Analyse structurale au sein du diagramme de phase de La2O3-WO3 et exploration des propriétés de conduction ionique

Une appréhension avec les outils cristallographiques permet de mettre en évidence les relations structures-propriétés si nécessaires à la compréhension des propriétés de conduction des matériaux. C est avec ce double objectif que mes travaux de thèse ont été menés : investigations structurales et études des propriétés de conduction ionique des composés définis de structure inconnue au sein du diagramme de phase La2O3-WO3. Les structures et les microstructures de ces composés, obtenus principalement par voie de synthèse solidesolide, ont été étudiés par diffraction des rayons X, des neutrons et des électrons (Microscopie Electronique en Transmission et à Balayage) sur poudre, par analyse thermique différentielle et gravimétrique, ainsi que par résonance magnétique nucléaire du 139La. Les propriétés de conduction ionique ont été caractérisées par spectroscopie d impédance. Le composé ß-La2WO6 (1:1), forme basse température a vu sa structure résolue de manière ab-initio. Il s agissait du dernier membre des composés Ln2WO6 (Ln = lanthanide + Yttrium) de structure inconnue, celui-ci ne présente aucune parenté cristallographique avec les composés de même formulation. Malgré des tests d hyper-trempe, la variété haute température n a pu être complètement isolée. Suite à la résolution structurale ab-initio du composé La14W8O45 (53,3%mol. WO3), nous avons dû reformuler ce composé vers La18W10O57 (52,6%mol. WO3). Ce composé cristallise dans une maille hexagonale de grand volume, avec un paramètre c six fois multiple du paramètre de la maille fluorine. Les trois variétés allotropiques du composé La6W2O15 (3:2) ont été étudiées. Lors de ce travail, nous avons mis en évidence une relation de parenté structurale entre ces trois formes. Les structures des formes haute température (a > 930C) et moyenne température (630C < ß < 930C) sont résolues. Lors de cette étude, nous avons mis en évidence un nouveau composé défini qui se situe à 42%mol. WO3. Ce nouveau composé présente une structure modulée (1D) très proche de la forme haute température de la phase La6W2O15. On constate l insertion d un oxygène supplémentaire. Enfin, le composé La10W2O21 (5:2) a également été l objet d une longue étude car celui-ci présente une propriété de conduction par ions O2- intéressante, à 700C, une conductivité de l ordre de 6.10-4 S.cm-1 a été mise en évidence. La résolution structurale a mis en évidence une structure lacunaire à la fois dans le sousréseau anionique et dans le sous-réseau cationique.The aim of this work deals with the structural exploration of the La2O3-WO3 phase diagram. Indeed, like many phase diagrams, La2O3-WO3 was built in the 70s, but definite compounds structures were not determined and are still unknown. This thesis focus on La2WO6, La18W10O57, La6W2O15 and La10W2O21. These compounds are synthesized as powder by solid state reaction and characterized by X-ray, neutron powder diffraction, gravimetric and differential thermal analysis, T.E.M. and 139La N.M.R.. The structure of the low temperature form named ß-La2WO6 has been determined from laboratory X-ray diffractometer, neutron (T.O.F) and electronic diffraction data. This tungstate crystallizes in a non-centrosymetric orthorhombic space group (n19) P212121 Z = 8, a = 7,5196(1) Å, b = 10,3476(1) Å, c = 12,7944(2) Å. La18W10O57 which presents 52.65 WO3 mol% in the binary system La2O3-WO3 in place of the previous reported compound: La14W8O45 53.3 WO3 mol%. The structure has been determined from synchrotron and laboratory X-ray, neutron, and electron diffraction data. This tungstate crystallizes in the non-centrosymmetric hexagonal space group (n190) P-62c , with Z = 2, a = 9.0448(1)Å, c= 32.6846(3)Å . This compound presents a sub cell c =c/6= 5.45Å. Polytypism have been observed concerning this compound by transmission electronic microscopy, ionic conductivity and thermal expansion have been explored. For the 3 allotropic La6W2O15 forms, a structural relation have been established. The a and ß form are solved. The crystal structure of La10W2O21, which has to be reformulated (La5.667W0.333)LaWO14??2, is best described, on average, by a 2 x 2 x 2 anion-deficient fluorite-related superstructure cubic cell, space group F 4 3m, Z = 4, a = 11.17932(6) Å. The La/W mixed site is nicely confirmed by 139La NMR. This compound exibits an interesting fast oxide-ion conducting properties, comparable with LAMOX at low temperature. Different from many ionics conductors, we don t notice any temperature structural transition. Its conductivity is about 6.4.10-4 S.cm-1 at 700C.LE MANS-BU Sciences (721812109) / SudocSudocFranceF

