Advanced Transmission Electron Microscopy of Ferroelectric Perovskites

The continuous advancement in materials science and electron microscopy techniques has significantly enhanced the study and application of lead-free functional ceramics. Ferroelectric ceramics, particularly barium titanate (BaTiO₃), are now positioned at the forefront of research due to their impressive dielectric, ferroelectric, and piezoelectric properties. Recent innovations in aberration-corrected transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) have enabled atomic-scale structural and chemical characterization. Building on these developments, further advances such as electron ptychography have pushed imaging resolution beyond the sub-ångström limit into the deep sub-ångström regime (below 0.5 Å). Together, these capabilities have marked a transformative period in the investigation of ferroelectric domain dynamics and domain wall–defect interactions. Historically, BaTiO₃ was among the first ceramics discovered to exhibit ferroelectricity in the 1940s. Despite decades of investigation, key questions about domain dynamics, particularly at the atomic scale, have persisted. Since the widespread implementation of Cs-corrected TEM in the early 2000s, together with continued advancements in in situ TEM technologies, researchers have gained unprecedented access to in situ observations of electromechanical responses in BaTiO₃ under thermal, electrical, and mechanical stimuli, enabling direct insight into polarization switching and domain wall behavior at the nanoscale. This thesis is positioned precisely at the intersection of these historically significant trends: environmental mandates driving lead-free alternatives, and the technological advancements enabling atomic-scale investigations. Specifically, by combining experimental TEM observations with theoretical calculations, this work deepens the understanding of BaTiO₃’s complex properties from atomic to bulk scales, particularly focusing on dislocation-mediated domain nucleation, switching behaviors, and domain wall–dislocation interactions. Firstly, this thesis presents the development and application of a MEMS-based in situ TEM platform for wide-temperature-range investigations on a reference single-crystal BaTiO₃ sample. By extending the continuously controllable temperature range from −175 °C to 200 °C within a single TEM option, we report, to the best of our knowledge, the first direct in situ TEM observation of phase transitions and domain evolution across all four phases of BaTiO₃. Based on previous developments of advanced sideentry in situ cryogenic/heating TEM holders, we additionally demonstrate exceptional thermal and mechanical stability in this system, with a mean drift rate of 1.52 nm/min. This enables robust atomic-resolution STEM imaging and reliable quantitative analysis under variable thermal conditions. These capabilities not only facilitate detailed investigation of BaTiO₃ but also pave the way for comprehensive in situ studies of a broad range of functional materials under cryogenic, heating, and electrical biasing stimuli—most critically at the atomic scale. Secondly, employing these MEMS-based in situ TEM technologies, the thesis presents an in-depth exploration of dislocation-mediated domain nucleation and domain wall pinning phenomena in uniaxially deformed BaTiO₃. We observed that imprinting well-aligned dislocations within the {100}⟨100⟩ slip system promotes the formation of in-plane a-type domains over c-type domains. This enables intrinsic engineering of domain configurations and tailoring of electromechanical properties by leveraging the inherent dielectric anisotropy (εₐ > ε꜀) of BaTiO₃. In addition, strong pinning effects on domain wall motion were observed due to these dislocations, which extrinsically contribute to the electromechanical response of single-crystal BaTiO₃. This combination of intrinsic domain engineering and extrinsic contributions from domain wall pinning significantly enhances the material’s electromechanical performance, suggesting innovative pathways for designing ferroelectric functionality through defect engineering. Lastly, by applying advanced MEMS-based in situ cryogenic and heating STEM techniques, we directly observed the pair interactions between dissociated dislocation cores of the [110]ₚ꜀-type across a wide temperature range (−175 °C to 200 °C). The dissociation of the [110]ₚ꜀ dislocation into two partial cores of 1/2[110]ₚ꜀ and a slip plane has been reported in other perovskite structures, such as SrTiO₃. Remarkably, when combined with theoretical calculations, we reveal temperature-dependent dislocation behaviors, specifically slip and climb, governed by the competing influences of mechanical and chemical driving forces, along with phase transitions. An abnormal dislocation climb at cryogenic temperatures was also observed and investigated. Our in situ observations provide the first direct evidence of the dynamic behavior and pair interactions of these dissociated dipoles at the atomic scale. We identify this interaction as an intrinsic feature of dislocation dynamics in BaTiO₃, with broader implications for the perovskite oxide family. Building on this discovery, our findings elucidate the intricate coupling between local atomic structure (lattice energy) and chemical composition (charge energy), providing crucial insights into dislocation-mediated polarization, domain nucleation, and domain wall–dislocation interactions. As such, this work offers a foundational framework for interpreting dislocation behavior across a broad range of perovskite materials and supports defect-driven design of next-generation functional devices. In summary, by bridging historical advances in ferroelectric research with cutting-edge TEM/STEM methodologies, this thesis establishes a comprehensive framework for understanding and engineering lead-free single-crystal BaTiO₃ ceramics, contributing significantly to the development of environmentally sustainable and technologically advanced functional materials.