Search

The spin crossover (SCO) efficiency of [ 57 Fe(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.

A low content of chromium (≤5 mol %) has been incorporated into a SnO 2 cassiterite by a coprecipitation route in a basic medium, followed by an annealing step under an O 2 flow at T = 800 °C and T = 1000 °C. Accurate UV-vis and EPR spectroscopy investigations show the coexistence of isolated Cr 4+ and Cr 3+ ions as well as ferromagnetic Cr 4+ -Cr 3+ and antiferromagnetic Cr 3+ -Cr 3+ interactions. The strong purple hue is related to the isolated Cr 4+ ions stabilized in a distorted octahedral site. This is thanks to the second-order Jahn-Teller (SOJT) effect with a crystal field splitting 10 Dq value around 2.4 eV, whereas the 10 Dq value is around 2 eV for isotropic Cr 3+ ions, partially substituted for Sn 4+ ions in cassiterite. Just after the coprecipitation process, only Cr 3+ species are stabilized in this rutile network with a poor crystallinity. The isolated Cr 4+ 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 Cr 3+ 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 Cr 4+ species and limiting the Cr 4+ -Cr 3+ ferromagnetic interactions, which are optimal for a 2% Cr content and also cause the color to darken. The key roles of the Cr 4+ rate and the Cr 4+ -Cr 3+ 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.

Tungsten trioxide (WO 3 ) 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 nanoWO 3 , 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., Competing Interests: The authors declare no conflict of interest.

We here present the synthesis of a new material, Na 3 (VO)Fe(PO 4 ) 2 F 2 , 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, 57 Fe Mössbauer, and 31 P solid-state nuclear magnetic resonance spectroscopy. The crystal structure of this newly obtained phase is similar to that of Na 3 (VO) 2 (PO 4 ) 2 F, with a random distribution of Fe 3+ ions over vanadium sites. Even though Fe 3+ and V 4+ ions situate on the same crystallographic position, their local environment can be studied separately using 57 Fe Mössbauer and X-ray absorption spectroscopy at Fe and V K edges, respectively. The Fe 3+ ion resides in a symmetric octahedral environment, while the octahedral site of V 4+ is greatly distorted due to the presence of the vanadyl bond. No electrochemical activity of the Fe 4+ /Fe 3+ redox couple is detected, at least up to 5 V, whereas the reduction of Fe 3+ to Fe 2+ has been observed at ∼1.5 V versus Na + /Na through the insertion of 0.5 Na + into Na 3 (VO)Fe(PO 4 ) 2 F 2 . Comparing to Na 3 (VO) 2 (PO 4 ) 2 F, the electrochemical profile of Na 3 (VO)Fe(PO 4 ) 2 F 2 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 Fe 3+ 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.

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 (FeF 4 )-O-(FeF 4 ) 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 FeF 3 parent material is correlated with a significant improvement in lithium insertion capability relative to a defect-free compound.

The mixed-valent iron arsenate hydroxide Fe 13.5 2.22+ (AsO 4- 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 Fe 2+/ Fe 3+ in double chains creating channel walls. Remarkably, hexagonal channels consist of chains of face-sharing Fe 2+ O 6 octahedra, 3/4th occupied, whereas AsO 4 tetrahedra occupy triangular ones with a single " up" orientation according to the polar P6 3 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 Fe 3+ -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.

The synthesis of a Co-doped or Fe-doped La(Ga,Al)O 3 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 LaGa 1-x Co x O 3 (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 Co 3+ (3d 6 ) ions show that these values allow the occurrence of SCO. Hence, this study shows the possibility of using LaGa 1-x Co x O 3 compounds as thermal sensors at low Co contents (x = 0.02). The competition between steric and electronic effects in LaGaO 3 in which Co 3+ 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 Co 3+ (LS) although its ionic radius is smaller in comparison with that of Ga 3+ .

The multiferroic LuFe 2.5+ 2 O 4 was recently proposed as a promising material for oxygen storage due to its easy reversible oxidation into LuFe 3+ 2 O 4.5 . We have investigated the similar scenario in YbFe 2 O 4+x , leading to a slightly greater oxygen storage (OSC) capacity of 1434 μmol O/g. For the first time, the structural model of LnFe 2 O 4.5 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 [YbO 2 ] sub-units controlled by the adaptive Ln/Fe oxygen coordination and the Fe 2/3+ redox. After oxidation, the rearrangement of the Fe coordination polyhedra is unique such that all available FeO n 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.

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/SnO x 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 β-Sn core and a SnO and SnO 2 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.

The thermosalient effect is still a rare and poorly understood phenomenon, where crystals suddenly jump, bend, twist or explode upon undergoing a thermally activated phase transition. The synthesis and characterisation of the new spin transition Fe(iii) compound [Fe(5-Br-salEen) 2 ][ClO 4 ] (salEen = N -ethyl- N -(2-aminoethyl)salicylaldiminate) is described and its thermosalient behaviour reported. It is the first example of a thermosalient effect with a spin transition and magnetic, calorimetric, diffraction, microscopy and computational studies are used to characterise these effects. Both thermosalient effect and spin transition occur around 320 K upon heating and are accompanied by an anisotropic unit cell change with conservation of crystal symmetry that causes a large enough stress of the crystal lattice to induce crystal explosion. This stress can ultimately be traced back to a diffusionless and distortive structural perturbation resulting in a coupled spin transition-thermosalient effect.

