铁电压电材料在现代高新技术领域具有非常广泛的应用，然而，以锆钛酸铅(PZT)为代表的铅基压电材料在制备、使用及后处理过程中会引发严重的铅污染问题，其应用正逐步受到限制。我国学者罗豪甦开发的钛酸铋钠（Na0.5Bi0.5TiO3, NBT）铁电单晶具有较好的压电性能，且能与BaTiO3（BT）或K0.5Bi0.5TiO3 （KBT）等组分形成准同型相界（MPB）而进一步提升其压电性能，是很有应用潜力的无铅压电材料体系之一。在罗豪甦研究员的研究基础上，本论文以其制备的高质量NBT基无铅铁电单晶为研究对象，利用先进电子显微学方法开展了多层次多尺度的静态结构和在外场下的动态结构演变研究，揭示其微观结构与宏观性能之间的关联。首先，研究了纯NBT单晶的微结构和成分特征。该材料的微结构从形貌上可分为片层铁电畴区和非片层区，电子衍射和球差校正扫描透射电镜实验结果表明片层铁电畴区为三方（R）相，而非片层区为R相和四方（T）相极化纳米微区（PNRs）共存，且非片层区的Na/Bi的比值比铁电畴区略高；两种区域间的成分起伏以及非片层区共存的两相PNRs被认为是NBT的介电弛豫现象的来源；此外，高的漏导、低的T相含量和低的畴璧密度是导致NBT压电性能差的重要原因。其次，深入研究了准同型相界附近0.95NBT-0.05BT单晶的微结构及其在外加电场作用下的结构演化与相变机理。该材料的微结构从形貌上可分为铁电区和弛豫铁电区，其中铁电区为R相和T相铁电畴共存（T相为主），弛豫铁电区为R相和T相PNRs共存，而弛豫铁电区的中Bi和Ba的含量略低于铁电区；通过对这两种区域在外电场下的结构演化过程的原位动态观察，揭示出外场诱导了铁电区中T相90°畴的可逆翻转以及弛豫铁电区的不可逆弛豫-铁电转变和可逆T相-R相相变，并对其压电性能增强有重要贡献。最后，结合电子显微学、变温Raman光谱和宏观性能测试深入研究了Mn: NBT单晶的T相片状结构及其与反铁电性能的联系。变温Raman光谱和介电铁电性能测试表明，T相与退极化现象和双电滞回线有紧密关联；首次从原子尺度直接观察到特征尺寸为1~2个单胞厚度的T相纳米片状结构，并揭示出不同极化态的T-T和T-R之间的连接方式；两个反平行极化的T相纳米片状结构以一个非常薄的非极性、无倾转的过渡层相连接可能形成特殊的反铁电结构，从而使NBT基材料在T相为主的成分或温度下出现双电滞回线。

Ferroelectric/piezoelectric materials have been widely used in modern high-tech fields. However, the applications of lead-based piezoelectric materials, represented by lead zirconate titanate (PZT), are gradually restricted, due to the serious lead pollution in the process of preparation, use and post-treatment. The bismuth sodium titanate (Na0.5Bi0.5TiO3, NBT) single crystals developed by Chinese scholar Luo Haosu show good piezoelectric properties and can form morphotropic phase boundary (MPB) with BaTiO3 (BT) or K0.5Bi0.5TiO3 (KBT) to further improve its piezoelectric properties. Therefore, it is considered as one of the potential lead-free piezoelectric materials. On the basis of Luo Haosu's research, this paper uses several advanced electron microscopy methods to study the static structure of NBT-based lead-free ferroelectric single crystals at multiple levels and scales and the dynamic structure evolution under the external field, revealing the relationship between the microstructure and macroscopic properties.Firstly, the microstructure and composition characteristics of pure NBT single crystals was studied. The microstructure of the material can be divided into two regions, i.e., the lamellar ferroelectric domain region and the non-lamellar region. Electron diffraction and spherical aberration-corrected scanning transmission electron microscopy have demonstrated that the lamellar ferroelectric domain is rhombohedral (R) phase, while the non-lamellar region is the coexistence of R-phase and tetragonal (T)-phase polar nano regions (PNRs). The Na/Bi ratio of the non-lamellar region is slightly higher than that of the lamellar ferroelectric domain region. The composition fluctuation between the two regions and the coexistence R-phase and T-phase PNRs in the non-lamellar region are thought to be the structural source of dielectric relaxor behavior of NBT. In addition, high leakage conductivity, low T-phase content and low domain wall density are important reasons for the poor piezoelectric performance of NBT.Secondly, the microstructure of 0.95NBT-0.05BT single crystals near the morphotropic phase boundary and its structural evolution and phase transformation mechanism under the applied electric field were studied in detail. The microstructure can be divided into the ferroelectric region and the relaxor ferroelectric region in morphology. The ferroelectric region is characterized by the coexistence of R-phase and T-phase (mainly) ferroelectric domains, and the relaxor ferroelectric region is the coexistence of R-phase and T-phase PNRs. The content of Bi and Ba in the relaxor ferroelectric region is slightly lower than that in the ferroelectric region. Based on the dynamic in situ observation of the structural evolution of these two regions under the external electric fields, it is revealed that the external field induces the reversible domain switching of the T-phase 90° domains in the ferroelectric region, the irreversible relaxor-ferroelectric transition and the reversible T-R phase transition in the relaxor ferroelectric region, which contribute significantly to the enhancement of the piezoelectric properties.Finally, the tetragonal sheet structure of Mn: NBT single crystals and its relationship with the antiferroelectric properties were studied by electron microscopy, Raman spectroscopy and macroscopic properties. Temperature-dependent Raman spectroscopy and dielectric/ferroelectric properties show that T phase is closely related to depolarization phenomenon and double hysteresis loop. For the first time, thin tetragonal platelets with characteristic size of 1~2 monocell thickness were directly observed at atomic scale, and the connection modes of T-T and T-R with different polar states were revealed. The special AFE-like structure may be formed by the connection of two antiparallel-polarized tetragonal platelets with a very thin non-polar and tilt-free transition layer, thus causing the double hysteresis loop of NBT-based materials at T phase dominant composition or temperature.