Hydrothermal Synthesis And Properties Of The Perovskite-type Manganites And Ferrites

Traditional synthesis of perovskite manganese oxide compound methods for the high-temperature solid state reaction method,in accordance with the upcoming raw materials required for a certain proportion of a mixture of burning at high temperatures(generally above 1000℃,and even 1400℃) for several hours or in a few days.The advantage is very obvious that a wide array of sources of raw materials,easy operation,easy to grasp.However,due to excessively high temperatures cause increased costs,and solid-state reaction required to do long-range atomic migration exercise,with the result that the reaction should not be thorough and often will produce multi-phase,mixed phase,repeated grinding will increase the opportunities for the introduction of impurities,these will greatly affect the physical properties of chemical products.In order to avoid these shortcomings,much more mild synthesis technology has been used,and hydrothermal method is one of excellent synthetic methods.Hydrothermal method 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 and ferrites.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 K⁺ ion-doped perovskite oxides manganate La_xA_yK_1-x-yMnO₃(A= Ca,Sr and Ba) and Sr_0.9K_0.1MnO₃ synthesized under mild hydrothermal conditions. Compared with the traditional solid state reaction methods,hydrothermal synthesis greatly reduced the reaction temperature,the samples of high purity,good crystallinity,grain uniformity,and some samples at electron microscope showing excellent morphology.Single crystal of K_0.1Sr_0.9MnO₃ was hydrothermally prepared and the crystal has a uniform size of 110-160μm.Sr ions are partially replaced by K ions in the structure.This increased the average oxidation state of Mn ions more than +4.The detection of Mn⁵⁺ by NIR,LS,and XPS further confirmed the Mn⁴⁺/Mn⁵⁺ mixed valence state in our sample.The dissolution experiments clearly showed the blue color coming from Mn⁵⁺ upon dissolution and its further oxidation to higher oxidation state in air.A series of rare earth orthoferrites were synthesized by mild hydrothermal synthesis in aqueous potassium hydroxide.The present method is advantageous due to the use of relatively mild conditions,one-step synthesis procedure and easy handling.The factors influencing the synthesis were also reported in detail, especially the alkalinity of the reaction system.The crystals sizes were sensitive to the alkalinity.The preliminary magnetic properties of LnFeO₃ were studied by VSM and SQUIDS.A series of double perovskites RSrMnFeO₆(R=La,Pr,Nd,Sm) have been synthesized by hydrothermal reactions and the crystals have a uniform size about 5～10μm.The compounds belong to the class of AA'BB'O₆ perovskite.The Rietveld refinement of powder X-ray diffraction data showed that the crystal structures of RSrMnFeO₆(R=La,Pr,Nd,Sm) are distorted orthorhombic with space group Pnma, Mn and Fe oxidation states equal to +4 and +3,respectively,as determined by XPS and iodometric titration.A series of the perovskite-type manganate oxides with Fe doping on Mn site have been synthesized by hydrothermal reaction technique and the crystal has a uniform size about 10μm.The substitution with Fe has the advantage that it does not cause lattice distortion since the Fe³⁺ ion has ionic radius of the same size as the Mn³⁺ ions,which is a special model to study the mechanism.