Domain Morphology and Atomic Structure of Antiferroelectric Perovskites

Antiferroelectrics are considered promising energy storage devices due to their reversible electric fieldinduced phase transitions between antiferroelectric and ferroelectric states. The transition manifests itself as the double polarization hysteresis loop, commonly observed in lead-based antiferroelectrics but rarely in lead-free antiferroelectrics. Thus, to achieve a comparable performance for lead-free antiferroelectrics is on-demand, simultaneously urged by environmental concerns. On the other hand, it is well-known that the property goes hand in hand with the microstructure, and transmission electron microscopy (TEM) allows a precise probing of the microstructure. In this work, the domain and atomic structure of the prototype PbZrO3 (PZ) and the well-established PLZST ceramics were examined utilizing (S)TEM techniques in combination with computational image analysis as a starting point. The complex incommensurate structure in the PLZST is resolved as a mixture of commensurate structures with different modulation lengths, manifested as the striation contrast, different from the antiphase boundaries (APB) morphology in PZ. The compositional dependence of the crystallography and domain state declares a promoted ferroelectricity with the increasing Ti/Sn content, complementing the macroscopic hysteresis loop behaviors. A superposition effect of the nearly unchanged AFE domain and reversible FE domain wall motion is hence postulated to rationalize the initial linear response of the polarization to the electric field in the FE dominant compositions. Furthermore, the atomistic studies provide a novel perspective of the polarization configuration that violates the long-accepted antiparallel and compensated configuration, which essentially belongs to the ferrielectric group. The methodology and gained knowledge are further applied to studying the prototype NaNbO3 (NN) and the newly designed solid solutions (NaNbO3-SrSnO3). A well-defined and distinct parallelogram domain morphology is observed in the NaNbO3 sample modified with 5 mol.% SrSnO3, coinciding with the pronounced double hysteresis loop at room temperature. Similar to PZ, APBs are recognized in all the investigated AFE domains in NN-based materials, responsible for the streaking in the electron diffraction patterns. Moreover, NN is atomically resolved to possess three types of commensurate structures, i.e., the 4-fold antiferroelectric phase, the 2-fold ferroelectric phase and the 2-fold antipolar APBs, in contrast to the complicated incommensurate modulations in PZ-based antiferroelectrics. Further experimental and theoretical findings indicate a strong stability of APBs against external stimuli and support the idea that the ferroelectric phase may be stabilized by the APBs with a high density, frustrating the reversible field-induced phase transitions. This study not only unveils the atomically resolved structure of antiferroelectric perovskites but more importantly highlights the importance of defect engineering in lead-free antiferroelectrics to achieve complete reversibility of the field-induced phase transition, paving the way for a future conception of compositional design as well as theoretical studies