Crystal Structures And Properties Of Perovskite LaMnO₃ And Its Doping System

Perovskite materials have become more and more popular in recent years.Perovskite materials are used in a wide range of applications in life.According to its unique magnetic properties and electrical properties,people have produced many products.For example,magnetic heads,magnetic switches and sensors,etc.Now people are paying more attention to perovskite materials'magnetoresistance effects and electronic transport properties.In this paper,LaMnO₃,LaMn?O,N?₃ and its relative oxides La_1-xSr_xMnO₃ and La_1-xSr_xMn?O,N?₃ have been synthesized by solid-state reaction method.Each sample has been measured by x-ray powder diffraction and the data has been refined by using FullProf software.All samples belong to the trigonal system and the space group of these samples was R-3c.By analyzing the unit cell parameters,magnetic properties and electrical properties,we can obtain the following conclusions:Analysis of unit cell parameters:Compared with LaMnO₃,the sample of LaMn?O,N?₃'s unit cell parameters has decreased,and unit cell volume has increased,the length of Mn-O and La-O bond has decreased and the angle of Mn-O-Mn has increased.As for the series of La_1-xSr_xMnO₃ samples,Its unit cell parameters,unit cell volume and the angle of Mn-O-Mn show a decreasing trend with doping.These mainly because the shrink of the MnO octahedron with the doping of Sr.The series samples of La_1-x-x Sr_xMn?O,N?₃'s unit cell parameters and unit cell volume also decreased with doping.Analysis of magnetic properties:LaMnO₃ and LaMn?O,N?₃ samples are paramagnetic materials.The magnetic properties of LaMnO₃ are larger than those of LaMn?O,N?₃.There are some Mn⁺22 in sample LaMnO₃,which make it more magnetic.The series samples of La_1-xSr_xMnO₃ and La_1-xSr_xMn?O,N?₃ are ferromagnetic materials.The samples'magnetization and Curie temperature have increased with the increasing of doping.The magnetic properties are correspondingly weakened when the doping exceeds the optimum doping ratio of 0.33.The series samples of La_1-xSr_xMnO₃ and La_1-x-x Sr_xMn?O,N?₃ have an obvious magnetoresistance effect between particles at room temperature,and the samples'magnetic phase transition belongs to the second-order phase transition.The series samples of La_1-xSr_xMnO₃ and La_1-xSr_xMn?O,N?₃ may conform to mean field theory.Analysis of electrical properties:LaMnO₃ and LaMn?O,N?₃ samples are all semiconductors,and the series samples of La_1-xSr_xMnO₃ and La_1-xSr_xMn?O,N?₃ have a change between metal conductor and semiconductor conversion with the increasing temperature.The samples'semiconductor conduction mechanism are the small polaron adiabatic transition model.La_0.6Sr_0.4MnO₃ and La_0.7Sr_0.3Mn?O,N?₃ samples show that electron and electron scattering play a major role in resistance at the temperature from 290 K to 330 K.The?-H diagrams of samples are measured.In the?-H diagrams,the negative magnetic part and the positive magnetic part show a little asymmetry.The change rate of magnetoresistance is relatively large near the zero field at room temperature,and the magnetoresistance effect between particles is obvious.The series samples of La_1-xSr_xMnO₃ and La_1-xSr_xMn?O,N?₃'s change of zero-field's and field-free's Seebeck coefficient with changing temperature have been tested.If the Seebeck coefficient is positive,it will show the conductive particles are hole,if the Seebeck coefficient is negative,it will show the conductive particles are electronics.The E_s value is obtained by fitting the Seebeck coefficient.We can obtain the band gap width of the samples by E_s and E_a that have been obtained from fitting Seebeck coefficient curve.