Structural Origin of the Deformation Propensity of Zeolitic Imidazolate Framework Glasses

Zeolitic imidazolate framework (ZIF) glasses featuring nanoscale porosity have attracted significant attention due to their potential applications in catalysis, energy storage, gas sorption, and separation. However, their mechanical properties may limit some of these applications. In this work, we investigate the structural origins of the variation of mechanical properties of zinc-based ZIF-62 (ZnIm(2-x)bIm(x)) glasses with different benzimidazolate (bIm) to imidazolate (Im) ratios. This is achieved using large-scale molecular dynamics simulations with a recent machine learning force field. We find that the simulated ZIF glass structures match those determined using X-ray total scattering. The relatively large bIm group is found to hinder the reconstruction of the coordination network during quenching of the ZIF melts, leading to more disordered Zn tetrahedra. Both Young's modulus and fracture toughness decrease with an increase in bIm content. Upon fracturing, all of the organic linkers remain intact, while Zn-N bond switching dissipates the strain energy. By correlating the atomic dynamics with the static structure, we find that the deformation propensity of ZIF glass is correlated with Zn mobility, which is in turn determined by the initial atomic volume before deformation across a variety of glass compositions and strain values. These findings could be helpful for designing more fracture-resistant metal-organic framework glasses in the future.