A coordination-ligand reorientation mechanism for ferroelectricity in Mn(II) hybrid complex and its utility for memory application

Moving beyond classical models of collective ion reorientation, we present a ligand-tilting design for molecular ferroelectrics. Using the asymmetric isothiocyanate (NCS⁻) ligand as a dynamic “molecular hinge,” we achieve facile polarization switching through geometric distortion and coordination-ligand flipping in the hybrid complex [Ph3PEt]3[Mn(NCS)5]. In this framework, the asymmetric cation stabilizes a polar phase in the triclinic P1 space group and a rare distorted trigonal-bipyramidal Mn(II) center, directed by the tilt of the coordinated NCS⁻ ligand, enabling room-temperature ferroelectricity with saturation polarization (Pₛ) of 1.57 µC·cm−2. Density functional theory (DFT) confirms that ligand-flipping modes are a key switching mechanism with a low barrier to polarization reversal. Piezoresponse force microscopy (PFM) provides visual validation of domain reversals via box-in-box lithography and single-point switching spectroscopy. Incorporation of [Ph3PEt]3[Mn(NCS)5] into ferroelectric field-effect transistors (FeFETs) yields a memory window (60 V) and an ON/OFF ratio (>104), with electrically controlled transitions between volatile and non-volatile states. The devices emulate synaptic functions, including long-term potentiation/depression (LTP/LTD) and multilevel conductance states (with 64 levels), demonstrating neuromorphic capabilities. Control experiments distinguish polarization-driven switching from charge trapping as the origin of the memtransistor response. Coordinate-ligand dynamics thus emerge as a design principle for molecular ferroelectrics, enabling memory and brain-inspired computing. Addressing the limitations of classical models in molecular ferroelectrics, the authors introduce a ligand-tilting design using asymmetric isothiocyanate ligands. This approach enables room-temperature ferroelectricity and neuromorphic capabilities in devices, with implications for memory and brain-inspired computing, as demonstrated by their integration into ferroelectric field-effect transistors.