Hydrothermal Synthesis And Characterization Of The Perovskite Manganites

The conventional syntheses for multicomponent perovskite manganites are through high temperature solid state reactions between oxides and/or carbonate precursors. Repeated cycles of milling and calcinations are carried out to achieve the completion of solid state reactions. Relatively high temperatures, typically above 1200℃are required because of the limited diffusion during calcinations. The advantages of solid state reaction are the ready availability of oxide precursors and the low cost for powder production. However, its disadvantages are clear. High temperature calcinations in some cases may lead to the decomposition of products of the formation of undesirable phases. Particle size reduction by milling can introduce chemical impurities into the ceramic product, which may severely influence the final properties of perovskite manganites.Hydrothermal method, involving heating metal salts, oxides or hydroxides as a solution or suspension in a liquid elevated temperature and pressure, offers an alternative synthesis route for the multicomponent ceramic oxides. Because the reactions are carried out in solution, the physicochemical properties of many reactions will change. Compared to high temperature solid state reactions, hydrothermal reactions need relatively low temperatures which favor the formation of low agglomeration, homogeneity and good sinterability ceramic powder. In addition, hydrothermal method is an open route to metastable phases.In this study, we focus on the hydrothermal synthesis of perovskite manganites. In the system that has been prepared by traditional methods, a more soft synthesis route is explored and the results are compared with those obtained by other synthesis methods for the purpose to establish the relationship between the synthesis method and structure and properties of final products. New compounds have also been prepared by hydrothermal method in a finely controlled condition. The purpose of the study is to investigate the influence of different synthesis methods on the structure and properties of final products.A series of rare earth manganites RMnO₃ (R= Er, Y) have been successfully synthesized by a hydrothermal method at 240℃for 48 hours. The pure metastable manganites may serve as a model for understanding the magnetismes of Jahn-Teller distortion and charge ordering. This new synthetic approach leaves many rooms for new doped or undoped RMnO₃ compounds.RMn₂O₅ (R=La, Nd and Pr) were successfully prepared through hydrothermal processing at 240℃and characterized by powder X-ray diffraction and Scan electron microscope .The influences of various hydrothermal conditions such as reaction time and temperature on the formation, morphology and crystalline size of products were investigated. Perovskite LaCrO₃ have been successfully prepared by hydrothermal synthesis method.