Improving ionic conductivity by Mg-doping of A2SnO3 (A=Li+, Na+)

The search for Li ions conducting ceramics is burgeoning, owing to the regain interest for solid state batteries. Here we investigate the effect of Mg substitutions on the ionic conductivity of the A2SnO3 (A=Li, Na) phases. Pure A1.8Mg0.1SnO3 and A2.2Mg0.1Sn0.9O3 were structurally characterized and their ionic conductivity was measured by AC impedance spectroscopy. We show a decrease of the activation energy with increasing the Mg substitution and found ionic conductivities three and two orders of magnitude higher for Li2.2Mg0.1Sn0.9O3 and Na1.8Mg0.1SnO3 as compared to pristine Li2SnO3 and Na2SnO3, respectively. Neutron diffraction was used to determine the Mg localization in the crystal structure and to provide a rationale for the ionic conductivity changes. Our results confirm the high sensitivity of the ionic conductivity on chemical substitutions, even limited ones

Vers l´obtention de nouveaux matériaux hiérarchisés par synthèse topotactiques

À l'heure actuelle, il y a un intérêt croissant pour l’adaptation des propriétés physique des solides contenant des métaux de transition en modifiant leurs réseaux ioniques. Ces changements topologiques modifient non seulement le degré d’oxydation du métal de transition, mais peuvent également modifier sa sphère de coordinence, ainsi que les connectivités cation-anion-cation à longue distance, pouvant conduire à des changements radicaux de leurs propriétés physiques.Dans ce contexte, mon travail de thèse, réalisé au laboratoire UCCS à Lille (France), a consisté en une étude approfondie de deux différents exemples de transformations topotactiques. Premièrement, je présente l´échange anionique ainsi que l'exfoliation en nano-feuillets chargés positivement, de l'oxo-bromure de cobalt 14H-Ba7Co6BrO17. Le processus d'exfoliation a été caractérisé structuralement à l’aide de techniques telles que les microscopies, la diffraction ou l’absorption des rayons X.Dans la deuxième partie de mon travail, une étude complète du processus d’exsolution reversible du fer dans BaFe2(PO4)2 a été effectuée, en établissant une relation entre les évolutions structurales et la quantité de fer exsoluté. Une étude sur l'insertion éventuelle du lithium dans la structure de la phase la plus lacunaire en fer, BaFe1.33(PO4)2, ainsi que ses capacités en tant que cathode pour batteries au lithium, a également été réalisée. En outre, des études structurales et magnétiques des solutions solides BaFe2-YMY (PO4)2 par substitution du métal par Co2+ ou Ni2+a aussi été réalisée.Enfin, dans la dernière partie de ce manuscrit, je présente trois phases différentes à base de fer qui ont été découvertes lors de prospection pour trouver de nouvelles phases ayant des capacités de transformations topotactique.At present, there is growing interest in tuning the physical behavior of solids containing transition metals by modifying their ionic lattices. These topological changes modify not only the electron count (through reduction or oxidation) but can also modify the local transition metal coordination sphere and the long-range cation-anion-cation connectivity, potentially leading to dramatic changes in their physical behavior. In this context, my PhD work developed in the UCCS laboratory at Lille (France), has consisted in a deep study of two different examples of topotactic transformations. Firstly I have performed anionic exchange as well as exfoliation into positively-charged nanoflakes, on the oxybromide cobaltites 15H-Ba7Co6BrO17. This exfoliation process has been structurally characterized by different techniques as microscopies, X-Rays diffractions and absorptions. For the last two, synchrotron radiation was used, the experiments being performed at SOLEIL.Secondely, a full study of the reversible iron exsolution process in BaFe2(PO4)2 has been carried out, establishing a relation between the structural features and the amount of iron exsoluted. A study about a possible lithium insertion in the lattice of the most exsoluted phase, BaFe1.33(PO4)2, as well as its capabilities determination as cathode in Li batteries was also performed. In addition, structural and magnetic studies has been realized in the substituted solid solutions of BaFe2-yMy(PO4)2 with M2+ = Co2+, Ni2+.Finally, the last part of the manuscript presents three different iron phases discovered during my prospecting work for new phases having abilities for topotactic transformation

New positive nanosheets from the exfoliation of bi-dimensional oxybromide for oriented thin films deposit

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Novel positive oxide nanosheets, from exfoliation of bidimensional oxybromide

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Diffraction des rayons X, DANES et XANES une complémentarité décisive pour la caractérisation de nanofeuillets : exemple de l’exfoliation d’oxybromures de cobalt bidimensionnels.

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Polymorphism in Li 4 Zn(PO 4 ) 2 and Stabilization of its Structural Disorder to Improve Ionic Conductivity

International audienceRealization of the vulnerability of current rechargeable battery systems drives the research of solid electrolytes. In the search for a new Li ion conductor, we explore the rich crystal chemistry of Li 4 Zn(PO 4) 2 which presents a low temperature monoclinic (α-) and a high temperature orthorhombic (β-) polymorph. We solved the crystal structure of the β-phase and found that it has a disordered Li/Zn-sublattice while showing the largest conductivity; however it could not be stabilized at room temperature by quenching. We discovered that the partial substitution of Zn 2+ with Ga 3+ in Li 4-x Zn 1-x Ga x (PO 4) 2 first leads to an intermediate β' phase. Increasing the Ga content to 0.5 mol pfu. enables to stabilize the pure β-phase at room temperature, which exhibits a conductivity by several orders of magnitudes higher than the pristine sample. The crystal structures of the new β'/β-phases have been solved to elucidate the conduction mechanism, which confirms the high sensitivity of ionic conductivity on disorder

Evidence of electrically charged cruystal Structures balanced by pairs

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Reversible Exsolution of Nanometric Fe 2 O 3 Particles in BaFe 2– x (PO 4 ) 2 (0 ≤ x ≤ 2/3): The Logic of Vacancy Ordering in Novel Metal-Depleted Two-Dimensional Lattices

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A collection of Fe-depleted hexagonal lattices stabilized by Fe-exsolution in the true 2D- Ising ferromagnetic BaFe2(PO4)2

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Polymorphs, phase transitions and stability in BaM 2 (PO 4 ) 2 M = Mn, Fe, Co systems

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