Halide Anions Tuning of Lead‐Free Perovskite‐Type Ferroelectric Semiconductors with Inverse High‐Temperature Symmetry Breaking

Abstract There is a noticeable gap in the literature regarding research on halogen‐substitution‐regulated ferroelectric semiconductors featuring multiple phase transitions. Here, a new category of 1D perovskite ferroelectrics (DFP)₂SbX₅ (DFP⁺ = 3,3‐difluoropyrrolidium, X⁻ = I⁻, Br⁻, abbreviated as I‐1 and Br‐2) with twophase transitions (PTs) is reported. The first low‐temperature PT is a mmmFmm2 ferroelectric PT, while the high‐temperature PT is a counterintuitive inverse temperature symmetry‐breaking PT. By the substitution of iodine with bromine, the Curie temperature (Tc) significantly increases from 348 K of I‐1 to 374 K of Br‐2. Their ferroelectricity and pyroelectricity are improved (Ps value from 1.3 to 4.0 µC cm⁻², p_e value from 0.2 to 0.48 µC cm⁻² K⁻¹ for I‐1 and Br‐2), while their optical bandgaps increased from 2.1 to 2.7 eV. A critical slowing down phenomenon is observed in the dielectric measurement of I‐1 while Br‐2 exhibits the ferroelastic domain. Structural and computational analyses elucidate that the order‐disorder movement of cations and the distortion of the chain perovskite [SbX₅]²⁻ anions skeleton lead to PT. The semiconductor properties are determined by [SbX₅]²⁻ anions. The findings contribute to the development of ferroelectric semiconductors and materials with multiple PTs and provide materials for potential applications in the optoelectronic field.