In Situ Tem Investigation Of Oxide Ion Migration In The Perovskite Oxide And Its Highly Oxygen-deficient Phase

Multivalent perovskite transition metal oxides(ABO_3-x)have attracted increasing attention due to their intriguing physical properties related to oxygen stoichiometry.By tuning the coordination environments of transition metal ions in perovskite oxides,the oxidation states of the transition metal,the metal-oxygen-metal bond angles and the overall charge carrier concentration could be altered,providing a route to control new functionalities ranging from magnetism,multiferroics,to superconductivity.The ability to precisely control oxygen nonstoichiometry is of great importance and a prerequisite for being able to control the properties of perovskite oxide reversibly.The nonstoichiometry in perovskite oxides is typically associated with the presence of oxygen vacancies in the lattice that have a strong impact on their electrical transport and reactivity properties.At present,the perovskite transition metal oxides（ABO₃）have been widely in solid oxide fuel cells,oxygen separation membranes,resistive random access memory,gas sensors,and catalysis etc,due to their high activity of oxide ions.The migration and transport of oxide ions in perovskite oxides are the heart process of perovskite oxides in device applications.The formation of oxygen vacancies and the migration of oxygen ions under external stimuli serve as the basis for functional oxide materials and related devices.Therefore,real-time visualization of active oxygen vacancies and the exploration of oxygen diffusion pathways on the atomic scale which will give rise to the deep understanding of the structure-property relationships,eventually facilitating the control of physical properties is highly desirable and of great importance.In situ aberration-corrected transmission electron microscopy（In situ aberration-corrected TEM）,as a new technology that can observe the dynamic process of materials in real time under the stimuli of external field（electric field,light field,strain field,thermal field,etc.）inside the TEM,makes it possible.It not only inherits the great advantages for the high spatial resolution of the crystal structure and the high energy resolution of the electronic structure,but also realizes the direct observation of the dynamic process of materials under the external stimuli inside the TEM.In this thesis,the in situ aberration-corrected TEM was used to investigate the dynamic process of oxide ion migration and structure phase transition in perovskite oxides and its high oxygen-deficient phase under the external electric field and strain field.The main contents and results in the thesis are as follows:1. In general,for the Jahn-Teller active transition metal ions with（e_g）¹ or（e_g）³configuration such as Cr²⁺,Mn³⁺（d⁴）and Cu²⁺,Ni⁺（d⁹）,the square planar coordinations have been found.Typically,a divalent iron（d⁶）is not Jahn-Teller active.The oxide ion migration with the change of the Fe coordination environment associated with the phase transition from the 3D brownmillerite SrFeO_2.5to the 2D infinite-layer phase SrFeO₂ is very puzzling thus desirable to be explored.Combined with the TEM-STEM holder,a sandwich structure consisting of W tip（positive electrode）/SrFeO_2.5 thin film/conductive 0.7%Nb:SrTiO₃ substrate（negative electrode）was constructed in the aberration-corrected TEM chamber with the high vaccum(10^-6 Pa).Our results indicate the electric filed could be taken as a strategy to induce the oxide ion migration in the3D brownmillerite SrFeO_2.5 to bear a 2D infinite-layer SrFeO₂ with a square-planar coordination in the TEM chamber.The structure transition procedure in the SrFeO_2.5thin film grown epitaxially on the 0.7%Nb:SrTiO₃ substrate can be divided into two steps.With the electric field applied to the SrFeO_2.5 film,the first step is to form the homogeneous domain with the b_||domain which has tilting FeO₆ octahedra along the[1 （?）0]direction of the NSTO substrate shrinking along the phase boundary and converting to a_||domain which is aligned flat along the[1（?）0]direction of the NSTO substrate in total.Subsequently,as the Fe³⁺were reduced by the electric field gradually,the apical oxygen atoms of the FeO₆ octahedra were released and some arranged in the FeO₄ tetrahedra to form the new infinite-layer phase SrFeO₂ layer by layer.2. By using a customized pulsed laser deposition system,the brownmillerite SrFeO_2.5 film with the in plane and out of plane 1D oxygen vacancy channels was achieved by adjusting the thermodynamic parameters of the film growth and selecting SrTiO₃ as the substrate,which was a better match to the lattice of the SrFeO_2.5 film.The dynamic behavior of oxide ion migration in SrFeO_2.5 films with in plane or out of plane oxygen vacancy channels was studied by in situ aberration-corrected transmission electron microscopy.Our results indicate that both the brownmillerite SrFeO_2.5 with in-plane and out of plane oxygen vacancy channel will be transformed to the infinite-layer SrFeO₂ with FeO₄ square plane parallel to the substrate,but the phase transition rate from the brownmillerite SrFeO_2.5 with out of plane oxygen vacancy channel to the infinite layer SrFeO₂ is faster and more efficient,about 6 times for in-plane oxygen vacancy channel.Oxide ion migration in SrFeO_2.5 has an obvious anisotropic behavior.3. Oxygen vacancy profile in LaCoO₃ exhibits rich phases with distinct structures,symmetries and magnetic properties.Exploring the lattice degree of freedom of LaCoO₃ in transition between these different structural phases may provide a route to enable new functionality in oxide materials with potential applications.Oxygen vacancy profile transition in LaCoO₃ has so far mainly been induced by transition-metal doping or thermal treating means;Epitaxial strain was proposed to compete with lattice freedom but has not yet been rationalized.Combined with the TEM-STEM holder,a sandwich structure consisting of W tip（positive electrode）/LaCoO₃ thin film/NSTO substrate（negative electrode）was constructed in the aberration-corrected TEM chamber.The experimental findings of strain-inhibited structural transition from perovskite to brownmillerite during the electromigration of oxygen vacancies in epitaxial LaCoO₃ thin films are demonstrated.The results indicate that the oxygen vacancy ordering phase induced by electric field is suppressed by both epitaxial strain field and external loads.The demonstrated complex interplay between electric and strain fields in structural transitions of LaCoO₃ opens up prospects for manipulating new physical properties by external excitations and/or strain engineering of a substrate.