Subtly tuning intermolecular hydrogen bonds in hybrid crystals to achieve ultrahigh-temperature molecular ferroelastic

Molecular-based ferroic phase-transition materials have attracted increasing attention in the past decades due to their promising potential as sensors, switches, and memory. One of the long-term challenges in the development of molecular-based ferroic materials is determining how to promote the ferroic phase-transition temperature (T_c). Herein, we present two new hexagonal molecular perovskites, (nortropinonium)[CdCl₃] (1) and (nortropinium)[CdCl₃] (2), to demonstrate a simple design principle for obtaining ultrahigh-T_c ferroelastic phase transitions. They consist of same host inorganic chains but subtly different guest organic cations featuring a rigid carbonyl and a flexible hydroxyl group in 1 and 2, respectively. With stronger hydrogen bonds involving the carbonyl but a relatively lower decomposition temperature (T_d, 480 K), 1 does not exhibit a crystalline phase transition before its decomposition. The hydroxyl group subtly changes the balance of intermolecular interactions in 2via reducing the attractive hydrogen bonds but increasing the repulsive interactions between adjacent organic cations, which finally endows 2 with an enhanced thermal stability (T_d = 570 K) and three structural phase transitions, including two ferroelastic phase transitions at ultrahigh T_c values of 463 K and 495 K, respectively. This finding provides important clues to judiciously tuning the intermolecular interactions in hybrid crystals for developing high-T_c ferroic materials.

Two new hexagonal molecular perovskites with the same inorganic chain but subtly different organic cations exhibit distinct phase-transition behaviours owing to the different intermolecular interactions.