Revealing domain configuration evolution at the phase transition of NaNbO3 lead-free antiferroelectric ceramics by in-situ transmission electron microscopy
Antiferroelectric (AFE) materials have potential applications in a wide range of areas, e.g., large-strain actuators, electrocaloric refrigeration and high-energy storage capacitors. When an electric field is applied to an AFE material, a ferroelectric (FE) state can be induced, which is manifested by the signature double polarization hysteresis loop. For this class of materials, the AFE-FE phase transition plays a critical role on their performance. Understanding the microstructure evolution at the phase transition is fundamental towards gaining an insight into the physical properties, and thus establishing structure-property correlations. In this contribution, we perform a transmission electron microscopy (TEM) study on lead-free NaNbO3 antiferroelectric ceramics before and after electric field-induced antiferroelectric-ferroelectric phase transition. By using electron diffraction, we confirmed the existence of the AFE state before and the co-existence of the AFE and FE states after the phase transition. Quarter-integer type and half-integer type superlattice reflections were observed, which are a signature of the AFE and FE states respectively. With centered dark field imaging, the domain morphology reveals typical micron-sized domain blocks present in NaNbO3 ceramics. Translational domain walls were also observed in both states. In addition, wedged shape domains were only observed in the FE state of the NaNbO3 ceramics that experiened phase transition. This study further aims towards obtaining a direct observation of the domain configuration evolution when in-situ biasing in TEM is applied together with 4D STEM to allow a detailed understanding of the phase transition.