpha-tocopherol) in atheromatous plaques. This study paves the way to non-invasive targeted imaging of atherosclerosis and synergistic therapeutic applications.

Photochromic Behavior of ZnO/MoO3 Interfaces.

ZnO/MoO3 powder mixture exhibits a huge photochromic effect in comparison with the corresponding single oxides. The coloring efficiency of such combined material after UV-light irradiation was studied in terms of intensity, kinetics, and ZnO/MoO3 powder ratio. Additionally, the incidence of the pretreatment step of the ZnO and MoO3 powders under different atmospheres (air, Ar or Ar/H2 flow) was analyzed. The huge photochromic effect discovered herein was interpreted as the creation of "self-closed Schottky barrier" at the solid/solid interfaces between the two oxides, associated with the full redox reaction which can be pictured by the equation ZnO1-ε + MoO3 → ZnO + MoO3-ε. Remarkable optical contrast between virgin and color states as well as self-bleaching in dark allowing the reversibility of the photochromism is emphasized. From this first discovery, deeper characterization of the self-bleaching process shows that the photochromic mechanism is complex with a bleaching efficiency (possibility to come back to the virgin material optical properties without any deterioration) and a bleaching kinetics, which are both dependent on the coloring irradiation time. This demonstrates that the oxygen exchange through the Schottky interface proceeds in at least two convoluted steps: an anionic surface exchange allowing a reversibility of the redox reaction followed by bulk diffusion of the exchanged anions which are then definitively trapped. An emergent "negative photochromism effect" (i.e., photochromism associated with a self-bleaching instead of a darkening under irradiation) is observed after a long irradiation time.

Synchrotron-based Mössbauer spectroscopy characterization of sublimated spin crossover molecules.

The spin crossover (SCO) efficiency of [57Fe(bpz)2(phen)] (where bpz = bis(pyrazol-1-yl)borohydride and phen = 9,10-phenantroline) molecules deposited on gold substrates was investigated by means of synchrotron Mössbauer spectroscopy. The spin transition was driven thermally, or light induced via the LIESST (light induced excited spin-state trapping) effect. Both sets of measurements show that, once deposited on a gold substrate, the efficiency of the SCO mechanism is modified with respect to molecules in the bulk phase. A correlation in the distribution of hyperfine parameters in the sublimated films, not evidenced so far in the bulk phase, is reported. This translates into geometrical distortions of the first coordination sphere of the iron atom that seem to correlate with the decreased spin conversion. The work reported clearly shows the potentiality of synchrotron Mössbauer spectroscopy for the characterization of nanostructured Fe-based SCO systems, thus resulting as a key tool in view of their applications in innovative nanoscale devices.

Tuning the CrIV/CrIII Valence States in Purple Cr-Doped SnO2 Nanopowders: The Key Role of CrIV Centers and Defects.

A low content of chromium (≤5 mol %) has been incorporated into a SnO2 cassiterite by a coprecipitation route in a basic medium, followed by an annealing step under an O2 flow at T = 800 °C and T = 1000 °C. Accurate UV-vis and EPR spectroscopy investigations show the coexistence of isolated Cr4+ and Cr3+ ions as well as ferromagnetic Cr4+-Cr3+ and antiferromagnetic Cr3+-Cr3+ interactions. The strong purple hue is related to the isolated Cr4+ ions stabilized in a distorted octahedral site. This is thanks to the second-order Jahn-Teller (SOJT) effect with a crystal field splitting 10Dq value around 2.4 eV, whereas the 10Dq value is around 2 eV for isotropic Cr3+ ions, partially substituted for Sn4+ ions in cassiterite. Just after the coprecipitation process, only Cr3+ species are stabilized in this rutile network with a poor crystallinity. The isolated Cr4+ content remains high after annealing at 800 °C for 2 days especially for the highest Cr rate (2 and 5 mol %), leading to a darker purple color, but unfortunately the Cr3+ content also increases for a higher Cr concentration. A lighter purple hue can be reached after calcination at a higher temperature (T = 1000 °C) for a shorter time (4 h) but with a lower Cr content to avoid Cr clusters. This is due to stabilizing a high content of isolated Cr4+ species and limiting the Cr4+-Cr3+ ferromagnetic interactions, which are optimal for a 2% Cr content and also cause the color to darken. The key roles of the Cr4+ rate and the Cr4+-Cr3+ clusters create local defects whose concentration strongly varies with a total Cr content, which have then been demonstrated to strongly influence the optical and magnetic properties.

