Impact of structure on terahertz vibrational modes (phonons) in metal-organic frameworks

• Applicant: Dr. Irena Senkovska
• Subject Area: Physical Chemistry of Solids and Surfaces, Material Characterisation
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 448809307

Metal-Organic Frameworks (MOFs) stand out among other porous materials due to their extremely high porosity and modular tunability. Compliance is often neglected for the majority of porous solids, however, MOFs are crystalline elastic ("soft") materials dominated by easily deformable coordinative bonds and organic linkers thus resulting in relatively low bulk and shear moduli.1 This softness of the crystalline solids can result in exotic properties, such as negative thermal expansion, phonon assisted phase transitions and even glass formation. The understanding of the underlying mechanisms is a crucial necessity for the further technological development of soft porous solids in optical and thin film applications. A unique feature of soft porous materials is that vibrations (phonons) of the ordered solid are strongly coupled to the liquid-like adsorbed fluid. The latter may vary in a wide range in terms of species (i.e. guest molecules) and pore filling degree and thus provide a wide range of partially ordered adsorbed states and possibly quasi-liquid like behavior. In this sense, one may expect a widely tunable elastic behavior and complex coupling behavior of phonon driven vs. adsorption assisted phase transitions. Today an understanding and analysis of cooperative lattice vibrations in MOFs is in its infancy. Their role in governing mechanical properties (compliance, elasticity) and phonon assisted (displacive) phase transitions is not well understood. The proposed project primarily addresses the investigation of low-frequency lattice vibrational modes, in representative MOF model systems, and their impact on exotic phenomena like negative thermal expansion and associated phonon driven phase transitions in MOFs. The model materials will be selected to understand a) the role of network constituents and b) topology (symmetry) on lattice vibrations. We select a complementary set of physical characterization tools: a) Brillouin spectroscopy, temperature and pressure variable in situ-Raman spectroscopy, b)X-ray thermodiffraction, and c) inelastic neutron scattering (INS) in order to analyze the lattice dynamics upon heating and adsorption/desorption. The spectroscopic analysis of idealized model materials (powders and single crystals) in combination with theoretical simulations of vibrational modes will provide the basis for a detailed understanding of the role of lattice vibrations for the thermomechanical properties of MOFs.

• DFG Programme: Research Grants
• International Connection: Russia
• Partner Organisation: Russian Foundation for Basic Research, until 3/2022
• Cooperation Partner: Dr. Alexander Krylov, until 3/2022