Ferroelectricity through Reversible Anion-Relay Polarization Switching in a Two-Dimensional Metal–Organic Framework

Abstract

Ferroelectric materials are central to next-generation electronics and energy technologies because of their ability to couple electrical, mechanical, and thermal signals. Metal–organic frameworks (MOFs) provide a versatile platform for such functionalities owing to their structural tunability; however, despite notable examples, the microscopic mechanisms governing polarization switching in MOFs remain poorly understood. Here we report a Cu(II)-based polar two-dimensional metal–organic framework [Cu(PhPO(NHCH23Py)2)](NO3)2·2H2O (1·2H2O), constructed from a low-symmetric flexible dipodal phosphoramide ligand, PhPO(NHCH23Py)2. Compound 1·2H2O exhibits robust ferroelectricity, confirmed by a well-defined rectangular P–E hysteresis loop with a saturation polarization of 1.2 μC/cm2. The ferroelectric polar domains, along with bias-dependent amplitude-butterfly and phase-hysteresis loops, were characterized by piezoresponse force microscopy (PFM). First-principles calculations uncover an unusual displacive polarization-switching pathway, in which two nitrate ions displace together along a field-defined direction, enabling reversible 180° dipole reversal through bonding reorganization at the Cu(II) center. This reversible anion-relay mechanism expands the catalog of microscopic ferroelectric processes and represents a new paradigm for MOFs. To demonstrate practical utility, flexible piezoelectric nanogenerators (PENGs) were fabricated by embedding 1·2H2O in thermoplastic polyurethane composites. The champion 10 wt % device delivered an open-circuit voltage of 25.1 V and a maximum power density of 48.7 μW/cm2, highlighting the potential of MOF-based ferroelectrics for piezoelectric energy harvesting applications.