High-Tc Realization of Lead-Free Halide Hybrid Ferroelectrics via Steric Confinement Modulation

Hybrid organic-inorganic perovskite (HOIP) ferroelectrics with high Curie temperature (T-c), typified by the lead halide hybrid perovskite ferroelectrics, are developing rapidly owing to their maneuverable ferroelectricity at high temperatures. However, acquiring high-T-c lead-free HOIP ferroelectrics via rational strategy still needs development. In this study, a brand-new program by modulating the steric confinement in a cuplike cavity to design high-T-c bismuth-halide ferroelectrics [H(2)mdap]BiX5 (H(2)mdap = N-Methyl-1,3-Propanediamine, X = Cl (1), Br (2), I(3)) is proposed. Emphatically, the T-c enhanced dramatically from 264 K of 3 to 318 K of 2 and 377 K of 1, induced by the substitution of Br and Cl to I, accompanied with an interesting transition from second-order phase transition (for 3) to the first-order one (for 1 and 2). The extent of T-c increase is up to 113 K, which far outweighs that of reported Pb-halide hybrid ferroelectrics. Structural and computational analyses elucidate that this unprecedented improvement of T-c is due to the higher phase transition energy barriers induced by modulating the steric confinement of cuplike cavity via halogen substitution. These results will provide new inspiration for designing high-T-c lead-free HOIP ferroelectrics.