Formation Mechanism of Topological Bubble Domains in Bi_0.5Na_0.5TiO₃-Based Piezoelectric Films

Nanotopological domains such as skyrmions, bubbles, and mesons in ferroelectric thin films offer fascinating physical properties as well as potential applications. Particularly, topological bubble domains demonstrate extraordinary piezoelectric response enhancement, yet the fundamental mechanism remains ambiguous. Here, bubble domains were induced in Bi_0.5Na_0.5TiO₃-based films by competition among charge, orbital, and lattice degrees of freedom, demonstrating domain-density-dependent piezoelectric enhancement. A combination of experimental and theoretical simulations elucidated that the strong coupling of lattice distortion and oxygen octahedral distortion leads to an augment in local inhomogeneity, resulting in the increased density of bubble domains. Bubble domains with low-angle domain walls and high electric field sensitivity promoted polarization rotation and thus improved the piezoelectric properties. This study establishes a structure-property relationship for topological domains while providing guidelines for designing high-performance nanoelectronic devices based on domain engineering strategies.