Experimental Evidence for Vibrational Entropy as Driving Parameter of Flexibility in the Metal-Organic Framework ZIF-4(Zn)

Large structural flexibility in some metal-organic frameworks (MOFs) is not only fascinating from a fundamental point of view but offers exciting opportunities in the areas of gas sensing, gas separation, and mechanocalorics. After intense interest of the MOF and materials community in these materials, scientists are starting to understand the details and principles that govern the thermodynamics of them from theoretical models, however experimental confirmation of possible models is still outstanding. We here use temperature-dependent inelastic neutron scattering to track the changes of lattice dynamics across the closed-pore to open-pore phase transition in ZIF-4(Zn). Our experiments are complemented by lattice dynamics calculations, allowing us to identify the important structural features that govern the free energy landscape in ZIF-4(Zn). We find that rigid unit modes dominate the low-energy lattice dynamics, suggesting the implementation of rigid unit modes in the search for guiding principles for the synthesis of flexible MOFs.