Research On The Short-Range A-Site Ionic Order And Correlated Magnetic&Dielectric Properties In La-Ba-Fe-O Double Perovskite Oxides

The ordering of the A-site ions of the double-layered perovskite oxide will cause two results.First,the ordering of the A-site ions causes the spatial symmetry of the compound crystal to decrease,and a local or overall structural phase transition occurs,which in turn leads to significant changes in macroscopic physical and chemical properties related to the crystal structure.Second,the order of the A-site ions causes the valence electron orbitals of the A and B cations to undergo further energy level splitting,changing the original orbital hybridization mode,and thus changing the electronic structure of the system.Therefore,the related research on the ordered type and order degree of the A-site ions in the two-layer perovskite oxide system has become a new means to achieve the fine-grained regulation of perovskite oxide macroscopic properties.In this thesis,the double-perovskite oxides LaBaFe₂O_5.5 and LaBaFe₂O₆ with A-site ion short-range order were obtained by topological reduction-oxidation method.LaBaFe₂O₅ was revealed by neutron powder diffraction structure refinement combined with high-angle annular dark field scanning transmission electron microscopy.There is a"La-Ba-Ba-La/Ba"type ion ordered structure in LaBaFe₂O_5.5.XPS characterization revealed that the A-site ions ordered the axial oxygen ion vacancies,resulting in an oxygen vacancy ordered structure.The ⁵⁷Fe M?ssbauer spectroscopy showed that the Fe ion in the LaBaFe₂O_5.5 sample was dominated by the+3 valence,while the Fe ion in the LaBaFe₂O₆sample exhibited the+3 and+5 valence coexistence and contained a small amount of non-integer valence Fe^?3+??+ions.The magnetic test shows that LaBaFe₂O₆ exhibits strong ferrimagnetic behavior,and its Curie transition temperature is greatly improved compared with the disordered La_0.5Ba_0.5FeO₃ system and the A-site short-range ordered LaBaFe₂O_5.5 system.This is because the orderly arrangement of La³⁺and Ba²⁺ions is beneficial to stabilize the B-site charge order,thus enhancing the Fe³⁺-O-Fe⁵⁺ferromagnetic super-exchange interaction.According to the above experimental conclusions,we believe that the orderly interaction of the A-site ion structure and the B-site ion charge order may be the main reason for the target compound to exhibit strong ferromagnetic coupling behavior.The dielectric properties test shows that the LaBaFe₂O_5.5 system has higher carrier activation energy than La_0.5Ba_0.5FeO₃ under the action of alternating electric field,and thus exhibits lower dielectric loss.The intrinsic reason obtained by micro-scale analysis may be due to the localized ordered domains formed by La³⁺ and Ba²⁺ ions inducing oxygen vacancies ordering and Fe³⁺-Fe⁵⁺charge ordering,thus forming localized charge wells and polarized electric fields.Suppress the jump behavior of the polaron.In addition,the LaBaFe₂O₆ system also exhibits strong extrinsic dielectric relaxation behavior,further demonstrating that oxygen vacancy order can significantly enhance the space charge effect near the grain boundary.In summary,the order of the A-site ions has a significant effect on the magnetic and dielectric properties of the La-Ba-Fe-O perovskite system.The in-depth study of the structure-property relationship of the system can be a new type of magneto-electricity.A useful reference is provided for device development and functional optimization of iron materials.