Understanding Phonon Properties in Metal-Organic Frameworks From First Principles

Metal-organic frameworks (MOFs) have been extensively studied during the last years due to their numerous possible applications exploiting the large amount of internal surface area (e.g. catalysis, storage, capture and separation of gases). Due to the relatively new trend to employ MOFs in functional devices [1-3], researchers have been gradually becoming more interested in their functional properties, many of which are typically dominated by contributions of phonons – i.e. quasi-particles of lattice vibration with energy and momentum. However, vibrational properties in MOFs, despite their importance for describing practically relevant quantities like thermal conductivity [4], mechanical behaviour [5], or thermal expansion [6], 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 metallic nodes and organic linkers in isoreticular MOFs (IRMOFs) to separately explore their influence on the phonon dispersion and the resulting properties. The goal of our study is to deduce structure-to-property relationships for phonon-related properties in MOFs: differences in physical observables (thermodynamic quantities, elastic constants, etc.) are explained by comparing vibrational modes amongst the studied systems and rationalising frequency shifts by structural arguments. Clear trends in the changes of phonon band dispersion and spatial localisation of modes can be observed. Our simulations have been performed in the framework of density functional theory using the PBE functional [7] and self-consistent charge density functional tight binding [8].

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