Nanoparticles (NPs) of WO3-x Compounds by Polyol Route with Enhanced Photochromic Properties.

Tungsten trioxide (WO3) is well-known as one of the most promising chromogenic compounds. It has a drastic change of coloration induced from different external stimuli and so its applications are developed as gas sensors, electrochromic panels or photochromic sensors. This paper focuses on the photochromic properties of nanoWO3, with tunable composition (with tunable oxygen sub-stoichiometry). Three reference samples with yellow, blue and black colors were prepared from polyol synthesis followed by post annealing under air, none post-annealing treatment, or a post-annealing under argon atmosphere. These three samples differ in terms of crystallographic structure (cubic system versus monoclinic system), oxygen vacancy concentration, electronic band diagram with occurrence of free or trapped electrons and their photochromic behavior. Constituting one main finding, it is shown that the photochromic behavior is highly dependent on the compound's composition/color. Rapid and important change of coloration under UV (ultraviolet) irradiation was evidenced especially on the blue compound, i.e., the photochromic coloring efficiency of this compound in terms of contrast between bleached and colored phase, as the kinetic aspect is high. The photochromism is reversible in a few hours. This hence opens a new window for the use of tungsten oxide as smart photochromic compounds.

Monitoring the Crystal Structure and the Electrochemical Properties of Na3(VO)2(PO4)2F through Fe3+ Substitution.

We here present the synthesis of a new material, Na3(VO)Fe(PO4)2F2, by the sol-gel method. Its atomic and electronic structural descriptions are determined by a combination of several diffraction and spectroscopy techniques such as synchrotron X-ray powder diffraction and synchrotron X-ray absorption spectroscopy at V and Fe K edges, 57Fe Mössbauer, and 31P solid-state nuclear magnetic resonance spectroscopy. The crystal structure of this newly obtained phase is similar to that of Na3(VO)2(PO4)2F, with a random distribution of Fe3+ ions over vanadium sites. Even though Fe3+ and V4+ ions situate on the same crystallographic position, their local environment can be studied separately using 57Fe Mössbauer and X-ray absorption spectroscopy at Fe and V K edges, respectively. The Fe3+ ion resides in a symmetric octahedral environment, while the octahedral site of V4+ is greatly distorted due to the presence of the vanadyl bond. No electrochemical activity of the Fe4+/Fe3+ redox couple is detected, at least up to 5 V, whereas the reduction of Fe3+ to Fe2+ has been observed at ∼1.5 V versus Na+/Na through the insertion of 0.5 Na+ into Na3(VO)Fe(PO4)2F2. Comparing to Na3(VO)2(PO4)2F, the electrochemical profile of Na3(VO)Fe(PO4)2F2 in the same cycling condition shows a smaller polarization which could be due to a slight improvement in Na+ diffusion process thanks to the presence of Fe3+ in the framework. Furthermore, the desodiation mechanism occurring upon charging is investigated by operando synchrotron X-ray diffraction and operando synchrotron X-ray absorption at V K edge.

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 pair distribution function and density functional theory analysis indicates that oxygen vacancy formation is accompanied by spontaneous rearrangement of fluorine anions and vacancies, producing dual pyramidal (FeF4)-O-(FeF4) structural units containing 5-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 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 a defect-free compound.

Mixed-Valence Iron Dumortierite Fe13.52.22+(As5+O4- x)8(OH)6 and Its Intricate Topotactic Exsolution at Mild Temperatures.

The mixed-valent iron arsenate hydroxide Fe13.52.22+(AsO4- x)8(OH)6, x = 0.25, was prepared using the reaction of iron metal with arsenate in aqueous solution and autogenous pressure. Its crystal structure reveals a dumortierite-like framework with mixed-valent Fe2+/Fe3+ in double chains creating channel walls. Remarkably, hexagonal channels consist of chains of face-sharing Fe2+O6 octahedra, 3/4th occupied, whereas AsO4 tetrahedra occupy triangular ones with a single " up" orientation according to the polar P63 mc symmetry. We have analyzed the transformation of this phase upon heating, in which several chemical processes interact, including dehydroxylation, arsenate to arsenite reduction, and oxidative exsolution of a significant part of iron (ca. 15%) found at the surface as hematite and amorphous Fe-rich surficial layer. It leaves a strongly disordered composite structure between several Fe3+-based subunits, in which ∼80% of them is ordered in a complex supercell. Because of the high degree of disorder, the crystal chemistry of the individual subunits and their plausible imbrication were considered to unravel the most plausible ideal 3D model.

