Synthesis,Structure,Defects And Properties Of LaM₂GaO₅-based Ionic Conductors

As the world continues to pay attention to energy and environmental issues,green and clean devices such as solid oxide fuel cells（SOFC）have become the research focus of scholars.The oxide electrolyte material is used to determine the SOFC operating temperature(oxygen ion conductivity,σ_500–700°C>10^-2 S·cm^-1)and operating efficiency（oxygen transport number,t_O2-～1）,which has naturally become a research hotspot.In recent years,La₂O₃/Sr O/Ga₂O₃ ternary systems have been reported successively for a variety of materials with significant oxide or proton ion conductivity.The reason why this type of material exhibits excellent electrical properties is that Ga can accommodate and transport different charge carriers due to the flexible and variable coordination number of Ga in different structures.However,in this ternary system,the layered La Sr₂（Ga O₄）O compound material is rarely studied,and only reported to have p-type electron/ion mixed conduction behavior,and no research progress has been made.At the same time,based on previous studies on ionic conductor materials,it has been shown that the isolated tetrahedra in the structure are beneficial to conduct ions and the excess free oxygen in the structure is beneficial to control structural defects.Therefore,in this thesis,we control the structural defects of La Sr₂（Ga O₄）O layered materials by means of acceptor doping,non-oxygen anion substitution,and extreme synthesis conditions,which purpose is to further explore and develop new ion conductor materials with excellent performance（high conductivity）,and at the same time provide feasible solutions and research ideas for controlling the local defects of materials and improving the electrical properties of materials.The main research contents are:1.The acceptor-doped La1-_xSr_2+x（Ga O₄）O1-0.5_x（0≤x≤0.3）system was synthesized by high-temperature solid-state method.The system was shown to crystallize in the tetragonal space group P4/ncc（previously reported as a body-centered tetragonal structure）by means of diffraction methods,and the Ga O₄tetrahedra were slightly tilted along the c-axis.When Sr²⁺replaces La³⁺,oxygen vacancies are generated at the loosely bound oxide sublattice.The material has p-type electron/ion mixed conductivity,and the conductivity isσ_800°C～10^-4 S·cm^-1,but it does not have proton conductivity due to its instability in a humid environment.Aided by computational simulations,the migration of oxygen ions in the system begins at oxygen vacancies,assisted by the coordinated rotation and deformation of adjacent Ga O₄ tetrahedra,and undergoes 2D long-range migration through angle-sharing Ga₂O₇ dimers that are constantly breaking and re-formation.2.The La Sr₂Ga O₅ material was H^-substituted with non-oxygen anions by the method of topotactic reduction,and the black product obtained was a metastable oxide/metal mixed material coexisting with La Sr₂Ga O₅/La₂O₃/Ga.With the help of thermal analysis technology,it is shown that there is no H in the product,and only 10%of the oxygen in the structure is taken away,forming a La Sr₂Ga O_5-y material with high oxygen defect concentration.The AC impedance data show that the material has obvious electronic conduction behavior,and the low temperature conductivity is improved by 4～5 orders of magnitude compared with the parent material.3.The non-oxygen anion-containing F^-ion La_1-xSr_2+xGa O_5-xF_x（x=0.5,1）materials were prepared by high temperature solid phase method,and a high temperature oxidation experiment was designed to expand the oxygen vacancy defect concentration in the material.The existence and oxidation of F^-ions in the structure were determined by energy spectroscopy.The AC impedance data showed that the vacancy defects enlarged by fluorination and reoxidation did not significantly improve the electrical properties of the material,and the electrical conductivity was similar to that of the La Sr₂（Ga O₄）O material;compared with the conductivity of the F-doped unoxidized material,it was only improved by one order of magnitude.4.Sr₃Ga O_4.5,La1-_xSr_2+x（Ga O₄）O1-0.5_x（x=0.3,0.4）materials like this super-solid solution region material,or La M₂Ga O₅（M=Mg,Ca）materials are difficult to be prepared by high temperature solid phase method,which are prepared by pneumatic suspension-laser heating technology,but the pure phase was not successfully obtained by diffraction characterization.