Hydrothermal Synthesis And Properties Of The Perovskite-type Iron Complex Oxides

With development of society, synthesis exploring of new materials have becomemore extensive and important. Perovskite-type iron complex oxides are very usefuland intensively studied oxides materials. The reason is that perovskite-type ironcomplex oxides included many physics phenomenon(optics, electrics, magnetic andso on ).The synthesis and preparation of materials are very important part in thematerials science, it can effect character of sample for different synthesis process.Traditional synthesis of perovskite-type iron complex oxides methods for the hightemperaturesolid state reaction method,the advantage is very obvious that a widearray of sources of raw materials, easy operation process,easy to grasp. But solid statesynthesis also has some defect and it was be determined by reaction mechanism. Themethod is diffusion process in thermodynamics, therefore, reaction rate are very slowand reaction required to do long-range atomic migration exercise. With the result thatthe reaction usually require higher reaction temperature, frequently grind and timeafter time high temperature calcinations, so it cause increased cost. It is difficult to getcompletely homogeneous samples. These will greatly affect the physical properties ofchemical products. In order to avoid shortcomings of high- temperature solid statereaction,much more mild synthesis technology has been used, and hydrothermal ismethod one of excellent synthetic methods.Hydrothermal technique has been widely applied to the synthesis of metastablephases and microporous solids. This method provides an attractive alternative to thesynthesis of complex oxides and fluorides, since it is carried out under relatively mildconditions with controllable particle size distribution of the product. Hydrothermalmethod, oxides or hydroxides as a solution or suspension in a liquid elevatedtemperature and pressure, offers an alternative synthesis route for the multicomponentceramic oxides prepared using the traditional, the use of a solvent would permit rapidmixing of several chemical elements, leading to homogeneous products, and alsooffers the potential for control of crystal growth leading to particles of desired morphologies, something rather difficult to achieve when using the high temperaturesthat must be employed in solid-state reactions. In particular, the hydrothermal methodhas great scope for the preparation of multinary oxide phases, i.e. containing two or moremetals, where the rapid mixing of the constituent elements would provide a great syntheticadvantage, as well as potentially leading to the discovery of new materials.In this study,we focus on the hydrothermal synthesis of perovskite-type iron complexoxides. In the system that has been prepared by traditional methods, a more soft synthesis route isexplored and the results are compared with those obtained by other synthesis methods for thepurpose to establish the relation ship between the synthesis method and structure andproperties of final products. The purpose of the study is to investigate the influence ofdifferent synthesis methods on the structure and properties of final products.A series of rare earth orthoferrites were synthesized by mild hydrothermalsynthesis in aqueous potassium hydroxide. The present method is advantageous due tothe use of relatively mild conditions,one-step synthesis procedure and easy handling.The factors influencing the synthesis were also reported in detail,especially thealkalinity of the reaction system. The crystals sizes were sensitive to the alkalinity.Rare earth ferrites were characterized by powder X-ray diffraction, scanningelectron microscopy, electrical conductivity. Conductivity exhibits thermally activatedprocesses and follows Jonsche's power law with increasing temperature. In differentoxygen partial pressure, Rare earth ferrites conductivity is contributed to oxygen vacancyand hole. The conductivity of hydrothermal synthesis sample is higher whenconductivity of hydrothermal and solid state synthesis sample are compared.A series of double perovskites Ba_2-xSr_xFeSbO₆ have been synthesized byhydrothermal reactions and the crystals have a uniform size .The compounds belongto the class of A2BB′O6 perovskite. The Rietveld refinement of Powder X- raydiffraction data showed that the crystal structures of Ba2FeSbO6 have a trigonalstructure with space group R ?3 m,Fe and Sb oxidation states equal to +3 and +5,respectively,as determined by XPS. The dielectric constant and loss tangent of thesolid at different temperatures revealed a relaxation process. The frequencydependence of electrical property led to the framework of conductivity and electricmodulus formalisms. The scaling behavior of imaginary part of electric modulussuggested that the relaxation describe the single mechanism at various temperatures.The variation tendency of the ac impedance indicated the thermally activated conduction process following Jonsche's power law. The magnetic properties ofBa_2-xSr_xFeSbO₆(x=0, 0.4, 0.8, 1.2) were studied by SQUIDS and showedantiferromagnetism.