Polarization Configuration-Regulated Bubble Domains for High Piezoelectric Response in Bi _0.5 Na _0.5 TiO ₃ -Based Films

Topological bubble domains exhibit rich physics and immense application potential. However, research has focused on lead-based ultrathin films requiring intricate boundary conditions for stabilization, significantly limiting their broad utilization in ferroelectric materials. Here, controllable bubble domain engineering in Bi_0.5Na_0.5TiO₃-based epitaxial films is achieved through a straightforward polarization configuration design. Phase-field simulations and multiscale experimental characterization reveal that bubble domain density and size increase with higher Miller indices of the film orientation. Unlike the singular in-plane polarization vectors in (100)- and (110)-oriented films, the (111)-oriented films exhibit a complex polarization configuration (coexisting in-plane and out-of-plane polarization vectors), dramatically enhancing the formation of toroidal bubble domains. The high electric field sensitivity of these domains promotes polarization rotation, leading to a significantly enhanced piezoelectric response in (111)-oriented epitaxial films. This work provides a simple strategy for manipulating topological domains and critical guidance for high-performance nanoelectronics.