Crystal structure of lanthanum bismuth silicate Bi<SUB>2-x</SUB>La<SUB>x</SUB>SiO<SUB>5</SUB> (x~0.1)

International audienceA melting and glass recrystallization route was carried out to stabilize a new tetragonal form of Bi2SiO5 with bismuth partially substituted by lanthanum. The crystal structure of Bi2–xLaxSiO5 (x~0.1) was determined from powder X-ray and neutron diffraction data (space group I4/mmm, a = b = 3.8307(3) Å, c=15.227(1) Å, V=224.18 Å3, Z=2; reliability factors: RBragg=5.65%, Rp=14.6%, Rwp=16.8%, Rexp=8.3%, χ2=8.3 (X-ray) and RBragg=2.40%, Rp=8.1%, Rwp=7.5%, Rexp=4.2%, χ2=3.3 (neutrons); 11 structural parameters refined). The main effect of lanthanum substitution is to introduce, by removing randomly some bismuth 6s2 lone pairs, a structural disorder in the surroundings of (Bi2O2)2+ layers, that is in the (SiO3)2− pyroxene files arrangement. It results in a symmetry increase relatively to the parent compound Bi2SiO5, which is orthorhombic. The two structures are compared

Design of lanthanide metal organic frameworks incorporating dicarboxylate ligands

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Effects of tungsten substitution on the transport properties and mechanism of fast oxide-ion conduction in La₂Mo₂O₉

International audienceThe oxide-ion conduction properties of the series La2Mo2−yWyO9 (y ≤ 1.4) are studied by impedance spectroscopy. For each studied composition (y = 0.25, 0.5, 0.75, 1.0, 1.2 and 1.4) two conduction regimes are evidenced: a low temperature regime of conventional Arrhenius-type, and a high temperature regime interpreted as VTF (Vogel–Tammann–Fulcher)-type, with highly mobile oxide-ions. While the characteristics of the Arrhenius-type regime evolve regularly with tungsten substitution, those of the VTF-type regime vary non linearly, with an extremum at y = 1. Both findings are analyzed in connection with similar trends previously evidenced in a crystallographic study [G. Corbel, Y Laligant, F. Goutenoire, E. Suard, P. Lacorre, Chem. Mater. 17 (2005) 4678], namely smooth evolution of O2/O3 sites occupancy, and non linear evolution of the cell volume and cationic framework characteristics, respectively. The best performance in term of conduction stability in a reducing atmosphere is obtained for doubly substituted La1.7Gd0.3Mo0.8W1.2O9 (stable at 696 °C down to P(O2) = 1.6 10− 16 Pa)

Supramolecular architecture based on [Fe(CN)6]3− metallotectons and melaminium synthons

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Investigation of the La2O3–Nb2O5–WO3 ternary phase diagram: Isolation and crystal structure determination of the original La3NbWO10 material

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Structure solution of the complex γ-La6W2O15

International audienceOxides in the Ln2O3-MO3 (M = Mo and W) system are of significant technological interest for their laser applications [1], ionic conduction [2], catalytic [3] and ferroelectric [4] properties. The La2O3-WO3 phase diagram has been studied by a number of groups [5–7], but little detailed crystallographic information was reported due to the lack of good single crystals. Some of the reported compositions have not been appropriately characterized. Recently, the structures of La2WO6, La18W10O57 and La10W2O21 were solved using X-ray powder diffraction (XRPD) [8–10].For the La6W2O15 compound phase transitions at 630 and 930 °C have been reported [1–3]. The structure of the high temperature phase α-La6W2O15 was determined ab-initio by XRPD [11]. The lower-temperature forms β and γ, however, couldn't be determined due to the large number of reflections in the X-ray powder diffraction pattern and the relatively low symmetry of the system. The existing literature on γ-La6W2O15 only relates two sets of unit cell parameters [5–6], that almost match the XRPD pattern of γ-La6W2O15, but some weak peaks remain without indexation and can't be explained by the presence of any impurity.Here we present the structure solution using transmission electron microscopy of the complex structure of γ-La6W2O15. From zone axis precession electron diffraction the unit cell was determined to be monoclinic with cell parameters a=1.57 nm, b=1.21 nm, c=1.57 nm, β=110°. As an example, the [100] zone axis is presented on figure 1. Due to the low symmetry of the crystal system and the large unit cell, a huge number of reflections needed to be acquired, so that electron diffraction tomography was used to record the intensities. The cation positions were obtained but the distribution of the cations on the sites was not evident. Z-contrast imaging showed that disorder on some cationic sites has to be considered (fig.2)

Structure solution of the complex γ-La6W2O15

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La10_{10}W2_2O21_{21} : An Anion-Deficient Fluorite-Related Superstructure with Oxide Ion Conduction

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Controllable microstructure tailoring for regulating conductivity in Al-doped ZnO ceramics

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