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]BiX 5 (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.