Halide Substitution Driven Symmetry Breaking in a Ferroelectric Mixed-Halide Halogenometallate for Piezoelectric Energy Harvesting.

Halide substitution provides a compelling yet underexplored route to engineer polar, noncentrosymmetric solids with emergent ferroelectric functionality. Herein, we report a new ammonium-based mixed-halide halogenometallate, [BTMA]2CdBr2I2, that crystallizes in the polar P21 space group and exhibits coupled ferroelectric and piezoelectric responses. The noncentrosymmetric nature of the structure is confirmed by its pronounced second harmonic generation (SHG) activity. Bulk ferroelectricity is evidenced by a well-saturated, rectangular polarization-electric field (P-E) hysteresis loop with a polarization of 4.43 μC cm-2, while piezoresponse force microscopy (PFM) directly visualizes switchable ferroelectric domains, confirming intrinsic polarization at the microscale. Beyond these fundamental properties, we further demonstrate device-level functionality by integrating [BTMA]2CdBr2I2 into flexible thermoplastic polyurethane (TPU) composites. The optimized 10 wt % composite delivers a high piezoelectric nanogenerator output voltage of 15.9 V, a power density of 10.24 μW cm-2, and an output work efficiency (OWE) of 23.7%. These results not only establish mixed halide engineering as a viable strategy to access polar molecular materials but also position halogenometallate-based hybrids as promising candidates for flexible, high-performance energy harvesting technologies.