Formation of Head/Tail-to-Body Charged Domain Walls by Mechanical Stress

Domain walls (DWs) in ferroelectric materials are interfaces that separate domains with different polarizations. Charged domain walls (CDWs) and neutral domain walls are commonly classified depending on the charge state at the DWs. CDWs are particularly attractive as they are configurable elements, which can enhance field susceptibility and enable functionalities such as conductance control. However, it is difficult to achieve CDWs in practice. Here, we demonstrate that applying mechanical stress is a robust and reproducible approach to generate CDWs. By mechanical compression, CDWs with a head/tail-to-body configuration were introduced in ultrathin BaTiO₃, which was revealed by in-situ transmission electron microscopy. Finite element analysis shows strong strain fluctuation in ultrathin BaTiO₃ under compressive mechanical stress. Molecular dynamics simulations suggest that the strain fluctuation is a critical factor in forming CDWs. This study provides insight into ferroelectric DWs and opens a pathway to creating CDWs in ferroelectric materials.

Manipulating ferroelectric behaviors via electron-beam induced crystalline defects

Ferroelectric nanoplates are attractive for applications in nanoelectronic devices. Defect engineering has been an effective way to control and manipulate ferroelectric properties in nanoscale devices. Defects can act as pinning centers for ferroelectric domain wall motion, altering the switching properties and domain dynamics of ferroelectrics. However, there is a lack of detailed investigation on the interactions between defects and domain walls in ferroelectric nanoplates due to the limitation of previous characterization techniques, which impedes the development of defect engineering in ferroelectric nanodevices. In this study, we applied in situ biasing transmission electron microscopy to explore how dislocation loops, which were judiciously introduced into barium titanate nanoplates via electron beam irradiation, affect the motion of ferroelectric domain walls. The results show that the motion was dramatically suppressed by these localized defects, because of the local strain fields induced by the defects. The pinning effect can be further enhanced by multiple domain walls embedded with defect arrays. These results indicate the possibility of manipulating domain switching in ferroelectric nanoplates via the electron beam.

Direct observation of nanoscale dynamics of ferroelectric degradation

Failure of polarization reversal, i.e., ferroelectric degradation, induced by cyclic electric loadings in ferroelectric materials, has been a long-standing challenge that negatively impacts the application of ferroelectrics in devices where reliability is critical. It is generally believed that space charges or injected charges dominate the ferroelectric degradation. However, the physics behind the phenomenon remains unclear. Here, using in-situ biasing transmission electron microscopy, we discover change of charge distribution in thin ferroelectrics during cyclic electric loadings. Charge accumulation at domain walls is the main reason of the formation of c domains, which are less responsive to the applied electric field. The rapid growth of the frozen c domains leads to the ferroelectric degradation. This finding gives insights into the nature of ferroelectric degradation in nanodevices, and reveals the role of the injected charges in polarization reversal.

Atomic-resolution electron microscopy of nanoscale local structure in lead-based relaxor ferroelectrics

Relaxor ferroelectrics, which can exhibit exceptional electromechanical coupling, are some of the most important functional materials, with applications ranging from ultrasound imaging to actuators. Since their discovery, their complex nanoscale chemical and structural heterogeneity has made the origins of their electromechanical properties extremely difficult to understand. Here, we employ aberration-corrected scanning transmission electron microscopy to quantify various types of nanoscale heterogeneities and their connection to local polarization in the prototypical relaxor ferroelectric system Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃. We identify three main contributions that each depend on Ti content: chemical order, oxygen octahedral tilt and oxygen octahedral distortion. These heterogeneities are found to be spatially correlated with low-angle polar domain walls, indicating their role in disrupting long-range polarization and leading to nanoscale domain formation and the relaxor response. We further locate nanoscale regions of monoclinic-like distortion that correlate directly with Ti content and electromechanical performance. Through this approach, the connections between chemical heterogeneity, structural heterogeneity and local polarization are revealed, validating models that are needed to develop the next generation of relaxor ferroelectrics.

Constructing phase boundary in AgNbO3 antiferroelectrics: pathway simultaneously achieving high energy density and efficiency

Dielectric capacitors with high energy storage density (W_rec) and efficiency (η) are in great demand for high/pulsed power electronic systems, but the state-of-the-art lead-free dielectric materials are facing the challenge of increasing one parameter at the cost of the other. Herein, we report that high W_rec of 6.3 J cm^-3 with η of 90% can be simultaneously achieved by constructing a room temperature M2-M3 phase boundary in (1-x)AgNbO₃-xAgTaO₃ solid solution system. The designed material exhibits high energy storage stability over a wide temperature range of 20-150 °C and excellent cycling reliability up to 10⁶ cycles. All these merits achieved in the studied solid solution are attributed to the unique relaxor antiferroelectric features relevant to the local structure heterogeneity and antiferroelectric ordering, being confirmed by scanning transmission electron microscopy and synchrotron X-ray diffraction. This work provides a good paradigm for developing new lead-free dielectrics for high-power energy storage applications.

Giant piezoelectricity of Sm-doped Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystals

High-performance piezoelectrics benefit transducers and sensors in a variety of electromechanical applications. The materials with the highest piezoelectric charge coefficients (d ₃₃) are relaxor-PbTiO₃ crystals, which were discovered two decades ago. We successfully grew Sm-doped Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (Sm-PMN-PT) single crystals with even higher d ₃₃ values ranging from 3400 to 4100 picocoulombs per newton, with variation below 20% over the as-grown crystal boule, exhibiting good property uniformity. We characterized the Sm-PMN-PT on the atomic scale with scanning transmission electron microscopy and made first-principles calculations to determine that the giant piezoelectric properties arise from the enhanced local structural heterogeneity introduced by Sm³⁺ dopants. Rare-earth doping is thus identified as a general strategy for introducing local structural heterogeneity in order to enhance the piezoelectricity of relaxor ferroelectric crystals.