Unlocking Cage-Confined Cations Molecular Dynamics toward High-T_c Perovskite Ferroelectrics

Cage-confinement effect that imposes great constriction on the dynamic behaviors of guest molecules is an established platform for tailoring physical properties. Herein, the strategy of enhancing cage-confinement effect to control molecular motion has been probed for the first time to exploit new high-T_c ferroelectrics of 2D hybrid perovskites. By fine-tailoring of the confined cations inside the perovskite cavities, we have successfully obtained new homologous ferroelectrics of (BA)₂(MA)₂Pb₃Cl₁₀ (1; BA=n-butylamine, MA=methylamine) and (BA)₂(EA)₂Pb₃Cl₁₀ (2; EA=ethylamine). Intriguingly, this dynamics modulation of cage-confined cations leads to a remarkable promotion of Curie temperature (T_c), boosting from 340 K (for 1) to 402 K (for 2). In situ solid-state NMR spectroscopy and theoretical simulations on energy barrier confirm that the larger EA cation in 2 is subject to stronger confinement effect, of which potential energy barrier of molecular motion is ~3.5 times that of MA cation. This dynamic behavior greatly suppresses the dynamic motions of EA cation, while MA cation can easily accelerate motion and reach a fast motion limit, thus accounting for an enhancement of T_c (ΔT~62 K) in 2. The finding sheds light on the understanding of cage-confined electric orders and the precise design of high-performance ferroelectrics.