Structural Phase Transition‐Associated Ferroelectricity and Piezoelectric Energy Harvesting in 0D Halogenozincate Hybrid via Cation's Terminal Group Lengthening

Halogenometallate hybrids have garnered significant research interest for their unique structural properties, making them promising candidates for practical applications. However, due to preparation challenges and stability issues, molecular-based materials with ferroelectric properties are needed. Herein, zero-dimensional (0D) A₂BX₄-type halogenozincate hybrids, (C₇H₁₃NR)₂ZnCl₄ (R = H for 1; methyl for 2; ethyl for 3, n-propyl for 4, and n-butyl for 5), are synthesized. Despite the sole difference in the cation, only 5 displays a reversible ferroelectric phase transition (P4₁↔P4₃2₁2). The ferroelectricity and its implementation in piezoelectric energy harvesting are demonstrated. Single-crystal structural analyses reveal the static-to-dynamic state change of the cation's terminal group drives the phase transition and associated properties. 5 achieves excellent ferroelectricity, including above room-temperature Curie temperature (330 K) and low coercive field (0.95 kV cm^-1). Furthermore, flexible piezoelectric nanogenerators are fabricated using polyvinyl alcohol films incorporated with 5 at varying weight percentages (wt%). The 15 wt% composite device exhibits the highest open-circuit peak voltage of 74 V, a short-circuit peak current of 4 µA, and a power density of 0.40 mW cm^-2. More importantly, it maintains stable performance over 5000 cycles. This finding provides a simple approach for designing low-dimensional hybrid ferroelectrics tailored for high-power output devices.