3D Chiral Perovskitoid‐to‐Perovskite Reconstructive Phase Transition Designed Molecular Ferroelectric Thin‐Film with out‐of‐Plane Polarization

Abstract The realization of pure out‐of‐plane polarization in ferroelectric thin films is crucial for next‐generation high‐performance microelectronics and miniaturized devices. Molecular ferroelectrics, which allow facile and large‐area thin‐film fabrication via solution processing, have gained tremendous attention in recent years. However, to date, molecular ferroelectric thin films with pure out‐of‐plane polarization have never been reported due to the inconsistency of their random orientation with the polarization direction. Herein, 3D chiral alkali metal halide ferroelectrics R / S ‐AQCsI 3 (AQ = 3‐aminoquinolinium) are designed, of which thin films exhibit a highly preferred (00 l ) orientation. Notably, R / S ‐AQCsI 3 can undergo an unprecedented reconstructive phase transition from hybrid 3D perovskitoid to perovskite structure while maintaining the thin films with the (00 l ) preferential orientation. More importantly, such a topological rearrangement of the 3D inorganic [CsI 3 ] n 2− framework triggers chiral AQ 2+ cation reorientation in the lattice, which changes the polarization direction from b ‐axis in the polar perovskitoid phase to c ‐axis in the perovskite phase, enabling the variation from pure in‐plane to pure out‐of‐plane polarization in the thin‐film. To the knowledge, this is the first report of molecular ferroelectric thin films with pure out‐of‐plane polarization. This work opens up an avenue for achieving pure out‐of‐plane polarization in molecular ferroelectric thin films.