EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Metal-organic frameworks (MOFs) are an emerging materials class which, due to their high (nano-)porosity, can be used for numerous applications such as gas separation/capture or encapsulation and release of drugs. Functional devices based on these hybrid materials are becoming increasingly important, calling for a precise knowledge of the materials’ mechanical, thermal, electronic, optical, etc. properties. Despite their current popularity and number of possible applications, the physical properties of MOFs are either relatively unknown or at least a fundamental understanding of the role of the different building blocks and their assembly is largely missing. Without understanding their mechanical properties, for example, one cannot assess the elastic resistance of a MOF, or estimate the potential success of heteroepitaxial growth processes on substrates. Thus, we are investigating the influence of (i) the metal ions, (ii) the organic inkers, and (iii) water as an adsorbate on the anisotropic mechanical properties (Young’s modulus, linear compressibility, Poisson’s ratio) in MOF-74 as an instructive (non-cubic) representative of this class of materials . The elasticity tensors of several related systems with varied structural constituents were calculated using density functional theory (DFT). The derived mechanical properties were comprehensively analyzed to find trends among the systems based on the structural variation. Moreover, external stresses and strains were applied to the systems to understand the atomistic relaxation mechanisms connected with the macroscopic deformation. Based on these, we are able to rationalize the observations that the mechanical stiffness (i) sensitively depends on the metal-oxygen bonding interactions, (ii) that it decreases for longer linkers and (iii) that the stiffness increases upon water adsorption. The presented insights regarding the fundamental mechanisms involved in the mechanical response of MOF-74-type systems form the basis for a future design of materials with tailor-made elastic properties.