| Single-step, low temperature (<250°C) and pressure (<20 atm) hydrothermal methods have been developed for the synthesis of copper and silver delafossite oxides, BiCuOS (transparent conductors), and hexagonal rare-earth manganites (multiferroics). These multifunctional materials are of interest because they combine multiple functional properties in a single-phase material, providing unique possibilities in numerous technological applications. Phase-pure products are achieved by proper selection of the reaction temperature, pH, and mineralizers to enhance the solubility and control the oxidation potential of the reactants.; The "Teflon-pouch" hydrothermal method was used to synthesize broad families of phase-pure copper and silver delafossite oxides in high yields. A minimum solubility of ∼10-4 mol L-1 for both the A- and B-site reagent metal oxides at the reaction temperature is necessary to yield the delafossite oxide. The solubility of the reactants can be estimated primarily from the acid-base character of the reagent metal oxide. The silver delafossite oxides AgAlO2, AgSeO2 and AgGaO2 were synthesized in high yield for the first time and their optical and electrical properties were examined, and compared to that of copper delafossite oxides, to elucidate the role of the A-site cation. While their conductivities (10 -2-10-4 S/cm) are slightly lower than values previously reported for polycrystalline powder samples of copper delafossite oxides, the optical band gaps are larger and visible light absorption is lower for the silver delafossite oxides.; The universality of the hydrothermal method to synthesize other multifunctional materials was demonstrated as rare-earth manganites, B CuOS, and CuS were also synthesized in a similar fashion as for delafossite oxides. Indeed, these syntheses highlight the advantages of the hydrothermal technique in solid states syntheses. For example, the hydrothermal route produces BiCuOS on a gram scale in only 48 h, which is much less time than the 600 h (25 d) required previously for the solid state reaction. Water clearly plays the crucial role in the formation of pure CuS phase because it alters the conditions of the solid phase reaction between copper and sulfur, allowing the complete reaction at low temperatures without surface passivation. |
