Metal-Free Perovskite Ferroelectrics with the Most Equivalent Polarization Axes

Ferroelectricity in metal-free perovskites (MFPs) has emerged as an academic hotspot for their lightweight, eco-friendly processability, flexibility, and degradability, with considerable progress including large spontaneous polarization, high Curie temperature, large piezoelectric response, and tailoring coercive field. However, their equivalent polarization axes as a key indicator are far from enough, although multiaxial ferroelectrics are highly preferred for performance output and application flexibility that profit from as many equivalent polarization directions as possible with easier reorientation. Here, by implementing the synergistic overlap of regulating anionic geometries (from spherical I^- to octahedral [PF₆]^- and to tetrahedral [ClO₄]^- or [BF₄]^-) and cationic asymmetric modification, we successfully designed multiaxial MFP ferroelectrics CMDABCO-NH₄-X₃ (CMDABCO = N-chloromethyl-N'-diazabicyclo[2.2.2]octonium; X = [ClO₄]^- or [BF₄]^-) with the lowest P1 symmetry. More impressively, systemic characterizations indicate that they possess 24 equivalent polarization axes (Aizu notations of 432F1 and m3̅mF1, respectively)─the maximum number achievable for ferroelectrics. Benefiting from the multiaxial feature, CMDABCO-NH₄-[ClO₄]₃ has been demonstrated to have excellent piezoelectric sensing performance in its polycrystalline sample and prepared composite device. Our study provides a feasible strategy for designing multiaxial MFP ferroelectrics and highlights their great promise for use in microelectromechanical, sensing, and body-compatible devices.