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

In this work, we report a polar organic-inorganic hybrid single crystal, 2-chloroethyltrimethylammonium tetrachloroferrate (III), [C5H13ClN][FeCl4], that distinctly integrates piezoelectric, nonlinear optical, and paramagnetic properties within an ammonium tetrachloroferrate framework. The [C5H13ClN][FeCl4] single crystal belongs to the polar non-centrosymmetric space group Cmc21. The non-centrosymmetric structure is evidenced by the second harmonic generation (SHG) response. The crystal exhibits a quasi-static piezoelectric charge coefficient (d33) 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 [C5H13ClN][FeCl4] 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 Fe3 + centers. These findings establish that discrete [FeCl4]- tetrahedra can be incorporated into a polar ammonium lattice to promote magnetic functionality without disrupting symmetry-dependent piezoelectric and nonlinear optical properties.