Phonon-Related Properties in Metal-Organic Frameworks

Metal-organic frameworks (MOFs) have been extensively studied during the last years due to their numerous possible applications exploiting the large internal surface area (e.g. catalysis, storage, capture and separation of gases). Due to the relatively new trend to employ MOFs in functional devices1-3, researchers have been gradually becoming more interested in their properties, many of which are typically dominated by contributions of phonons. However, vibrational properties in MOFs, despite their importance for describing practically relevant quantities like thermal conductivity4, mechanical behaviour5, or thermal expansion6, are still largely unexplored. Here, the phonon picture provides a convenient framework to associate various materials properties with individual vibrational modes and helps to understand why certain properties can be observed. By exploiting knowledge about the phonons, specific building blocks can be combined to engineer phonon band structures and the resulting properties. Therefore, we studied the influences of different constituents on the (an)harmonic vibrational properties of a variety of MOFs by means of atomistic simulations. We systematically varied the constituents in isoreticular MOFs (IRMOFs) to separately explore their influence on the phonon dispersion. The goal of our study is to deduce structure-to-property relationships for phonon-related properties (thermodynamic quantities, elastic constants, etc.) in MOFs. Our simulations have been performed in the framework of density functional theory using the PBE functional7 and self-consistent charge density functional tight binding8.

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