Topology-Controlled Phonon Dielectric Response Beyond Density Scaling in Metal-Organic Frameworks

Abstract

Effective-medium theory treats material porosity as passive dilution. Using ab initio density functional theory and high-resolution synchrotron terahertz (THz) spectroscopy on isochemical zeolitic-imidazolate frameworks, we show that while the electronic permittivity obeys Clausius-Mossotti density scaling, the phonon contribution violates the conventional density scaling rules. Identical Born charges rule out the role of local chemistry. Instead, the framework connectivity localizes the THz response, where the coherency of phonon eigenvectors determines the mode-effective charges. