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Yttrium aluminium garnet Y3Al5O12 (YAG) is a widely used phosphor host. Its optical properties are tuned by chemical substitution at its YO8 or AlO6/AlO4 sublattices, with emission wavelengths defined by a finite number of rare-earth (YO8 sublattice) and transition-metal (AlO6/AlO4 sublattice) dopants which have been explored extensively. Non-stoichiometric compositions Y3+xAl5-xO12 (x ≠ 0) may offer a route to new emission wavelengths by distributing dopants over multiple crystallographic sites, but deviation from Y3Al5O12 stoichiometry is difficult to achieve and limited generally to < 1% excess Y3+. Here we report a series of highly non-stoichiometric YAG ceramics Y3+xAl5-xO12 (0 ≤ x ≤ 0.4), with up to 20% of the AlO6 sublattice substituted by Y3+, synthesised by advanced melt-quenching techniques. This impacts the up-conversion luminescence of Yb3+/Er3+-doped systems, whose yellow-green emission contrasts with the red-orange emission of their stoichiometric counterparts. This contrasts with YAG:Ce3+ where the dopant ions occupy the YO8 sublattice exclusively, with down-conversion luminescence that is hardly affected by host nonstoichiometry. Beyond YAG, analogous highly non-stoichiometric systems should be obtainable for a range of functional garnets, demonstrated here by the successful synthesis of Gd3.2Al4.8O12 and Gd3.2Ga4.8O12. This opens the way to property tuning by control of garnet host stoichiometry, and the prospect of improved performance or new applications for garnet-type optical and magnetic materials.

International audience; The charge and electron transport properties of molten ionic systems are among the most relevant properties to consider in the control of several electrochemical processes. First-principles-based equilibrium molecular dynamics (EMD) can provide reliable predictions of both the total and partial charge transport properties. In this work we calculate the charge transport properties of the electrolytic bath (Na 3 AlF 6-AlF 3-Al 2 O 3) of the Hall-Héroult electrolysis cells. We predict both the individual and collective charge transport properties (total, partial conductivity and self diffusion coefficients) for 11 different compositions typical of industrial conditions via a series of EMD simulations. The predicted total and partial ionic conductivities and their composition dependence are compared to available experimental data. A good agreement is obtained for all studied compositions. From a more fundamental point of view, the microscopic aspect of the charge transport properties of cryolitic melts is discussed through its correlation to the local structure of different melts. Deviations between the calculated partial conductivities and those derived via the Nernst-Einstein approximation can be explained by the presence of strong short-range ordering within the melts.

International audience; In situ high temperature NMR spectroscopy was used to characterize the NaF-ScF3 melt over a wide range of compositions. 19 F, 23 Na, and 45 Sc NMR spectra were acquired in NaF-ScF3 melts of up to 70 mol% of ScF3. The interpretation of all experimental results obtained in situ in the melt is significantly enhanced by the contribution of Molecular Dynamics (MD) calculations. A new interatomic potential for the molten NaF-ScF3 system was developed by using a Polarizable Ion Model (PIM). The potential parameters were obtained by force-fitting to density functional theory (DFT) reference data. MD simulations were combined with further DFT calculations to determine NMR chemical shifts for 19 F, 23 Na, and 45 Sc. The agreement between the experimental NMR data and the corresponding calculated data from our applied computational protocol indicated the polymerization and network formation in the melt. Additionally, the density and the electrical conductivity in the molten state were calculated from the statistical analysis of ionic trajectories obtained through MD simulations.

International audience; Analytical methods for glass characterization. In the glass industry, knowledge of the chemical composition and atomic structural of glasses is important for quality control on production sites or for the research and development of new products. In this chapter, we will briefly describe different methods that can be used to easily obtain pieces of information regarding the glass atomic structure and composition, the composition of unmelted particles, or even the oxidation state of the multivalent elements influencing the optical properties of the final glass products.; Au sein de l’industrie verrière, connaître la composition et l’état structural des verres est un enjeu important pour le contrôle qualité sur les sites de production ou encore la recherche et le développement de nouveaux produits. Dans ce chapitre, nous décrirons brièvement différentes méthodes pouvant être utilisées par l’industrie verrière pour obtenir des informations structurales, chimiques, ou encore sur la composition des infondus ou le degré d’oxydation des éléments multivalents influençant les propriétés optiques des produits finis.

To understand the behavior of rare earth elements (REE) in magmatic systems it is important to characterize in a systematic way their incorporation into silicate melts and glasses. Here, we study the structural environment of the REE Y and La in four aluminosilicate glasses and melts with varying REE content, using a combined simulation and diffraction approach. Glasses are investigated by X-ray and neutron diffraction as well as classical molecular dynamics simulations using two different polarizable ion potentials. Structure models of the corresponding melts are derived from classical and first-principles molecular dynamics simulations. We discuss the effect of temperature on coordination numbers and rationalize the structural changes in response to variations in melt/glass composition in terms of cation field strengths. We find robust evidence that REE and Al coordination numbers decrease with increasing REE content in the investigated melts and glasses. Comparing the two classical potentials, one of them is able to reproduce features of the experimental structure factors with a mixture of corner- and edge-sharing Al-O/REE-O polyhedra, whereas the other potential predicts corner-sharing Al-O/REE-O polyhedra only.

Knowing the charge-transport properties of molten oxides is essential for industrial applications, particularly when attempting to control the energy required to separate a metal from its ore concentrate. Nowadays, in the context of a drastic increase of computational resources, research in industrial process simulation and their optimization is gaining popularity. Such simulations require accurate data as input for properties in a wide range of compositions, temperatures, and mechanical stresses. Unfortunately, due to their high melting points, we observe a severe lack of (reproducible) experimental data for many of the molten oxides. An alternative consists in using molecular dynamic simulations employing nonempirical force fields to predict the charge-transport properties of molten oxides and thus alleviate the lack of experimental data. Here, we study molten alumina using two polarizable force fields, with different levels of sophistication, parameterized on electronic structure calculations only. After validating the models against the experimental sets of density and electrical conductivity, we are able to determine the various ionic contributions to the overall charge transport in a wide range of temperatures.

International audience; Molten cryolite (NaF-AlF 3) plays a key role for aluminum production since it is used as a solvent. Among the various leads to improve the electrochemical processes, adding other fluoride salts is one of the most promising. However there is very few data on the impact of these additional species on the physico-chemical properties of the melt. In order to fill this gap we investigate the properties of high temperature KF-AlF 3 liquids by combining NMR spectroscopy, molecular dynamics and DFT calculations. Our results show that the speciation, and consequently the transport properties differ from that of cryolite. This work will allow to assess the interest of adding KF in aluminium production processes.

This article focuses on the speciation of molten fluoride mixtures based on ThF4 and UF4 actinides used in molten salt reactors. The local structure of molten AF-MF4 systems (A = Li+, Na+, K+; M = Th4+, U4+) was studied in situ by combining measurements by high temperature X-ray absorption spectroscopy and molecular dynamics simulations. In the molten state, 20 higher than the melting temperature, these mixtures are composed of free fluorine and anionic species [MF7]3−, [MF8]4− and [MF9]5− whose distribution varies with the amount of MF4 (M = Th4+, U4+). Regardless of the cation, these complexes consist of an average of 8 F− neighbors for MF4 content is lower than 35 mol%. This value decreases to 7 as the size of the alkaline ion A (A = Li+, Na+, K+) increases. The [MFx]4−x species are linked together by bridging fluorine ions to form long chains [MxFY]4x−y. The addition of a few percent UF4 to the eutectic LiF-ThF4 composition (77.5 mol% - 22.5 mol%) at leads to a slight increase in the population of free fluorine ions due to the breakdown of the Th-F links to form complexes [UFx]4−x isolated or connected to the (ThxFy)4x−y chains. This change in the structure of the liquid results in a slight decrease in viscosity when a few mol. % of UF4 is added.

For aluminum production, the alumina (Al2O3) dissolution in the electrolyte is one of the important step in the industrial process. The electrolyte is a cryolitic bath containing mainly NaF and AlF3 at around 1000 °C. In this liquid, the main anionic species are fluoroaluminates ions such as [AlF6]3–, [AlF5]2–, [AlF4]−, and F–. During the Al2O3 dissolution, different kinds of oxyfluoroaluminates species are formed. However, due to the difficult experimental conditions and the large number of potential ions, no quantitative speciation could be made up to now. Here, we propose a speciation of alumina–cryolite melts combining in situ NMR experiments, classical molecular dynamics simulations, and electronic structure calculations. This allows us to establish the nature and quantities of each species, depending on the Al2O3 concentration, for two different initial cryolitic compositions. In particular, we show that the O atoms are always linked to at least two Al atoms, leading to the formation of polymeric ox...