Because of a very low thermodynamic stability, obtaining a pure monophasic compound of ferric pseudobrookite is quite difficult to achieve. Indeed, the low reticular energy of this phase leads easily to its decomposition and the occurrence of the secondary phases: hematite (Fe2O3) and/or rutile (TiO2). Samples with global composition Fe2-xTi1+xO5 (x = 0, 0.05, and 0.10) have been synthesized by the Pechini route and, thereafter, thermally treated at different temperatures. The concentrations of Fe2O3 and TiO2 secondary phases were accurately determined and correlated with the target compositions and the synthesis parameters, especially the thermal treatment temperature. As revealed by Mössbauer spectroscopy, all iron ions are at the III+ oxidation state. Thus, the formation of hematite or rutile as a secondary phase may be related to the occurrence of cationic vacancies within the pseudobrookite structure, with the amount of vacancies depending on the annealing temperature. In light of the presented results, it appears unreasonable to propose a "fixed" binary phase diagram for such a complex system. Furthermore, the occurrence of cationic vacancies induces a coloration change (darkening), preventing any industrial use of this reddish-brown pseudobrookite as a ceramic pigment.

Iron fluoride trihydrate can be used to prepare iron hydroxyfluoride with the hexagonal-tungsten-bronze (HTB) type structure, a potential cathode material for batteries. To understand this phase transformation, a structural description of β-FeF3·3H2O is first performed by means of DFT calculations and Mössbauer spectroscopy. The structure of this compound consists of infinite chains of [FeF6]n and [FeF2(H2O)4]n. The decomposition of FeF3·3H2O induces a collapse and condensation of these chains, which lead to the stabilization, under specific conditions, of a hydroxyfluoride network FeF3-x(OH)x with the HTB structure. The release of H2O and HF was monitored by thermal analysis and physical characterizations during the decomposition of FeF3·3H2O. An average distribution of FeF4(OH)2 distorted octahedra in HTB-FeF3-x(OH)x was obtained subsequent to the thermal hydrolysis/olation of equatorial anionic positions involving F(-) and H2O. This study provides a clear understanding of the structure and thermal properties of FeF3·3H2O, a material that can potentially bridge the recycling of pickling sludge from the steel industry by preparing battery electrodes.

The mechanism of lithium insertion that occurs in an iron oxyfluoride sample with a hexagonal-tungsten-bronze (HTB)-type structure was investigated by the pair distribution function. This study reveals that upon lithiation, the HTB framework collapses to yield disordered rutile and rock salt phases followed by a conversion reaction of the fluoride phase toward lithium fluoride and nanometer-sized metallic iron. The occurrence of anionic vacancies in the pristine framework was shown to strongly impact the electrochemical activity, that is, the reversible capacity scales with the content of anionic vacancies. Similar to FeOF-type electrodes, upon de-lithiation, a disordered rutile phase forms, showing that the anionic chemistry dictates the atomic arrangement of the re-oxidized phase. Finally, it was shown that the nanoscaling and structural rearrangement induced by the conversion reaction allow the in situ formation of new electrode materials with enhanced electrochemical properties.

The very fast and efficient water vapor absorption of the dimeric fluorous copper(II)-carboxylate complex [Cu(2)(C(8)F(17)CO(2))(4)(acetone)(2)] (1) leads, in the solid state, to a dramatic decrease of the exchange magnetic coupling between the copper(II) ions and to a drastic change of its powder EPR spectrum.

Background: Most historical buildings in Bordeaux city are made of limestone. This yellowish-white rock is rather porous and highly sensitive to pollution. As a consequence of local weathering conditions, these buildings present a dark appearance due to the development of a superficial dark grey to black crust., Methods: For the last decade, a campaign has been underway to clean these buildings. Eleven techniques of surface treatment have been used, including laser beam technology. As a contribution to the study of laser beam effects on stone buildings, two analytical methods have been used on clean versus unclean surfaces: Cathodoluminescence (CL) and Electron Paramagnetic Resonance (EPR), in addition to SEM-EDX and XRD., Results: The black crust is composed of different types of particles: carbon porous micro-particles of industrial origin, atmospheric dust due to the erosion of soils and rocks, alumino-silicate particles from urban pollution; all these particles being cemented by gypsum., Discussion: As far as heritage conservation is concerned, the laser surface treatment not only preserves the original patina of the stone, but also leaves surface smoothness unaltered., Conclusions and Perspectives: CL and EPR data confirm that lasers--with highly controlled parameters--only get rid of the black crust and, thus, reveal the underneath layer, the so-called patina. This patina shows no luminescence, whereas the limestone on which it has grown shows a bright orange emission of CL. This indicates CL to be a fast and easy way to provide a high quality control for the restoration of polluted ancient stones.