Hydrogen-bonding engineering in a 3D cyano-bridged double-perovskite ferroelastic greatly improves the phase-transition temperature

Three-dimensional (3D) cyano-bridged double perovskites have attracted increasing attention in recent years for their potential applications in optoelectronic materials, such as dielectric and nonlinear optical switches, and ferroelectrics. However, due to the restricted dimensions of inorganic cavities and the weak intramolecular interactions, designing and developing 3D cyano-bridged double perovskites with excellent performances is a long-term challenge. Herein, we present two new 3D cyano-bridged double-perovskite ferroelastics, (HOCH2CH2NH3)2[KFe(CN)6] (1) and (CH3CH2NH3)2[KFe(CN)6] (2), to demonstrate a simple design principle of hydrogen-bonding engineering for obtaining 3D cyano-bridged double perovskites with a high phase-transition temperature (Tc) and high-performance dielectric switching. The introduction of hydroxyl groups causes minimal disturbance to the 3D [KFe(CN)6]2- framework, and thus, both 1 and 2 undergo the same paraelastic-to-ferroelastic phase transition with the Aizu notation of m3mF2/m. The difference is that compared with the parent compound 2, the cations containing hydroxyl groups in 1 not only form hydrogen-bonding interactions with the [KFe(CN)6]2- framework but also form an unprecedented 1D hydrogen-bond chain with surrounding cations along the crystallographic b-axis, which effectively raises the rotational energy barrier of the cations, thereby increasing the Tc from 288 K (2) to 376 K (1). Moreover, the introduction of the hydroxyl group increases the molecular dipole, leading to a large dielectric-switching effect, with a variation in the dielectric constant (Delta epsilon ') of up to 45 at 1 MHz, far exceeding those of reported 3D cyano analogues. We believe that this work will open an avenue for designing high-performance 3D perovskite materials. Introducing a hydroxyl group on the ethylamine cation in three-dimensional cyano-bridged double perovskites effectively enhances cation-cation and cation-framework interactions, thus increasing the phase-transition temperature by 88 K.