Individual transport numbers provide a measure of the charge ratio which is carried by each ionic species of an electrolyte. However, unlike the electrical conductivity or diffusion coefficients, this quantity lacks a clear definition from the statistical mechanics point of view. In practice, it is measured via complex experimental setups, and most of the interpretation of the available data is made by using the Nernst–Einstein approximation. Here we show that this approach is not suitable in molten salts due to the large contributions of the cross-terms due to the ionic interactions in the total ionic conductivity. We propose a partition scheme that allows to attribute these cross-terms to the various ions, allowing for an estimate of the transport numbers in a series of molten salt formed by mixing NaF and AlF3 at various compositions. The results are interpreted using the speciation of the melt, which is characterized by the formation of various AlFx clusters. At large AlF3, the Na+ ions are shown to contribute almost totally to the ionic conductivity.

A thermodynamic evaluation and optimization of the CALPHAD type of the Cu-O-F system, together with phase diagram measurements, are considered in the present work as the first step towards a complete evaluation of Cu in the multicomponent system Cu-Fe-Ni-Na-Al-Ca-O-F. Differential Scanning Calorimetry experiments were conducted using a closed crucible to identify the phase transitions in the CuF2-Cu and CuF2-Pt pseudo-binary systems in the presence of oxide impurities. The thermograms and the diffractograms, as measured by X-Ray Diffraction, showed that reactions occurred in both systems, where the CuO impurity from the CuF2 reactant led to the formation of Cu2O after three or four runs. All new model parameters of the fluoride-rich region of the Cu-O-F system were optimized based on this study’s experimental results. Measured and calculated results are in good agreement and provide a new interpretation of the phase transitions reported in the literature.

International audience; Improvement of the industrial electrolytic process for aluminum production necessitates a thorough understanding of the underlying ionic structure of the electrolyte, which mainly comprises NaF and AlF3 at around 965 °C. The chemical and physical properties of this melt strongly depend on the aluminum speciation, which requires a multipronged approach in order to clarify its properties. Here we parametrize a new polarizable ion model (PIM) interatomic potential for the molten NaF−AlF3 system, which is used to study the liquid properties up to 50 mol % of AlF$_3$ at high temperatures. The potential parameters are obtained by force fitting to density functional theory (DFT) reference data. Molecular dynamics (MD) simulations are combined with further DFT calculations to determine NMR chemical shifts for $^{27}$Al, $^{23}$Na, and $^{19}$F. An excellent agreement is obtained with experimental values. This enables the study of the dynamic properties of the melts such as viscosity, electrical conductivity, and self-diffusion coefficient.

We used the aerodynamic levitation technique combined with CO2 laser heating to study the structure of liquid yttrium aluminates above their melting point with neutron diffraction. For various yttria contents, we determined the structure factors and corresponding pair correlation functions describing the short-range order in the liquids. In particular, we derived Al–O and Y–O bond distances and coordination numbers. Experimental data are compared with ab initio molecular dynamics, carried out using the VASP code where the interatomic forces are obtained from density functional theory. In particular, partial pair correlation functions have been calculated and are in relatively good agreement with the experimental observations.

The combination of neutron diffraction with aerodynamic levitation and laser heating, pioneered by Neville Greaves and co-workers about 15years ago, is an important tool for studying the structure of liquid melts. Since the first work on liquid Al2O3 published in 2001, the technique has been largely improved and experiments are now routinely performed at neutron sources, providing interesting structural information on various materials.In this paper, the structure of glass-forming compounds in the system CaO-Al2O3-SiO2 was measured by applying neutron diffraction with aerodynamic levitation. Results obtained in the liquid state above the melting point and from the glass at room temperatures are presented. Various compositions were studied by increasing the silica content and by changing the ratio CaO/Al2O3. As observed using other methods, the main structural changes relate to modification of the Al-O short range order.

