Adaptive Chemical Strain Boundary in Freestanding BiSmFe ₂ O ₆ Double‐Perovskite Membranes

Double‐perovskite multiferroic systems have attracted increasing attention due to their highly tunable structures and chemical adaptability. The uncontrollable performance changes caused by the release of boundary conditions are an important factor hindering the development of freestanding membranes. Here, the freestanding BiSmFe 2 O 6 (BSFO) double‐perovskite membranes with a fixed mechanical boundary are proposed, leveraging adaptive chemical modification, a distortion that breaks intrinsic properties, during the boundary creation process. This is attributed to the tendency of film systems with abnormal Poisson deformation to develop chemical strain during the lattice volume change process. We employ electrodynamic decomposition, an approach enabling intact release of wrinkle‐free membranes, which enhances the quality of freestanding membranes and the controllability of the mechanical boundary. Compared to the epitaxial films, the freestanding BSFO double‐perovskite membranes retain the same ferroelectric origin and lattice parameters, with comparable electrical and optical behaviors, all reflecting the preservation of the about 5.06 GPa chemical strain boundary post‐peeling. These findings underscore the effectiveness of adaptive chemical modulation in achieving constant mechanical boundary conditions for strained materials with outstanding performance.