Study Of The Interfacial Structure And Domain Structure Of Transition Metal Oxides

Transition metal oxides show a series of novel physical properties,such as superconductivity,ferroelectricity,ferromagnetism,magnetoresistive effect and magnetostriction,because of their special atomic and electronic structure.They are very important oxide materials with wide application prospects in semiconductor and microelectronics industry.These physical phenomena of transition metal oxides are closely related to the microstructure characteristics,and the physical properties can be regulated by changing the microstructure,such as Jahn-Teller distortion,oxygen octahedral rotation,lattice stress,domain structure,etc.Transmission electron microscopy has a high spatial resolution and the ability to study the atomic and electronic structures of materials at the atomic scale,which has a unique advantage in characterizing the microstructure of transition metal oxides.In this thesis,transmission electron microscopy with spherical aberration corrector is used to characterize the special interfacial random strain fields and domain structure in the microstructure of transition metal oxides.The main research work is as follow:（1）Based on scanning transmission electron microscopy（STEM）techniques,we successfully imaged the random strain field at the interface of core-shell ZnO nanowires.Combining experimental observations and image simulations,we find that the strain contrast originates from dechanneling of electrons and increased diffuse scattering induced by static atomic displacements.For a thin sample with a random strain field,a positive strain contrast appears in the low-angle annular dark-field（LAADF）image and a negative contrast in the high-angle annular dark-field（HAADF）image,but for a thick sample（>120 nm）,the positive contrast always occurs in both the LAADF and HAADF images.Through the analysis of the relationship between strain contrast and various parameters,we also discuss the optimum experimental condition for imaging random strain fields.（2）Ruddlesden-Popper hybrid improper ferroelectricity Ca_3-xSr_xTi₂O₇（X=0,0.1,0.3,0.5）and Ca₃Ti_2-xMn_xO₇（X=0,0.1,0.3,0.5）ceramics were synthesized by traditional solid-state sintering method,and their properties and structures were characterized by XRD measurements.it shows that the samples still maintain the original structure after doping with different elements,and the peak location is slightly shifted due to the difference of ion radius.The hysteresis loops of the samples show that the samples are ferroelectric,and the ferroelectric-phoelectric transition temperature decreases with the increase of Sr²⁺substitute,but the leakage current is large due to the poor insulation.Intergrowth of A₄B₃O₁₀ with n=3 and ABO₃ with n=∞can be observed in HAADF images with atomic resolution.By analyzing the Ca/Sr atomic displacement,the ferroelectric domain with 90 degree polarization direction perpendicular to each other is observed.The polarization strength of the sample is mainly due to the polarity shift of the A（Ca/Sr）atom,and the polarity shift and ferroelectric-paramoelectric transition temperature can be changed by doping.