This work aims at studying the volume change of bauxite corroded by a molten slag. Cylindrical samples were prepared by mixing ground bauxite with slag. Optical measurement at high temperature (1450 °C) of deformation with a high-resolution camera has been developed. Image processing allowed for determining the change in diameter of the sample. We showed that the deformation was induced by the precipitation of new expansive crystallised phases observed by SEM-EDS analyses. Adding pellets of the same slag upon the samples allowed to emphasize the effect of the slag amount on the size change. The change in diameter significantly increased in the impregnated area.

In aluminum production, the electrolyte is a molten fluorides mixture typically around 1000°C. In order to have a better understanding of the industrial process, it is necessary to have a model which will describe the molten salts on a wide range of compositions and temperatures, to accurately cover all the combinations that may be encountered in an operating electrolysis vessel. The aim of this study is to describe the speciation in the electrolyte in terms of anionic species in the bulk materials far from electrodes. To determine the speciation in situ at high temperature in the absence of an electrical field, we develop an original approach combining experimental methods such as Nuclear Magnetic Resonance spectroscopy (NMR) at high temperature with Molecular Dynamics (MD) simulation coupled with first principle calculations based on Density Functional Theory (DFT). This approach allows the calculation of NMR parameters and the comparison with the experimental ones. It will be provide an additional validation and constraint of the model used for MD. We test this approach on the model NaF-AlF3 system.

Aerodynamic levitation is an effective way to suspend samples which can be heated with CO2 lasers. The advantages of this containerless technique are the simplicity and compactness of the device, making it possible to integrate it easily in different kinds of experiments. In addition, all types of materials can be used, including metals and oxides. The integration of aerodynamic levitation at synchrotron and neutron sources provides powerful tools to study the structure and dynamics of molten materials. We present here an overview of the existing techniques and of the developments made at the CEMHTI in Orleans, as well as a few examples of experimental results already obtained.

In situ high temperature nuclear magnetic resonance in molten fluoride mixtures gives some structural picture of the complexes existing in the melt, i.e. of their nature and relative proportion. Thanks to the development of a laser heating system associated with a close crucible in boron nitride, we can describe experimentally the evolution of these complexes from the anions and the cations point of view. By 19F NMR, we have shown the existence of three kinds of fluorine atoms depending on the composition: free fluorine like in pure LiF (non-bonded), bridging fluorine in melts rich in LnF3 in addition with terminal fluorine singly bonded to one rare earth. Data obtained by NMR spectroscopy are also combined with EXAFS measurements, again thanks to a specific development of the sample holder adapted with molten fluorides and high temperature. This study is a part of our systematic investigation of the different Alk–LnF3 systems by NMR and EXAFS spectroscopy.

International audience; The interpretation of the experimental data of the atomic structure of complex materials is often difficult. Because of the absence of long range order, the experiments can give only a partial and incomplete description of the atomic structure of the molten liquids (interatomic distances and a number of neighbors). Thus, the coupling of experimental and simulation techniques become essential for defining structural models consistent with all the available experimental data. We have investigated the structure of liquid Y3Al5O12 (YAG) above its melting point (Tmp = 2220 K) by ab-initio molecular dynamics (AIMD), carried out using the VASP code where the interatomic forces are obtained from density functional theory. In this paper, neutron diffraction and simulation results will be compared. By using the AIMD model we determined structural parameters like coordination numbers, interatomic distances and angle distributions.

The objective of this paper is to describe the local structure in fluoride melts of nuclear interest. Our experimental approach combines NMR and EXAFS spectroscopies both sensitive to the microstructure of solid and liquid materials. These techniques allow identifying the complex formation in molten media. Thanks to the development of specific cells adapted to molten fluorides up to 1300 °C, we give some structural description of LiF–LnF3 systems (Ln = Y, Lu, Ce) and discuss the evolution of 19F spectra in terms of different fluorine configurations depending on the composition. This approach is further extended to the characterization of LiF–ThF4 system by 19F NMR in melts from 0 to 100 mol% ThF4 at temperatures ranging from 600 to 1100 °C. We report also very first EXAFS spectra obtained in situ at high temperature, at the Thorium LIII edge, in LiF–ThF4 molten mixtures.

