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

## TL;DR

A new two-dimensional metal-organic framework shows strong ferroelectric properties, with potential for use in energy-harvesting devices.

## Contribution

A reversible anion-relay mechanism for polarization switching in a Cu(II)-based MOF is discovered and demonstrated.

## Key findings

- The MOF compound 1·2H2O exhibits robust ferroelectricity with a saturation polarization of 1.2 μC/cm².
- Flexible piezoelectric nanogenerators using the MOF achieved an open-circuit voltage of 25.1 V and a power density of 48.7 μW/cm².

## 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(NHCH2
3Py)2)]­(NO3)2·2H2O (1·2H

2

O), constructed from a low-symmetric
flexible dipodal phosphoramide ligand, PhPO­(NHCH2
3Py)2. Compound 1·2H

2

O 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 1·2H

2

O 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.