Hydrothermal Synthesis And Properties Of The Doped Perovskite Manganites

The manganese perovskites have been studied for almost 60 years for their attractive structure and properties. Giant magnetoresistance effects have been observed in manganese perovskite thin films with genetic formulation La_1-xA_xMnO₃, with A=Ca, Sr. Ba, Pb. This phenomenon is of great interest because of its potential technological applications in magnetic recording, actuators and sensors. Copper perovskite characterized by the mixed valence Cu(II)-Cu(III) shows superconductivity at high temperature. There is the relation between the manganese perovskites' structure, electronic and magnetic properties. Mixed valence manganites with the perovskite structure (R_1-xA_x)MnO₃ (R rare-earth cation, A alkali or alkaline earth cation), with a structure similar to that of perovskite CaTiO₃, exhibit a rich variety of crystallographic, electronic and magnetic phases.Mixed-valence manganese oxides led to the formulation of new physical concepts such as double exchange and the Jahn-Teller polaron. They have a lot of interesting phenomenons, such as charge ordering, orbital ordering, spin ordering and phase separation. These materials have great potential applications, and their chemical applications include catalysis, such as catalysts for automobile exhausts. They can also be used as bolometers, magnetic sensors, solid electrolytes in fuel cells.There are many methods used for preparing magnetic oxides, and they have been prepared as polycrystalline ceramics, single crystals and thin films. Polycrystalline ceramics are relatively easy to prepare by solid-state reactions. The standard ceramic method involves repeated grinding, compaction and firing of component oxides until a single-phase material is achieved. However, this method needs a high temperature, a long reaction time, and the products are easily impure. The hydrothermal method compared with solid-state reactions has many advantages. This method only needs a lower temperature, and one-step synthesis to prepare manganites. The hydrothermal method therefore has great potential for the preparation of materials that would usually be prepared using the traditional, high-temperature methods of solid-state chemistry.More recent work has been driven by a desire to understand and exploit the large negative magnetoresistance effects which appear near and below the Curie temperature. The large negative magnetoresistance effects and related properties essentially arise from the double-exchange. In the ferromagnetic phase (T < Tc), the material becomes metallic, but is an insulator in the paramagnetic phase (T > Tc).Further studies should be done to understand their physical properties. We need more single crystals of different compositions The phase diagrams need to be completed and more chemical systems added. We pay great attention on preparing mixed-valence manganites by hydrothermal method in this study. Under hydrothermal conditions, we successfully prepared single crystal of manganese perovskites. We focus on whether the structures and properties of our products are different from others by using different synthesis methods.A series of A-doped Perovskite Manganites Nd_1-xA_xMnO₃(A=Ca,Sr)and Sm_0.4Ca_0.6MnO₃ were synthesized under mild hydrothermal conditions. The temperature is much lower, and the reaction steps are reduced compared with solid-state reactions. The use of water would permit rapid mixing of several chemical elements, leading to homogeneous products, and also offers the potential for control of crystal growth leading to particles of desired morphologies. Our products have high purity, good crystallinity, and show excellent morphology. We measured the I-V curves of our samples, and we find they are semiconductors. Then we studied the electrical resistivity measurements from 4K to 300K to observe the phenomenon of negative magnetoresistance effects.Two mixed-valence perovskites Nd_0.4Ca_0.6Mn_0.6Fe_0.4O₃,Sm_0.4Ca_0.6Mn_0.6Fe_0.4O₃ were selectively synthesized by hydrothermal method. They are distorted orthorhombic with space group Pbnm. In particular, the hydrothermal method has great scope for the preparation of multinary oxide phases, i.e. containing two or more metals. We also measured the electrical resistivity measurements from 4K to 300K to study the negative magnetoresistance effects, but we failed to get the curves.We successfully prepared the single crystals of 9R-BaMnO₃ with a perovskite structure under the hydrothermal conditions. The space group is R-3m. By using strong alkali media, we got the samples with relatively good size and quality. The SEM photographs show that its homogenous crystal size is about 70μm.