Ni thin films deposited on (111) Al thick samples have been submitted to annealing treatment. The goal is to identify the phases formed in the early stage of mixing between Ni and Al. Combining complementary techniques such as x-ray absorption spectroscopy and x-ray photoelectron spectroscopy, we characterized two different systems depending on the temperature conditions. For a thermal treatment at 110 °C, a system formed of small AlNi3 clusters embedded in the Al matrix was identified. This was interpreted as due to the existence of a spontaneous Al/Ni interface mixed layer acting as a seed for the AlNi3 cluster formation. After annealing at 300 °C, the Al3Ni intermetallic compound was detected, in agreement with results in the literature.

Granular Co/AlN multilayers, with Co thicknesses varying from 0.2 to 3 nm, were prepared by sputtering. The goal was to prepare Co clusters embedded in a ceramic matrix and to study their magnetic properties. The local surrounding of the Co atoms was characterized via x-ray absorption spectroscopy. The magnetic properties were found to depend markedly on the Co thickness. The findings can be explained in terms of strong chemical interaction between Co and AlN.

Ti, Fe, and Co ions were implanted in two ceramics, SiC and Si3N4, to reach concentrations on the order of 10% over a depth of about 50–60 nm. X-ray absorption spectroscopy was performed at the K edge of the implanted ions to identify their local environment at the end of the implantation process. Ti was found to form Ti–C and Ti–N bonds whereas Co and Fe precipitated and formed clusters in Si3N4. CoSi was detected in SiC whereas, in the same matrix, Fe clusters coexist with FeSi. A coherent interpretation of these results is given in terms of the heat of reaction for all possible systems. We also successfully interpret in the same way some results found in literature in the case of implanted oxides.

Ni ions were implanted in bulk AlN with the goal to form embedded metallic clusters. Combining several characterisation techniques such as X-ray absorption spectroscopy, X-ray diffraction and high resolution transmission electron microscopy, we determined the lattice parameter of the Ni clusters that display a fcc crystalline structure. The average size increases when the ion fluence is increased or after a thermal treatment. Thanks to moire fringes observed by high resolution transmission electron microscopy and to satellite peaks seen on the diffraction patterns, we concluded that the annealed Ni clusters orientate their (002) planes on the (101) of AlN. Moreover, the satellite positions allowed us to calculate Ni cluster average diameters, that are in agreement with average sizes deduced by X-ray absorption spectroscopy.

Films of ZrN were prepared using Zr sputtering assisted by N bombardment on heated substrates. They were submitted to Co implantation at 300 K with various fluences to reach concentrations as high as 10%–30% in the doped zone. The long and short range orders were studied via X-ray absorption spectroscopy performed at the Zr and Co K edges and X-ray diffraction. ZrN appears stable under the amount of energy deposited in the lattice, no amorphization is observed. Co ions precipitate to form clusters. This is interpreted as due to the great stability of the matrix together with the fact that Co–N bonds are not chemically favoured. Such a precipitation was already seen in the case of Cu and Ni implantation in the AlN matrix.

The structure of AF-ZrF(4) system (A(+) = Li(+), Na(+), K(+)) compounds in the liquid state is studied using an approach combining EXAFS spectroscopy with molecular dynamics simulations. A very good agreement is observed between the two techniques, which allows us to propose a quantitative description of the liquids. From the Zr(4+) solvation shell point of view, we observe a progressive stabilization of the 7-fold and then of the 6-fold coordinated complexes when passing from Li(+) to Na(+) and K(+) as a "counterion". Particular attention is given to the systems consisting of 35 mol % of ZrF(4). At that particular composition, the ZrF(6)(2-) complex predominates largely whatever the nature of the alkali. The calculated vibrational properties of this complex are in excellent agreement with a previous Raman spectroscopy experiment on molten KF-ZrF(4). The most important differences are observed for the lifetime of these octahedral units, which increases importantly with the size of the monovalent cation. On a larger scale, an intense first sharp diffraction peak is observed for the Zr(4+)-Zr(4+) partial structure factor, which can be attributed to the correlations between the octahedral units formed.

Si3N4 amorphous thin layers prepared by sputtering have been implanted either with Cu or with Fe ions. X-ray absorption spectroscopy was performed at the Si K edge to characterise the electronic empty states of p character, the structural state of the initial layers and the modifications around Si induced by implantation and a post-annealing treatment. We show that the energy deposition process mainly leads to a reorganisation of the second coordination shell around Si, i.e. concerns the Si-Si bonds.

