Lone pair expression in fluorinated two-dimensional hybrid perovskite enables multiaxial ferroelectricity and nonlinear optical response

Two-dimensional (2D) hybrid organic-inorganic perovskites (HOIPs) with tunable octahedra are promising for flexible devices due to spontaneous-polarization-driven functionalities like ferroelectric memory, piezoelectric sensing and optical switches. However, achieving large spontaneous polarization and strong second harmonic generation (SHG) remains challenging for 2D HOIP ferroelectrics. Here, we report a 2D HOIP ferroelectric (4,4-difluoropiperidinium)₂GeBr₄ designed by molecular fluorination. Remarkably, (4,4-difluoropiperidinium)₂GeBr₄ exhibits large saturation polarization (>15 μC/cm²), high Curie temperature (401 K) and the strongest SHG intensity among 2D HOIP ferroelectrics (about 6.2 times of KH₂PO₄). These properties arise from the synergy of two mechanisms: the 4s² lone pair on Ge²⁺ and the orientational ordering of 4,4-difluoropiperidinium cations. Moreover, (4,4-difluoropiperidinium)₂GeBr₄ features eight equivalent polarization directions (the maximum number among reported 2D HOIP ferroelectrics), enabling its composite (4,4-difluoropiperidinium)₂GeBr₄@thermoplastic polyurethane device to exhibit prominent piezoelectric sensing. Our work offers a reliable route to high-performance 2D hybrid ferroelectrics and highlights their potential in flexible devices.