Transmission Electron Microscopy Studies On Oxides And Heterostructures

Transition metal oxides exhibit a variety of physical properties,such as giant magnetoresistance effect,high temperature superconductivity,ferroelectricity,multiferroics,etc.,and show great potential applications in future electronic devices.Different electronic phases caused by the strong coupling among lattice,charge,spin and orbital degrees of freedom would make the physical properties extremely sensitive to lattice distortions.In particular,the lattice mismatch between the film and the substrate in oxide heterostructures and superlattices could be utilized to tune lattice distortions by hydrostatic pressure,chemical doping and epitaxial strains.However,the interfacial coupling occurs in only several unit cells.Therefore,in order to understand the interfacial mechanism,it is necessary to characterize the interfacial atomic and electronic structures at atomic scale.In this dissertation,aberration-corrected scanning transmission electron microscope（STEM）was utilized to characterize the atomic and electronic structures for oxide heterostructures and superlattices.With further help from the density functional theory（DFT）calculations,the influence of the orientation and doping of the substrate on the atomic and electronic structures of the film was revealed,corresponding to peculiar physical properties of the films.For perovskite oxide heterostructures,the impact of the ordering and structure distortions of the cations on the space group and ferroelectric properties was investigated.The dissertation mainly contains three parts as follows:1.The substrate orientation dependence on oxygen octahedral tilting of La_0.67Ca_0.33Mn O₃（LCMO）films grown on Nd Ga O₃ substrates was studied by using STEM.Interestingly,the epitaxial strains,which lead to the reorientation of the oxygen octahedral tilting axis,were found to occur at the interface between the film and substrate for the（0 1 0）-oriented substrate or inside the film for the（1 （?） 2）-oriented substrate,which was confirmed by the DFT calculations as well as electron energy loss spectroscopy measurements.These findings suggest that the interfacial oxygen octahedral coupling in oxide heterostructures plays a significant role in thickness-and orientation-dependent physical properties of thin films.2.The thickness and doping dependence of the Ca Ru O₃（CRO）on oxygen octahedral tilting and electronic structures of LCMO films grown（0 0 1）-oriented NGO substrates was investigated by STEM associated techniques.The interfacial effect of the superlattices on the antiferromagnetic coupling in LCMO films was studied systematically.The thickness of the CRTO layers would affect the oxygen octahedral tilting via interfacial oxygen octahedral coupling,which could be utilized to tune the antiferromagnetic coupling at interfaces.3.A-and B-site ordered double perovskites Na Ln BWO₆（Ln=La,Nd,and B=Mg,Mn）ceramics were successfully synthesized and characterized by X-ray diffraction,STEM,and optical second harmonic generation measurements.Na La Mg WO₆ and Na Nd Mg WO₆ ceramics possess the space group of P2₁/m,while Na La Mn WO₆ has a polar crystal structure（P2₁）without inversion symmetry at room temperature.All three samples exhibit the A-site layered ordering and B-site rock-salt ordering as directly obtained by the HAADF-STEM images.The observed non-switchable ferroelectric polarization is due to a large energy barrier by the structural domains arising from the cation ordering.