Giant mechanical tunability by a coordination bond strategy in a 3D hybrid cyanide double perovskite ferroelastic with reconstructive phase transition.

Three-dimensional (3D) cyanide hybrid organic-inorganic double perovskites (CHOIPs) have abundant electrical, optical, thermal, and magnetic properties due to their diverse chemical variability and structural flexibility, making them promising for applications in transducers, memories, and switch materials. However, the mechanical properties, crucial for practical applications, have long been overlooked. Here, we present a substantial improvement in the mechanical strength of a 3D CHOIP using the coordination bond strategy. Through introducing the hydroxyl group into the parent compound (CHCHNH)[KFe(CN)] (EA), two isomeric (HOCHCHNH)[KFe(CN)] (EAOH-1 and EAOH-2) materials that both crystallized in the 2/ space group at room temperature can be obtained. Notably, EAOH-1, featuring C-O-K coordination bonds between organic cations and the [KFe(CN)] framework, exhibits a nearly 300% increase in the elastic modulus () and hardness () compared to EA. In contrast, EAOH-2, which relies on weak 1D hydrogen bond interactions, shows approximate enhancements of 140% in and 50% in over EA. More interestingly, the significant decreases in both and induced by the thermally induced reconstructive phase transition from EAOH-1 to EAOH-2 further demonstrate the significant effect of the coordination bond strategy on the mechanical properties. This study highlights the potential of the coordination bond strategy to enhance the mechanical properties of CHOIPs, paving the way for the design of advanced materials with tailored mechanical performance.