X-ray absorption fine structure (XAFS) measurements on terbium fluoride in molten lithium fluoride and in molten lithium–calcium fluoride mixtures, (e.g. 0.20TbF3–0.80LiF, 0.20TbF3–0.62LiF–0.18CaF2, 0.20TbF3–0.48LiF–0.32CaF2, 0.50TbF3–0.50LiF, and 0.50TbF3–0.38LiF–0.12CaF2), have been carried out. In the solid state, coordination number of terbium (Ni) and inter ionic distances between terbium and fluorine in the first neighbor (ri) are nearly constant in all mixtures. In 0.20TbF3–0.80LiF, 0.20TbF3–0.62LiF–0.18CaF2 and 0.50TbF3–0.50LiF mixtures, Ni's decrease from ca. 8 to 6 and ri's also decrease from ca. 2.29 to 2.26 A on melting. On the other hands, in molten 0.20TbF3–0.48LiF–0.32CaF2 and 0.50TbF3–0.38LiF–0.12CaF2 mixtures, Ni's are slightly larger than 6 and ri's do not change. These facts correspond to the amount of F− supplied by solvent melts, i.e. the effect of CaF2 becomes predominant at bCaF2 > 0.32 in ternary 0.20TbF3–aLiF–bCaF2 mixtures and at bCaF2 > 0.12 in ternary 0.50TbF3–aLiF–bCaF2 mixtures.

We propose in this paper an original approach to study the structure of the molten LiF-ZrF(4) system up to 50 mol % ZrF(4), combining high-temperature nuclear magnetic resonance (NMR) and extended X-ray absorption fine structure (EXAFS) experiments with molecular dynamics (MD) calculations. (91)Zr high-temperature NMR experiments give an average coordination of 7 for the zirconium ion on all domains of composition. MD simulations, in agreement with EXAFS experiments at the K-edge of Zr, provide evidence for the coexistence of three different Zr-based complexes, [ZrF(6)](2-), [ZrF(7)](3-), and [ZrF(8)](4-), in the melt; the evolution of the concentration of these species upon addition of ZrF(4) is quantified. Smooth variations are observed, apart from a given composition at 35 mol % ZrF(4), for which an anomalous point is observed. Concerning the anion coordination, we observe a predominance of free fluorides at low concentrations in ZrF(4), and an increase of the number of bridging fluoride ions between complexes with addition of ZrF(4).

The combination of experimental and simulation techniques becomes essential for refining structural models that are consistent with all the available experimental data. In this work we investigated the structure of liquid CaAl2O4 above its melting point by ab initio molecular dynamics (AIMD), carried out using the VASP code where the interatomic forces are obtained from density functional theory. Neutron diffraction and simulation results are compared. By using the AIMD model we determined structural parameters such as coordination numbers, interatomic distances and bond angle distributions.

We describe a new high temperature environment based on aerodynamic levitation and CO 2 laser heating designed for high energy X-ray diffraction at the ID11 beamline at ESRF. The use of two lasers simultaneously provides a homogeneous temperature and makes it possible to explore the supercooled state with a high degree of control (about 500 °C below the melting point in this work) and to work very close to T g . The scattered beam is measured over a relatively wide Q -range of 0.5–19 A −1 with the ESRF Frelon 2D CDD camera. Using 1-min measurements we obtained good quality structure factors at various temperatures in the liquid and supercooled state of CaAl 2 O 4 . We also performed time-resolved measurements using an acquisition time of 100 ms and we obtained good statistics even at large Q . The Al–O and Ca–O bond lengths and coordination numbers derived from the calculated pair distribution functions are in good agreement with our previous X-ray and neutron works.

The chemical short-range structure was studied in liquid Al–Ni alloys by x-ray absorption spectroscopy as a function of temperature and composition. A containerless technique, combining aerodynamic levitation and inductive heating, was used to position and melt the samples. The fluorescence yield x-ray absorption at the Ni K edge was measured by a multichannel solid-state Ge detector. The number of heteroatomic pairs around the scatterer is higher than for a homogeneous alloy.

Using small-angle neutron scattering combined with a containerless aerodynamic levitation technique for high temperatures, we have measured the temperature dependence of the correlation length ξ of near-critical magnetic fluctuations in the solid phase of the completely miscible fcc alloy Co80Pd20. A fit to our data yields a critical exponent ν = 0.76 ± 0.05 for the divergence of ξ(T) above the ferromagnetic transition temperature Tc. This value of ν is consistent with the prediction of the three-dimensional Heisenberg model for magnetic critical scattering.

