Ion Regulated Topotactic Phase Transitions And Physical Properties In Cobaltate Films

The transition-metal oxide is a member of strongly correlated electronic system.The strong coupling among lattice,charge,orbit and spin inside the transition-metal oxide have produced many important physical phenomena,such as superconductivity,giant magnetoresistance effect,metal-insulator transition,charge-orbit ordering,etc.In recent years,with the study of ion regulation in materials,more and more physical and chemical properties of materials can be modulated by controlling the ion transfer.Therefore,for transition-metal oxides,ions should also be considered as a degree of freedom to determine the physical properties of materials.Ion regulation has been regarded as an important means for controlling the physical properties of materials.Moreover,the process of controlling the ion transfer in the material itself has a promising application in energy fields such as solid oxide fuel cells and hydrogen storage materials.In natural world,the cobalt element exhibits pretty rich valence state,which provides colbat oxides special advantage in studying the ion regulation.Therefore,the cobalt oxide has been regarded as a breakthrough in the study of transition-metal oxide ion regulation.In this thesis,we take cobalt oxide as an example,and use three methods of current effect,vacuum annealing,and ionic liquid gating to regulate the ion transfer in materials.The main results obtained include:1. By combining current effect and temperature cycling to achieve local oxygendefect in the film,a strain-dependent new method is proposed for oxygen defect modulation.The experimental results show that under continuous cyclic transport measurement,the electrical transport properties of?011?-La_0.7Sr_0.3Co O₃/Pb(Mg_1/3Nb_2/3)_0.7Ti_0.3O₃ film gradually changed from metallicity to insulation.This indicates that the Co ions in the film changed from+4 valence state to+3,leading to the reduction in the ratio of Co⁴⁺/Co³⁺and thus a transition in the transport behavior.The high-resolution TEM study demonstrated that local oxygen defects were generated in the film,resulting in a decrease in the ratio of Co⁴⁺/Co³⁺.Further electrical transport experiments show that the combination of current effects and temperature cycling plays a key role in generating the local oxygen defects in the film.In addition,through comparing with the repeated transport measurements on the La_0.7Sr_0.3Co O₃ bulk,it was found that the generation of oxygen defects in the film is highly strain-dependent.The combination of current effect and temperature cycling is an effective way to accurately modulate oxygen defects.This work provides new opportunities for the research of high-efficiency solid oxide catalysts.2. The transfer of A-site and O ions in the ABO_x systems has achieved greatsuccess in inducing or enhancing the functional properties,while the B-site cation directly determines the physical properties of the materials.By vacuum annealing,the concerted migration of O ions and B-site Co cations was achieved for the first time in the La_0.7Sr_0.3Co O₃ film.The film undergoes a tri-state topotactic phase transition from perovskite La_0.7Sr_0.3Co O₃ to brownmillerite La_0.7Sr_0.3Co O_2.5 and to single-layered perovskite La_1.4Sr_0.6Co O₄ via the concerted migration of oxygen and B-site Co-ions.The spherical aberration scanning transmission electron microscopy?STEM?,X-ray photoelectron spectroscopy?XPS?,X-ray absorption spectroscopy?XAS?,and electron energy loss spectroscopy?EELS?results indicated that the B-site Co ions migrate out of the lattice framework along the Co O₄ tetrahedral sublayer of the brownmillerite structure,which are embedded in the film as elementary Co-clusters.First-principles calculations and analysis on the oxidation state of Co ions indicated that the doping of the La element at the A site reduces the valence state of Co ions in the Co O₄ tetrahedral sublayer,thereby effectively reducing the strength of the Co-O bond and the cohesive energy of Co-ions of the Co O₄ sublayer.As the vacuum annealing temperature increases,the Co-O bond in the Co O₄ tetrahedral sublayer breaks,forming a Co ion transfer channel,which leads to the concerted migration of O ions and Co ions.Meanwhile,the concerted transfer of dual ions also leads to the rearrangement of Co 3d electrons,which changes the material properties from ferromagnetic metal state to ferrimagnetic insulation state and then to ferromagnetic insulation state.In this work,the types of controllable ions are expanded from A-site ions and O-site ions to B-site ions for the first time,and a successive tri-state topotatic phase transition is realized.This work opens up a new avenue for the research and application of topotatic phase transitions in ion electronics,electrocatalysis,ion batteries and other fields.3. Ionic liquid gating regulation has become an important method of regulating the properties of materials,and improving its regulation efficiencyat room temperature has become a core problem in the research of ionic liquid gating.By utilizing the out-of-plane tensile strain to promote the regulation efficiency of the ionic liquid on the brownmillerite-perovskite transition in La_0.7Sr_0.3Co O_x film,the reversible regulation of the topotatic phase transition is realized at room temperature in the La_0.7Sr_0.3Co O_x/La Al O₃ film.Different out-of-plane strains were introduced to the perovskite structure film by growing it on different substrates,which were used as the pristine samples and regulated by ionic liquid gating under same conditions.It was found that when the gate voltage was-3 V,all the films were converted to perovskite structure.While the voltage become+3 V,the ionic liquid gating efficiency on the film could increase when the film undergoes out-of-plane tensile strain.XAS results show that the oxygen defect concentration of perovskite La_0.7Sr_0.3Co O_x film increases with the increase of out-of-plane tensile strain.The out-of-plane tensile strain reduces the Gibbs free energy of oxygen defects,which promotes the transition from perovskite to brownmillerite structure.With the reversible topotatic phase transition of La_0.7Sr_0.3Co O_x/La Al O₃ film regulated by the ionic liquid gating,the film also undergoes a reversible transition from the ferrimagnetic insulation state to the ferromagnetic semiconductor state.This work demonstrates that the regulation efficiency of the ionic liquid gating at room temperature on La_0.7Sr_0.3Co O_x films can be greatly improved by introducing the out-of-plane tensile strain,which is also helpful to promoting ionic liquid gating to other more materials.4. Thecobalt oxide with brownmillerite structure has important potential applications in the field of catalysis due to its one-dimensional oxygen defect channels.However,the oxygen defect channels are usually lying in the in-plane direction,which greatly limits the catalytic efficiency of the material.Based on the knowledge of previous research,we comprehensively study the evolution of the lattice structure and physical properties of the La_0.7Sr_0.3Co O_x film involving perovskite-brownmillerite topotactic phase transition process by utilizing vacuum annealing.Our experiments evidence that the direction of one-dimensional oxygen defect channel of the brownmillerite structure La_0.7Sr_0.3Co O_2.5 film can turn from in-plane to out-of-plane under proper vacuum annealing.It is found that the perovskite-brownmillerite topotactic phase transition of La_0.7Sr_0.3Co O_x film mainly occurs within the annealing temperature range from 350 to 400?.Along with the topotactic phase transition in this annealing temperature range,the resistance of the film largely increased by nearly two orders of magnitude.Meanwhile,the saturation magnetization of the film at low temperature shows a slow increasing trend with the increase of annealing temperature.XRD results show that when the annealing temperature increases to 500?,the one-dimensional oxygen defect channels in La_0.7Sr_0.3Co O_2.5/La Al O₃ turn from in-plane to out-of-plane.Further analysis shows that when the temperature is 500?,the in-plane compressive strain can make the Gibbs free energy of the oxygen defect at the apex of Co O₆ octahedral equivalent to that of the equatorial plane.Oxygen defects begin to occur at the apex of Co O₆octahedron,and thus the oxygen defect channels of La_0.7Sr_0.3Co O_2.5 film shifts to be along out of plane direction.This discovery is of great significance for improving the redox catalytic efficiency of topotactic phase transition materials and controlling the rate of redox reactions.