Probing Co- and Fe-doped LaMO3 (M = Ga, Al) perovskites as thermal sensors.

The synthesis of a Co-doped or Fe-doped La(Ga,Al)O3 perovskite via the Pechini process aimed to achieve a color change induced by temperature and associated with spin crossover (SCO). In Fe-doped samples, iron was shown to be in the high-spin state, whereas SCO from the low-spin to the high-spin configuration was detected in Co-doped compounds when the temperature increased. Fe-doped compounds clearly adopted the high-spin configuration even down to 4 K on the basis of Mössbauer spectroscopic analysis. The original SCO phenomenon in the Co-doped compounds LaGa1-xCoxO3 (0 < x < 0.1) was evidenced and discussed on the basis of in situ X-ray diffraction analysis and UV-vis spectroscopy. This SCO is progressive as a function of temperature and occurs over a broad range of temperatures between roughly 300 °C and 600 °C. The determination of a crystal field strength of about 2 eV and a Racah parameter B of about 500 cm-1 for Co3+ (3d6) ions show that these values allow the occurrence of SCO. Hence, this study shows the possibility of using LaGa1-xCoxO3 compounds as thermal sensors at low Co contents (x = 0.02). The competition between steric and electronic effects in LaGaO3 in which Co3+ is stabilized in the LS state shows that electronic effects with the creation of M-O covalent bonds are predominant and contribute to the stabilization of a high crystal field around Co3+ (LS) although its ionic radius is smaller in comparison with that of Ga3+.

Comprehensive Study of Oxygen Storage in YbFe2O4+x (x ≤ 0.5): Unprecedented Coexistence of FeOn Polyhedra in One Single Phase.

The multiferroic LuFe2.5+2O4 was recently proposed as a promising material for oxygen storage due to its easy reversible oxidation into LuFe3+2O4.5. We have investigated the similar scenario in YbFe2O4+x, leading to a slightly greater oxygen storage (OSC) capacity of 1434 µmol O/g. For the first time, the structural model of LnFe2O4.5 was fully understood by high-resolution microscopy images, and synchrotron and neutron diffraction experiments, as well as maximum entropy method. The oxygen uptake promotes a reconstructive shearing of the [YbO2] sub-units controlled by the adaptive Ln/Fe oxygen coordination and the Fe2/3+ redox. After oxidation, the rearrangement of the Fe coordination polyhedra is unique such that all available FeOn units (n = 6, 5, 4 in octahedra, square pyramids, trigonal bipyramids, tetrahedra) were identified in modulated rows growing in plane. This complex pseudo-ordering gives rise to short-range antiferromagnetic correlation within an insulating state.

Structural and Morphological Description of Sn/SnOx Core-Shell Nanoparticles Synthesized and Isolated from Ionic Liquid.

The potential application of high capacity Sn-based electrode materials for energy storage, particularly in rechargeable batteries, has led to extensive research activities. In this scope, the development of an innovative synthesis route allowing to downsize particles to the nanoscale is of particular interest owing to the ability of such nanomaterial to better accommodate volume changes upon electrochemical reactions. Here, we report on the use of room temperature ionic liquid (i.e., [EMIm+][TFSI-]) as solvent, template, and stabilizer for Sn-based nanoparticles. In such a media, we observed, using Cryo-TEM, that pure Sn nanoparticles can be stabilized. Further washing steps are, however, mandatory to remove residual ionic liquid. It is shown that the washing steps are accompanied by the partial oxidation of the surface, leading to a core-shell structured Sn/SnOx composite. To understand the structural features of such a complex architecture, HRTEM, Mössbauer spectroscopy, and the pair distribution function were employed to reveal a crystallized ß-Sn core and a SnO and SnO2 amorphous shell. The proportion of oxidized phases increases with the final washing step with water, which appeared necessary to remove not only salts but also the final surface impurities made of the cationic moieties of the ionic liquid. This work highlights the strong oxidation reactivity of Sn-based nanoparticles, which needs to be taken into account when evaluating their electrochemical properties.