In-Situ TEM Study of Microstructure Evolution in Ferroelectric/ferroelastic Materials under Stress

In ferroelastic-ferroelectrics (FMs), stress-induced microstructure such as phase and domain can effectively enhance properties significantly. We prepared free-standing single-crystal BaTiO₃ sub-micrometer pillar as a model system to investigate microstructural evolutions in FMs under high stress loading. We directly observed and quantitatively analyzed in situ in a transmission electron microscope with 4D-STEM the microstructural evolutions in the pillar under various strain loading (different intensity, direction, and rate). We found that dozens of slow compression cycles (strain rate of 10^-2/s, at 520 MPa) can induce multiple-nanodomain and multiple-phase coexistence in BaTiO₃. After unloading, the pinned boundaries and domain walls by mobile point defects can stabilize these microstructures, including metastable ones, therefore improve both functional and mechanical performance of FMs. The "brittle" FMs can in fact withstand GPa level stress without fracture, resulting in large strain (higher than 5% in BaTiO₃) under the specially designed loading ways. Our work elucidates the complex multiscale (from micrometer to unit cell scale) evolution of phase, domain microstructures and their interactions in FMs, as well as the corresponding improvement in properties under the large strain loading. Based on this, we propose a novel method for domain engineering in FMs.