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The introduction of chemical disorder by substitutional chemistry into ionic conductors is the most commonly used strategy to stabilize high-symmetric phases while maintaining ionic conductivity at lower temperatures. In recent years, hydride materials have received much attention owing to their potential for new energy applications, but there remains room for development in ionic conductivity below 300°C. Here, we show that layered anion-ordered Ba2−δH3−2δX (X = Cl, Br, and I) exhibit a remarkable conductivity, reaching 1 mS cm⁻¹ at 200°C, with low activation barriers allowing H⁻ conduction even at room temperature. In contrast to structurally related BaH2 (i.e., Ba2H4), the layered anion order in Ba2−δH3−2δX, along with Schottky defects, likely suppresses a structural transition, rather than the traditional chemical disorder, while retaining a highly symmetric hexagonal lattice. This discovery could open a new direction in electrochemical use of hydrogen in synthetic processes and energy devices., H¯イオンの低温高速伝導を実現. 京都大学プレスリリース. 2021-06-03., Bringing order to hydrogen energy devices. 京都大学プレスリリース. 2021-06-03.

International audience; The local and electronic structures of Na3V2(PO4)2F3–Na3V2(PO4)2FO2 electrode materials have been investigated by a combination of 23Na and 31P magic-angle spinning NMR spectroscopy and density functional theory calculations. The spin distributions and the 31P NMR Fermi contact shifts in these materials are calculated based on the projector augmented wave approach implemented in the VASP code. Upon oxygen substitution, V4+ ions are formed and involved in highly covalent vanadyl bonds. We show that they exhibit a very specific electronic structure with a single electron on the 3dxy orbital perpendicular to the bi-octahedra axis. The V3+ ions, on the other hand, exhibit a partial occupation of the t2g orbitals by two electrons. The peculiar electronic structure of the V ions is at the origin of the complex spin transfer mechanisms observed in the Na3V2(PO4)2F3–Na3V2(PO4)2FO2 materials and results in the existence of several 23Na and 31P MAS NMR resonances. Owing to the proper signal assignment achieved using DFT calculations, we could estimate the degree of oxygen substitution for fluorine in the materials and discuss the local distribution of V3+/V4+ ions. Furthermore, through the 31P NMR study on the Na3V2(PO4)2FO2 composition, we here demonstrate that a 31P NMR resonance close to 0 ppm can also be observed in paramagnetic materials if there is no proper orbital overlap for the electron spin transfer to occur. Thanks to the proper signal assignment achieved using DFT calculations, we couldestimate the degree of substitution of oxygen for fluorine in the materialsand discuss the local distribution of V3+/V4+ions

Two novel high-pressure polymorphs of lanthanum oxyhydride have been successfully predicted and stabilized under pressure. When reacted at 3 GPa, the fluorite structure of LaHO with anion-centered tetrahedral (HLa4/OLa4) geometry is transformed to the PbCl2-type structure involving coordination number increase of H- to five (HLa5 square pyramids). Upon further application of pressure to 5 GPa, LaHO changed into the anti Fe2P-type structure. Interestingly, the 5 GPa phase contains tetrahedral HLa4 and square-pyramidal OLa5 geometry, meaning coordination switching, as confirmed by ab initio calculations. The structural analysis shows that this unprecedented phenomenon is enabled by higher compressibility of hydride anion and emphasizes its potential in the search for new high-pressure forms of hydride-based mixed anion materials.

International audience; Polyanionic materials such as Na3V2(PO4)2F3–yOy (0 ≤ y ≤ 2) are of high interest as positive electrode for Na-ion batteries since they offer competitive electrochemical performances compared to sodiated transition metal oxides. The composition Na3V2(PO4)2F3 (y = 0) has the highest theoretical energy density among the series, but surprisingly a lot of discrepancies are reported throughout the literature considering its structure and its electrochemical properties. We will show that most of the compounds reported as being Na3VIII2(PO4)2F3 are in fact slightly oxidized due to synthesis conditions resulting in a partial oxygen substitution for fluorine. To get an in-depth understanding of this system, a series of compositions Na3V2(PO4)2F3–yOy (0 ≤ y ≤ 0.5, i.e., near the fluorine-rich composition) was synthesized and characterized combining synchrotron X-ray diffraction, X-ray absorption spectroscopy, solid state nuclear magnetic resonance spectroscopy, and galvanostatic electrochemical tests. The structural features and electrochemical signatures of these oxidized compounds will be carefully compared to those recently obtained for Na3VIII2(PO4)2F3 by Bianchini et al.

