Accessing a Forbidden Disordered State of a Zeolitic Imidazolate Framework with Higher Stiffness and Toughness through Irradiation

Metal-organic frameworks (MOFs) with nanoscale porosity have numerous potential applications, for example, within catalysis, shock absorption, ion transportation, gas sorption, and separation. Recently, the disordered state of MOFs has also attracted significant attention, since some MOF crystals can melt and be quenched into bulk glasses, as first discovered for zeolitic imidazolate frameworks (ZIFs, a subset of MOFs). Another potential route to disorder in MOFs that so far remains unexplored is irradiation, e.g., silicate minerals are well-known to rearrange into a disordered state upon irradiation. Here, we investigate the structural disordering of both ZIF-4 crystals and glasses under irradiation using reactive molecular dynamics simulations. Our results show that the two phases converge toward a new "forbidden"disordered state that is not accessible through any thermal path. The evolution of the structure upon irradiation allows us to establish a relationship between structural disorder and mechanical properties to explain why the irradiated ZIF-4 state is both stiffer and tougher than that of both the parent ZIF-4 crystal and glass.