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Transformation to Unique properties and New oxides by Electrochemistry – TUNE

Submission summary

Transition metal oxides are widely studied these last two decades due to their exceptional physical properties such as superconductivity, magnetism (Colossal magnetoresistance), thermoelectricity, dielectric, multiferroique or for their insertion properties for the energy storage. as electrode materials for batteries. These last ones are essentially due to their ability to show various oxidation states (mixed valence) in different coordinations -tetrahedral, octahedral, pyramid- allowed that way the formation of new phases with original structure 2D or 3D and generally with unique physical properties. In those oxides, the oxidation state of the transition metal is directly related to the oxygen content. The chemist has a lot of tools to control the oxygen content and/or the oxidation state of the transition metal. Indeed, we can use the cationic substitution or the annealing under oxygen or hydrogen pressure. Another route that could be use either to control the oxygen content or to directly generate new framework remain the soft chemistry methods. For example, tt was already demonstrated several years ago by Grenier et al. on the perovskite SrCoO3-d, where they could stabilize the stoichiometric phase SrCoO3 by electrochemical oxidation in alkaline media, leading for the latter to ferromagnetic properties. These approaches are extremely powerful to better understand the relation structure'properties. In addition, the low temperature approach is known to be the best way to succeed in the stabilization of highly (or very low) oxygenated phase with high (or low) valence for the transition element or original coordination; as it was demonstrated by Tsujimoto et al. in the course of the reduction of SrFeO3 by CaH2 at 300°C leading to the phase SrFeO2 with plan square coordination for the iron. Within this context, we propose to explore and generate new frameworks in mixed valent cobalt and iron oxides. Our approach to generate new materials is based on soft chemistry and electrochemistry. Indeed, through these routes, we whish to stabilize metastable phases or unusual oxidation states of the transition metal element and hence to discover unique physical properties. In addition, the low temperature used for these syntheses is in perfect appropriateness with today's environmental concerns. In the strongly correlated systems, two families based on non-stoichiometric cobaltites are recently under intensive investigation for their unique magnetic and transport properties: the LnBaCo2O5+d ('112') and the LnBaCo4O7+d ('114') with Ln= rare earth. These two families exhibit not only remarkable abilities to oxygen sorption/desorption below 400°C with 0<d<1 but also significant thermoelectric, transport properties (insulator transition), magnetic behavior (ferrimagnetism and magnetic frustrations due to the presence of kagome layers in the structure for the '114' type phases) and also metal nature of the spin state transitions for the '112' type phases and interesting magnetic frustration. The '112 family' is closely related to the perovskite structure, whereas the '114 family' is closely related to the spinel and hexagonal ferrite system. Thus, in this project, we propose to control the oxygen stoichiometry of these systems through soft chemistry and electrochemistry in order to get a better access to the composition-structure-properties relationships together with the formation of unique frameworks, formed either after oxidation, reduction or ionic exchange, using topotactic reactions. We believe also that exploring other transition metal like iron and manganese will lead to the formation of new frameworks and exotic phenomena. The synergy between the structural and physical characterizations in solid state chemistry performed at Caen and the new competence in soft chemistry and solid state electrochemistry that we want to develop is the ideal multidisciplinary needed to succeed in preparing new phases. The generation of original frameworks, is one important direction of research for the discovery of new magnetic and transport properties. The originality of our project stems to the use of soft chemistry and electrochemistry for the stabilization of new metastable frameworks in view of discovery new magnetic and transport properties and in optimizing the latter by controlling carefully the oxygen stoichiometry during the synthesis.

Project coordinator

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

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Beginning and duration of the scientific project: - 0 Months

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