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Understanding Phonon-Related Properties in Metal-Organic Frameworks for Controlling Their Mechanical and Thermal Characteristics
Metal-organic frameworks (MOFs) are an emerging materials class which, due to their exceptionally large (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 – especially those ones which are intimately related to phonons –are either relatively unknown, or at least a fundamental understanding of the role of the different building blocks and their assembly is largely missing. Thus, understanding how the specific building blocks affect the phonons of a MOFs is crucially important to be able to design the related properties. Therefore, we studied the influences of different constituents on the (an)harmonic phonon properties of a variety of MOFs by means of atomistic ab initio simulations. As a starting point, we systematically varied the constituents in isoreticular MOFs (IRMOFs) to separately explore their influence on the phonon dispersion. Here we identified several trends, which can be understood based on classical arguments. Additionally, the acoustic bands (closely connected to the elastic tensor of a crystal) typically show the most notable dependence on the chemical composition of the MOFs. Thus, in a second step, we focused on the elastic properties in MOF-74 as an instructive (non-cubic) representative of this class of materials.  Here, we investigated 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). The derived mechanical properties were comprehensively analyzed to find trends among the systems based on the structural variation connected to deformation mechanisms at a microscopic scale. Building on those results, in a last step, also a typical anharmonic property, the thermal expansion of MOF-74, was investigated.  This was achieved by means of a combined experimental and theoretical approach using powder x-ray diffraction and a theoretical treatment within the Grüneisen theory of thermal expansion. Here, we found significant anharmonicities in the interactions of linkers and nodes, suggesting that these are relevant screws to turn to manipulate anharmonic properties. The presented insights form the basis for a future design of materials with tailor-made vibrational, elastic, and anharmonic properties.