Hydrothermal Synthesis And Characterization Of Rare Earth Garnet-structure Ferrite Materials

It is well-known that the synthesis procedures play an important role in characteristics of materials, moreover, the physical properties significantly vary with the preparation condition and defect properties. A variety of methods have been used to prepare rare earth iron garnets, including solid-state reaction, sol-gel process, co-precipitation route, low temperature liquid phase epitaxy (LPE) and so on. Each method has its own disadvantages and advantages.The classical solid state reactions involve high temperature procedure, lengthy sintering time and grinding steps between RE2O3 andα-Fe2O3. The sol-gel method is time-consuming. In microemulsion processing, large amount of solvent is used to synthesize small amount of material. The LPE method can obtain single crystal, besides employing matched lattice substrate and needing flux (usually PbO), and it has to get corresponding perovskite as first phase, which make this method complicated and hard handing. For preparative methods mentioned above, firing is an essential crystallization step, which usually results in particle agglomerates.For overcoming the drawbacks of these methods, we adopted hydrothermal synthesis which provides a promising route to prepare a well-crystalline and phase-pure rare earth iron garnet in one-step in a tightly closed vessel. The hydrothermal synthesis technique is promising in the preparation of complex oxides in terms of relatively low reaction temperature, one-step synthesis procedure, easy handling and controllable particle size. Moreover, the hydrothermal technique has been widely applied to the synthesis of metastable phases and unstable phases at high temperature. Therefore, in the process of developing and preparing garnet of high-temperature metastable phases, the synthetic chemistry has become a challenging scientific and technical issue. In 1961, Geller and Espinosa have concluded that the maximum lattice parameter for unsubstituted rare earth iron garnets obtained through conventional sintering method could not be greater than 12.540 A. A reasonable explanation based on the free energy calculations for the high temperature solid state reaction was given by Kimizuka et al in 1983. However, a rigorous literature search reveals that few papers devoted to the study of Pr3Fe5O12 and Nd3Fe5O12 exist. Sm3Fe5O12 and Eu3Fe5O12 were reported a lot, but have not yet been synthesized by hydrothermal synthesis. In the paper, we have expanded on hydrothermal method. We successfully synthesized a series of compounds.1. Large single crystals Pr3Fe5O12 and Nd3Fe5O12 have successfully synthesized under mild hydrothermal conditions and structurally characterized by XRD SEM-EDS,IR,Raman,XPS,DSC,VSM (given Curie temperature,SQUID (given curves of dependent of temperature and field) and single crystal structure analysis. Thermal analysis shows that LS-HS transition of Fe3+occurs in the sample, and physical properties have been changed before and after transition. For example, Curie point increases by about 20℃.2. We have synthesized Sm3Fe5O12 and Eu3Fe5O12. The purity and crystallinity predominate over the other ones. The samples have also characterized by mentioned above method. The alkalinity, reaction temperature and reaction time of the system play key roles in influencing the crystallization and composition of the products.3. In order to meet the requirements of application to improve material properties, we had substitution of elements. The Mn3+ion substituted Fe3+ion in the parent compounds RE3Fe5Oi2 (RE= Pr,Nd,Sm,Eu) that have previously synthesized, and the amount of substitution increases with rare earth ionic radius. The samples have characterized by mentioned above method.Compared with parent compounds RE3Fe5O12 (RE= Pr,Nd,Sm,Eu),4πMs become larger and Tc become smaller.