A Polar Magnetic Tetrachloroferrate Hybrid Crystal Enabling Piezoelectric Energy Harvesting and Nonlinear Optical Functionality

ABSTRACT In this work, we report a polar organic‐inorganic hybrid single crystal, 2‐chloroethyltrimethylammonium tetrachloroferrate (III), [C 5 H 13 ClN][FeCl 4 ], that distinctly integrates piezoelectric, nonlinear optical, and paramagnetic properties within an ammonium tetrachloroferrate framework. The [C 5 H 13 ClN][FeCl 4 ] single crystal belongs to the polar non‐centrosymmetric space group Cmc2 1 . The non‐centrosymmetric structure is evidenced by the second harmonic generation (SHG) response. The crystal exhibits a quasi‐static piezoelectric charge coefficient (d 33 ) of 4.1 pC N −1 , further supported by local piezoresponse force microscopy (PFM) showing characteristic amplitude butterfly and phase hysteresis loops. The Piezoelectric generator (PEG) fabricated using [C 5 H 13 ClN][FeCl 4 ] generates an electrical output voltage corresponding to the applied periodic mechanical force, demonstrating the energy‐harvesting capability. Magnetic measurements performed using SQUID magnetometry indicate paramagnetic behavior arising from tetrahedrally coordinated high‐spin Fe 3 + centers. These findings establish that discrete [FeCl 4 ] − tetrahedra can be incorporated into a polar ammonium lattice to promote magnetic functionality without disrupting symmetry‐dependent piezoelectric and nonlinear optical properties.