Synthesis And Properties Of Doped Perovskite Manganites

Perovskite manganites are very useful and intensively studied oxides materials. The outstanding feature of the compounds is that their structure and properties are sensitively depends on the composition. When the composition of the materials is modulated little, their properties will change greatly. In view of the facts, ion doping has been regarded as a kind of study methods. By doping at A site or B site of perovskite, researches have not only improved the properties of existing materials, but also got lots of useful compounds and found many interesting physical phenomena.The synthesis and preparation of materials are very important part in the materials science. Firstly, without materials, properties research can not be carried on. Secondly, the synthesis methods will have great effect on the properties of the materials. The compounds that have the same composition can have different structure and exhibit different properties if they have been synthesized by the different method. In the synthesis methods of perovskite composite oxide, the solid state synthesis is kind of used widely methods. The method has a lot of advantages. For example, reaction materials usually are cheap and facile oxides, the operations are sample and large scale production can carry out. However, the solid state synthesis also has some defects. Solid state reactions are diffusion process in thermodynamics, reaction rates are very slow, so usually require higher reaction temperatures with excess of 1000℃and frequently grind. Moreover, it is difficult to get completely homogeneous samples. Therefore, some soft chemical methods, such as flux synthesis and hydrothermal technique have been adopted for the synthesis of oxide. The method are simple, versatile, and cost-effective approaches available for obtaining crystalline, chemically purified, single-phase powders at lower temperatures compared to conventional solid-state reactions. The reactions in molten media or hydrothermal system are usually controlled by chemical equilibrium, so proceed much faster than diffusion controlled solid-state reactions.The reaction process of flux method is that the reaction materials are firstly molten in the flux with lower melting point at higher temperature, and then cool slowly. After forming supersaturated solution, the flux keeps on cooling crystal, and finally the crystal are precipitated. The flux method usually has two kinds, including flux and self-flex method. The self-flux method is that the flux is both reaction medium and reaction materials. For example, Bi₂O₃ is both reaction medium and reaction materials in the synthesis of BiMn₂O₅. In the ordinary flux method, the flux is often some salts or bases with lower melting point, such as KCl, NaCl, KOH and NaOH etc. Many compounds have been prepared by the flux method, such as some nickelate and high temperature superconducting cuprate, etc. The hydrothermal synthesis reaction is the one that is carried out in the closed system, with water used as the medium. Under certain temperature and self-temperature, the physical and chemical properties of water will change greatly, for example, the density, viscidity and surface tension lower, but the boil point raise. In the hydrothermal reaction, the water act as the medium and all the reaction go on in the homogeneous dispersion system. So the compounds crystallize well, have no defects, and their composition is homogeneous by the method. The hydrothermal synthesis method has been used widely in both single crystal, but also nano-scale materials, such as TiO2, BaTiO3 and La_1-xBa_xMnO₃ etc. In addition, the hydrothermal reaction need lower temperature, so some metastable and intermediate state materials, which can not be got by solid state method, can be synthesized by the hydrothermal synthesis. In recent years, the flux method and hydrothermal synthesis have used more and more in the preparation of perovskite composite oxide. For example, some multiferroic Bi_1-xLa_xFeO₃, alkali metal doped manganite La_1-xNa_xMnO₃ and nickelate La_1-xK_xNiO₃ are prepared by the flux method; some dielectric materials BaTiO3 and magnates with alkaline earth metal doping at A-site are prepared by the hydrothermal synthesis. The flux and hydrothermal synthesis can prepare high pure and good crystallized compounds, which is helpful to the further research in properties.In the article, we prepare A-site and B-site doped perovskite manganite by the flux method and hydrothermal synthesis, and characterize the structure and properties of the compound. In the first chapter, some basic knowledge about the research background are presented. In the second chapter, we prepare perovskite manganite Gd_1-xHo_xMnO₃ (0≤x≤1), explore the optimum synthesis condition and study the evolvement in the structure and magnetic properties with the modulation of Ho doping content. In the third chapter, we prepare perovskite manganite Gd_1-xHo_xMnO₃ (0≤x≤1), explore the optimum synthesis condition and study the evolvement in the structure, morphology and magnetic properties with the modulation of Fe doping content. In the forth chapter, we prepare Bi_0.5Ca_0.5-xLa_xFe_0.3Mn_0.7O₃ (x = 0-0.25) by the flux method, and study the magnetic and transport properties. In the fifth chapter, we prepare two samples La_0.59Ca_0.21K_0.08MnO₃ and La_0.63Ca_0.1K_0.15MnO₃, and characterize their structure and magnetic properties.