Pressure-induced phase transition of isoreticular MOFs: Mechanical instability due to ligand buckling

Since MOFs are often upon varied pressures in gas adsorption/desorption process, understanding the mechanical stability of these ultraporous frameworks becomes extremely crucial. In this paper, taking the isoreticular DUT material as an example, the relation between the mechanical stability of isoreticular MOFs and their ligands is investigated by real-time molecular dynamics simulations as well as three state of art computational approaches including Born stability criteria, anisotropy in elastic moduli and pressure-versus-volume equations. It is found that the global instability of DUT materials is driven by the local buckling of their ligand backbones. Inspired by this finding, we develop here a mechanical model based on three parameters including the topology constant, the elastic modulus of ligand and the ligand length. Through comparing to the results extracted from computational methods, it is found that our mechanical model is reliable in describing the critical transition pressure of various isoreticular MOF materials such as DUT and IRMOF materials. The mechanical model developed here not only provides a design criterion for isoreticular MOF crystals to modify their rigid/flexible properties, but also can be utilized to give a quick prediction of the critical transition pressure of isoreticular MOFs.