Quantum chemical calculations to elucidate the electronic and elastic properties of topologically equivalent metal organic frameworks

We have theoretically investigated the elastic properties of three topologically identical zeolitic imidazolate frameworks: ZIF-4, ZIF-62 and TIF-4, by means of ab initio calculations. The ZIFs are a subset of metal organic frameworks (MOFs), whose versatile functionality is providing exciting opportunities in solid state science. Understanding the relationship between structure and elastic properties of hybrid materials is making it possible to develop a new generation of functional materials. In this work we have determined the bulk ($K$), shear ($G$) and Young's moduli ($E$), together with Poisson's ratio($\upsilon$), and the universal elastic anisotropy index ($A$$^{\textbf{U}}$) from the computed elastic coefficients. Tensorial analysis of the elastic constants reveals unusual and highly anisotropic elastic behaviour. All three materials exhibit low Young's and shear moduli. In addition, their flexibility incorporates regions of negative Poisson's ratio (NPR) and negative linear compressibility (NLC). The elastic properties of these ZIF crystals are are affected by the substituted organic linkages.