Explore On Preparation And Electric And Magnetic Properties Of Perovskite Cr-Based Oxide RE_1-xCa_xCrO₃

In this thesis,the RE₂CaCr₃O₉ with superlattice structure was proposed and constructed from the similarity of crystal structure and electronic orbit.Due to the in-plan distribution,the type of hybrid orbitals and the number of possible thermal excited electrons of Cu ion in YBa₂Cu₃O₇（YBCO）lattice are similar or same to the Cr ion in RE₂CaCr₃O₉ lattice,the Cr-O surface of RE₂CaCr₃O₉ and Cu-O surface of YBCO should have similar basic band structure,therefore RE₂CaCr₃O₉ should be superconductivity which is similar to YBCO.In this thesis,the modified sol-gel method（Pechini method）was used to prepare the powder precursor.After pre-sintered at 700℃ for 2h,the powder precursor was grinding and pressed to circular slice,then the sample was heated at 1000 ℃ for 8h to synthesize the target material RE₁-xCaxCrO₃The X-ray diffraction patterns of the L₀.67Ca₀.33CrO₃ samples in nominal composition sintered at different temperatures shows,in our experimental temperature range,the Ca atom replace La atom into LaCrO₃ lattice is primarily inserted into the phase mechanism,the sample mainly should be superlattice orthorhombic La₂CaCr₃O₉ phase.The x-T results of sample show that it has obvious diamagnetic shift,and the anti-magnetic signal is much larger than the existing ordinary anti-magnetic materials.The diamagnetic shift can be explained by superconducting transition.The XRD and x-T of La₁-xCaxCrO₃ samples with different nominal components show that:in addition to the superlattice orthogonal La₂CaCr₃O₉ phase,there may also be superlattice orthorhombic La₄CaCr₅O₁5（x = 0.2）and LaCaCr₂O₆（x =0.5）phase with a diamagnetic transition.The results of transport measurements of polycrystalline samples show that La₀.67Ca₀.33CrO₃,La₀.8Ca₀.2CrO₃ and La₀.5Ca₀.5CrO₃ may be the material with high electrical transport anisotropy.The resistance signal of polycrystalline samples mainly comes from the disorder grain boundaries.The results of magnetic transport measurements of La₄CaCr₅O₁5 polycrystalline sample show that the grain boundary resistance has a negative magnetoresistance effect due to grain diamagnetic transformation below 181K and the resistance jump of up to 500 times after metal semiconductor transformation.Such a large resistance jump,which can be explained by using the superconducting electron tunneling effect at present.Y,Sm could completely replace La in La0.67Ca0.33CrO3,which result in forming superlattice orthorhombicY₂CaCr₃O₉ and Sm₂CaCr₃O₉ phase.Ce completely substitutes La site in La₀.67Ca₀.33CrO₃ can not form the superlattice orthorhombic Ce₂CaCr₃O₉ under our heat treatment conditions and without the diamagnetic shift.This phenomenon,on the other hand.indicates that the diamagnetic shift in all samples is indeed related to the superlattice structure.In summary,there are diamagnetic properties of the superlattice materials such as La₂CaCr₃O₉,La₄CaCr₅O₁5,LaCaCr₂O₆,Y₂CaCr₃O₉ and Sm₂CaCr₃O₉ and so on.The diamagnetic shift in these materials can be explained by superconducting transition at present data,which are may likely to be a series of novel high temperature superconducting materials.The results of this thesis can provide a useful reference for the exploration of novel superconducting materials and the research on the mechanism of high temperature superconductor.