Oxidation control of fluxes for mixed-valent inorganic oxide materials synthesis

This dissertation is concerned with controlling the flux synthesis and ensuing physical properties of mixed-valence metal oxides. Molten alkali metal nitrates and hydroxides have been explored to determine and exploit their variable redox chemistries for the synthesis of mixed-valent oxide materials. Cationic and anionic additives have been utilized in these molten salts to control the relative concentrations of the redox-active species present to effectively tune and cap the electrochemical potential of the flux.; Atoms like bismuth, copper, and manganese are capable of providing different numbers of electrons for bonding. With appropriate doping near the metal-insulator transition, many of these mixed-valent inorganic metal oxides exhibit extraordinary electronic and magnetic properties. Traditionally, these materials have been prepared by classical high temperature solid state routes where microscopic homogeneity is hard to attain. In these routes, the starting composition dictates the doping level, and in turn, the formal oxidation state achieved. Molten flux syntheses developed in this work have provided the potential for preparing single-phase, homogeneous, and crystalline materials. The redox-active fluxes provide a medium for enhanced doping and mixed-valency control in which the electrochemical potential adjusts the formal oxidation state, and the doping takes place to maintain charge neutrality.; The two superconductor systems investigated are: (1) the potassium-doped barium bismuth oxides, and (2) the alkali metal- and alkaline earth metal-doped lanthanum copper oxides. Controlled oxidative doping has been achieved in both systems by two different approaches. The superconducting properties of these materials have been assessed, and the materials have been characterized by powder X-ray diffraction and e-beam microprobe elemental analyses. In the course of these studies, several other materials have been identified. Analysis of these materials, and the conditions necessary to prepare them, have further aided in developing a model for use in controlling the electrochemical potential of the flux.; The alkali metal hydroxide fluxes have large electrochemical windows, and a variety of chemical reducers have been explored in the copper system. Control of the electrochemical potential has been developed through compositional control of the flux whereby the entire range of copper oxidation states, including the metal, has been achieved at a single temperature, in a single flux system.; Environmentally-friendly copper ore mimics have been prepared for thermodynamic analysis to aid in mineral transport modeling. The hydrothermally-prepared homogeneous copper- and cobalt-doped birnessites have been structurally, compositionally, and physically analyzed.