Thèse de doctorat en himie des matériaux
Sous la direction de Denis Pelloquin.
Soutenue en 2012
à Caen .
This memory is devoted to the research and characterization of new complex cobalt-rich oxides, by transmission electron microscopy (TEM) techniques. A part of the work presents the study of a “114”-type compound, non-stoichiometric at the level of the oxygen content, YBaCo4O7+δ, for the case when δ has a value of approximately 1. 5. When an extra oxygen content is applied to the original, stoichiometric material, it has been found during the TEM studies that these extra oxygen atoms cause the formation of a complex superstructure that has been characterized in terms of the lattice parameters. This is accompanied by a reduction of the original trigonal symmetry to an orthorhombic one. Layered, cobalt-rich materials in the Ba–Co–O system, formed by the intergrowth of the hexagonal perovskite, more exactly the Ba2Co9O14 and Ba3Co10O17 compounds, are presented in the second part of the memory. These materials are part of a larger structural family, which corresponds to the [BaCoO3]n[BaCo8O11] chemical formula, where n denotes the thickness (number) of hexagonal perovskite blocs [BaCoO3] in their structure. The second part of the formula represents the chemical composition of another block, common to all these materials. Which is a CdI2-type one, responsible for the magnetic properties at low temperature, that these compounds exhibit. Concerning the n = 2 term, it is impossible to synthesize the pure phase, systematically obtaining secondary phases (like BaCoO3 or Co3O4). Chemical doping, by replacing a part of the cobalt species with iron, and using the sol-gel technique, leads to obtaining a nearly pure phase Ba3(Fe,Co)10O17, which has been analyzed by X-ray diffraction, neutron diffraction and transmission electron microscopy.
Non stoichiometric effects in cobalt rich complex oxides
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