International audience

International audience

This thesis is focused on oxides related to perovskite such as K2NiF4 structure-type, double perovskite and brownmillerite with mixed conduction properties. This ability to conduct both oxygen ions and electrons is relevant for electrochemical devices operating at high temperature, particularly as an electrode for solid oxide fuel cell. Specifically, this thesis deals with the synthesis and advanced crystal structure characterization of the reactivity of these materials mainly through large scale facilities by means of neutron powder diffraction (NPD) and X-ray synchrotron. Preliminary work in these studies involves inorganic synthesis by solid-state or by sol-gel route, thermogravimetric analysis and the iodometric titration. Original reactivity cells have been developed at the ISCR to study redox behavior under different gas flow and as a function of temperature for both neutron diffraction and X-ray synchrotron experiment. In situ study by NPD of La2-xSrxMnO4 ± δ compounds where x = 2.0 and x = 0.8 which derived from the compound cathode reference La1-xSrxMnO3 allowed to follow the structural evolution as a function of δ in reducing conditions for x = 2.0 and oxidizing conditions for x = 0.8. The synchrotron study of Pr2NiO4.22 helped to highlight the monoclinic symmetry at room temperature while previous studies announced an orthorhombic symmetry. Besides, structural changes including the transition to the HTT phase are accompanied by an incommensurable modulation that persists at least up to 900 °C. The study of double perovskites NdBaCo2-xMnxO5+δ where 0 ≤ x ≤ 2 showed that these materials exhibit a promising electrical conductivities for SOFC applications as cathode. In addition, the comparison of the molecular dynamics and NDP combined with MEM for x = 0 compound has elucidated the oxygen diffusion mechanism in these compounds. The study by NPD in reducing condition of LaSrFeCoO6 to the brownmillerite LaSrFeCoO5 has showed that the reduced structure persists at high temperatures and allowed to follow the evolution in the ordering of the magnetic moments while cooling LaSrFeCoO5., Ce travail de thèse se situe dans la thématique des oxydes dérivés de la pérovskite ayant des propriétés de conduction mixte tels que les structures de type K2NiF4, les pérovskites doubles et la brownmillérite. Cette aptitude à conduire à la fois l'oxygène et les électrons présente un intérêt pour des dispositifs électrochimiques fonctionnant à haute température et notamment en tant qu'électrode pour les piles à combustible à oxyde solide. Plus précisément, cette thèse concerne la synthèse et l'étude cristallochimique avancée de la réactivité de ces matériaux essentiellement par les grands instruments par le biais de la diffraction de neutrons (NPD) et des rayons X synchrotron. Le travail préliminaire à ces études implique de la synthèse inorganique par voie solide ou par voie sol-gel, l'analyse thermogravimétrique et la titration iodométrique. Des cellules de réactivité originales ont été développées spécialement à l'ISCR pour l'étude in situ du comportement redox sous différents flux gazeux et en fonction de la température à la fois dans le cadre de la diffraction des neutrons et rayons X synchrotron. L'étude in situ par NPD des composés La2-xSrxMnO4±δ où x = 2,0 et x = 0,8 qui dérivent du composé de cathode de référence La1-xSrxMnO3 a permis de suivre l'évolution structurale en fonction du δ en conditions réductrices pour x = 2,0 et en conditions oxydantes pour x = 0,8. L'étude DRX synchrotron de Pr2NiO4,22 a permis de mettre en évidence la symétrie monoclinique à température ambiante alors que les études précédentes annonçaient une symétrie orthorhombique. Les variations structurales notamment la transition vers la phase HTT sont accompagnées d'une modulation incommensurable qui persiste jusqu'à au moins 900 °C. L'étude des pérovskites doubles NdBaCo2−xMnxO5+δ où 0 ≤ x ≤ 2 a permis de montrer que ces matériaux présentent des conductivités électriques totales très prometteuses pour des applications en tant que cathode de SOFC. De plus, la confrontation de la dynamique moléculaire et de la NPD combinée à la MEM pour le composé x = 0 a permis d'élucider le mécanisme de diffusion de l'oxygène dans cette famille de composés. L'étude par NPD de la réduction de LaSrFeCoO6 vers LaSrFeCoO5 de structure brownmillérite a permis de mettre en évidence que la structure réduite persiste à haute température et l'évolution de la mise en ordre des moments magnétiques lors du refroidissement de LaSrFeCoO5.