Halogen-Tuned Band Gaps and Net-Dipole Polarization in 0D Hybrid Ferroelectrics

Abstract While halogen substitution routinely tunes hybrid metal halides, its influence on the microscopic origin of ferroelectricity remains underexplored. Here, we synthesize two zero-dimensional (0D) bismuth-based hybrids (3FEA)3BiX6 (X = I, Br), denoted 1-I and 2-Br, and show that the polarization mechanism shifts from one jointly governed by the organic and inorganic components to a displacive process dominated by the inorganic lattice. 2-Br exhibits an enhanced spontaneous polarization (∼4.35 μC/cm2), which originates mainly from the uncompensated net dipole of the strongly distorted [BiBr6]3– sublattices. This structural locking corresponds to a massive kinetic barrier (∼464 kJ/mol) during its anomalous phase transition. Furthermore, the inherent interoctahedral charge-transfer architecture in 1-I fosters broadband self-trapped exciton emission, which undergoes abrupt thermal quenching precisely synchronized with its macroscopic structural phase transition. These results establish halogen engineering as an effective strategy for simultaneously tailoring polarization mechanisms, phase-transition behavior, and optoelectronic responses in 0D hybrid ferroelectrics, providing design principles for multifunctional ferroic semiconductors.