The local structure of liquid Y 2 O 3 at 2800 K has been investigated by combining aerodynamic levitation and laser heating with the neutron scattering technique. Despite relatively low counting statistics, we have been able to determine the short-range order in the liquid. In particular, we found Y–O and Y–Y coordination numbers around 6 and 12, confirming our previous study by anomalous X-ray scattering. The unusually sharp main diffraction peak observed with X-ray measurements, and characteristic of a high degree of chemical ordering, is strongly reduced in the neutron structure factor and thus indicates that it is mostly due to Y–Y correlations.

Aerodynamic levitation is a simple way to suspend samples which can be heated with CO2 lasers. The advantages of this technique are the simplicity and compactness of the device, making it possible to integrate the device easily into different kinds of experiments. In addition, all types of sample can be used, including metals and oxides. The integration of this technique at synchrotron and neutron sources provides powerful tools to study molten materials.

The short-range order in liquid binary Al-rich alloys (Al-Fe, Al-Ti) was studied by x-ray diffraction. The measurements were performed using a novel containerless technique which combines aerodynamic levitation with inductive heating. The average structure factors, S(Q), have been determined for various temperatures and compositions in the stable liquid state. From S(Q), the pair correlation functions, g(r), have been calculated. The first interatomic distance is nearly temperature-independent, whereas the first-shell coordination number decreases with increasing temperature for all the alloys investigated. For the Al-Fe alloys, room-temperature scanning electron microscropy (SEM) studies show the formation of a microstructure, namely the existence of Al(13)Fe(4) inclusions in the Al matrix.

The local short-to-intermediate range structure was studied in liquid Al-Ni and Al-Cu alloys by X-ray diffraction as a function of temperature and composition. A containerless technique, combining aerodynamic levitation and inductive heating, was used to position and melt the samples. The scattered intensity was recorded by an image plate. Liquid samples with compositions corresponding to an intermetallic solid phase show a pronounced prepeak in the structure function S(Q).

AlN bulk ceramic has been implanted with energetic Co ions. In order to accurately characterise the atomic surrounding of the implanted ions, X-ray absorption measurements were carried out at 80 K in the fluorescence mode at the Co K edge in the as-implanted and annealed states. Simulation of the EXAFS oscillations allowed us to identify a first stage where Co is inserted in the AlN matrix followed by a second stage where Co precipitates form., Facultad de Ciencias Exactas, Instituto de Física La Plata

Nanoparticles of cobalt and nickel embedded in AlN matrix have been prepared by ion implantation or sputter deposition with average diameters ranging from 0.8 to 9 nm. The average magnetic moment per atom versus the average cluster diameter was measured either by x-ray magnetic circular dichroism and/or by SQUID magnetometry. The resulting values are always smaller than the bulk ones. An interpretation is given in terms of an interaction between the atoms located at the cluster surface and the AlN matrix, leading to a magnetically dead layer about 0.1 nm thick.

Small Ni, Cu, Fe, Ag, Pt, Au nanoparticles with average diameters in the nanometer range have been studied by infrared spectroscopy. The Ni, Cu, Fe, Ag, Au nanoparticles were embedded in transparent matrices, AlN or Si3N4, or deposited on Si3N4 in the case of Pt. They were previously characterized by several techniques such as X-ray absorption spectroscopy, grazing incidence small-angle X-ray scattering or transmission electron microscopy. Infrared absorption depending on the metal is detected in the 60 meV to 140 meV energy range. It is interpreted as due to low-energy collective electron excitations related to the 2D confinement occurring at the metal-matrix interface.

The dynamic structure factor S(Q,omega) of the refractory oxide melts MgAl2O4 and MgAl4O7 is studied by inelastic x-ray scattering with aerodynamic levitation and laser heating. This technique allows the authors to measure simultaneously the elastic response and transport properties of melts under extreme temperatures. Over the wave vector Q range of 1-8 nm-1 the data can be fitted with a generalized hydrodynamic model that incorporates a slow component described by a single relaxation time and an effectively instantaneous fast component. Their study provides estimates of high-frequency sound velocities and viscosities of the Mg-Al-O melts. In contrast to liquid metals, the dispersion of the high-frequency sound mode is found to be linear, and the generalized viscosity to be Q independent. Both experiment and simulation show a weak viscosity maximum around the MgAl4O7 composition.

