Study On The Microstructure And Magnetic Properties Of A-site Doped LaMnO₃ Materials Prepared By Sol Gel Method

Lanthanum manganate(LaMnO3)is a typical perovskite material with ABO3 structure,which is the focus of the research on the perovskite manganites materials.The La1-xAxMnO3(A =Ca,Sr,Mg,Ba,Ce,or other rare earth elements)is modified by doping A-site elements with LaMnO3 as matrix,which lead to generation of both Mn3+ and Mn4+ in the B-positionmixed valence state.Thus a series of new physical chemical properties appear,such as magnetoresistance,magnetothermal effect,etc.Studies have shown that the conversion temperature of most manganese oxide materials is lower than normal temperature,so the application of this type of materials is restricted.The Pb2+ doping LaMnO3 samples which is different from most elements such as Mg2+ and Ca2+ show the ferromagnetic properties at room temperature have the high phase transition temperature,and have a big magnetoresistance and magnetic entropy change near the Curie temperature.Therefore,there is potential application value in the field sensors,magnetic refrigeration materials and so on.Studies have shown that these particular physical and chemical properties may be related to various phase transitions and interactions in the system,such as phase transition from paramagnetic to ferromagnetic,phase transition from ferromagnetic to antiferromagnetic,electron-electron interaction,electron-lattice coupling effect and so on.Therefore the Pb2+ doping becomes an important topic of perovskite manganese oxide research.The microstructure,morphology and magnetic of pure phase LaMnO3,Pb A-position doping LaMnO3,and Pb/Pr A-position double doped LaMnO3 composite oxide materials was researched systematically by sol-gel method.On the one hand,citric acid sol-gel method is processed on the molecular level,and the operation is simple and easy to control.Thus we can study the material structure and performance of the system change rule,and get superior performance products with high purity,small particle size and less reunion by controlling the different process conditions.On the other hand,samples show ionic radius effect and valence state effect,such as Mn-O bond length and Mn-O-Mn bond Angle changes,Mn ions mixed valence,La vacancy and oxygen defects and so on by unit or dual doping of material A position.That leads to Lattice distortion which results to the spin structure tilt and residual magnetic moment.So the samples show ferromagnetic.Therefore,we regulate and control the microstructure and magnetic properties of the composites by changing the technological conditions and the doping content of the elements,in order to reduce the agglomeration of the samples effectively and improve the magnetic properties of the samples at room temperature.That is of great significance to further explore the internal mechanism of A-doped LaMnO3 composite oxide material and to improve the application value of materials in the field of magnetism,which lays the foundation for the development of new materials.In this paper,the crystal structure,chemical bond,functional group,microstructure and magnetic parameters of the precursor were analyzed and characterized by TG-DSC,XRD,FT-IR,SEM and VSM.The main research results of this paper are as follows:1.The effects of different water bath temperature,different pH value,different calcination time and calcination temperature on the structure and morphology of LaMnO3 prepared by Sol-gel method were studied by XRD and SEM.The optimum preparation scheme was as follows:citric acid as complexing agent,50? water bath,PH = 7,and calcined for 10 hours.And the samples with small average particle size and morphology were obtained.2.The Lai1-xPbxMnO3 polycrystalline powder was fabricated by sol-gel method in this paper.The study on the structure and magnetism of La1-xPbxMnO3(x=0,0.1,0.2,0.3,0.4,0.5,0.6)with different proportions by using XRD patterns,TG-DSC spectrum,FTIR spectrum,SEM images and VSM spectrum.The effect of calcination temperature and calcine time was intensively investigated on the structure and magnetism of La1-xPbxMnO3.Based on the analysis of the experimental data,specific saturation magnetization of La0.7Pb0.3MnO3 was controlled effectively by changing calcine temperature.The specific saturation magnetization of La0.7Pb0.3MnO3 calcined at 900? for 2 hours is 55.49 emu/g,At the same time,the grain size is uniform and the particle aggregation is small.3.The La0.7-xPrxPb0.3MnO3 polycrystalline powder was fabricated by sol-gel method in this paper.The experimental conditions were calcined at 950 ? for 10 hours.The study on the structure and magnetism of La0.7-xPrxPb0.3MnO3(x=0,0.1,0.2,0.3,0.4,0.5,0.6,0.7)with different proportions by using XRD patterns,TG-DSC spectrum,FTIR spectrum,SEM images and VSM spectrum.It is found that with the increase of the doping amount of rare earth element Pr,the structure of the sample changes from the cubic structure of the rhyme,and the magnetic properties change from ferromagnetism to paramagnetism at room temperature,in which the saturation magnetization decrease rapidly with the increase of the doping ratio,but the coercivity increases.In addition,the samples prepared by Sol-gel method have the characteristics of uniform particle size and clear grain boundary.