We describe a new high temperature environment based on aerodynamic levitation and laser heating designed for neutron scattering experiments up to 3000°C. The sample is heated to the desired temperature with three CO2 lasers from different directions in order to obtain a homogeneous temperature distribution. The apparent temperature of the sample is measured with an optical pyrometer, and two video cameras are employed to monitor the sample behavior during heating. The levitation setup is enclosed in a vacuum-tight chamber, enabling a high degree of gas purity and a reproducible sample environment for structural investigations on both oxide and metallic melts. High-quality neutron diffraction data have been obtained on liquid Y3Al5O12 and ZrNi alloy for relatively short counting times (1.5h).

Molten fluorides are used in Molten Salt Reactors MSR such as the non moderated fast reactor MSFR, where the molten salt LiF-ThF4 is the fuel and the coolant. The formation of fission products (FP) such as lanthanides, during the reactor operation, possibly modifies the physicochemical properties of the melt. It is therefore important to characterize the melt from the structural and the dynamics point of view in order to determine its properties. Because of problems related to the radioactivity of thorium, as well as requirements related to spectroscopic methods, the system studied in this thesis is the LiF-ZrF4-LaF3 (zirconium and lanthanum are possible FP). The approach followed in this thesis combines measurements by NMR spectroscopy and EXAFS at 850 °C with molecular dynamics simulations. In the molten salt, we have shown the existence of zirconium and lanthanum complexes with different coordination numbers, whose proportions depend on the composition. Depending on the content of ZrF4, [ZrF7]3- species are dominant but change slightly and are further connected between each other’s via bridging fluorine. The addition of LaF3 to the mixture stabilizes the 7 coordination number around the zirconium and tends to enrich the environment of lanthanum with fluorides. A medium-range order is established between the various complexes containing zirconium and lanthanum due to bridging fluorine. Species dynamics is slower when the amount of either ZrF4 or LaF3 is higher. We noted a significant effect on the structure and dynamics of species starting 10 mol%. LaF3 added to the medium. The data obtained by this novel approach concerning the chemistry of the molten salt in MSR containing FP, are fundamental to improve the separation of these products and optimize the process.; Les fluorures fondus sont utilisés dans les Réacteurs à Sels Fondus tels que le réacteur non modéré, à neutrons rapides, MSFR où le sel fondu LiF-ThF4 joue le rôle du combustible et du liquide caloporteur. La formation des produits de fissions (PF) tels que les lanthanides, au cours du fonctionnement de ce réacteur, peut modifier les propriétés physicochimiques du bain fondu. Il est ainsi important de caractériser le bain fondu de point de vue structural et dynamique afin de remonter à ses propriétés. En raison des problèmes de radioactivité liés au thorium, et des conditions requises liées aux méthodes spectroscopiques utilisées, le système étudié dans ce manuscrit est le LiF-ZrF4-LaF3 (le zirconium et le lanthane étant des PF potentiels). L'approche développée dans cette thèse combine des mesures par spectroscopies RMN et EXAFS à 850 °C avec des simulations de dynamique moléculaire. Dans le bain fondu, nous avons montré la coexistence de complexes à base de zirconium et de lanthane de différentes coordinences, dont les proportions et les interactions dépendent de la composition. En fonction de la teneur en ZrF4, les espèces [ZrF7]3- majoritaires évoluent peu mais se connectent davantage via des fluors pontants. L’ajout de LaF3 au mélange stabilise la coordinence 7 autour du zirconium et tend à enrichir l’environnement du lanthane en fluors. Un ordre à moyenne portée s’établit entre les différents complexes à base de zirconium et de lanthane par l’intermédiaire des fluors pontants. La dynamique des espèces est ralentie en fonction de l’ajout de ZrF4 et LaF3. Nous avons noté un effet important sur la structure et la dynamique des espèces à partir de 10% mol. LaF3 ajouté au mélange. Les données obtenues par cette approche originale de la chimie du bain fondu dans le RSF en présence des PF, sont fondamentales pour améliorer la séparation de ces derniers et optimiser le procédé.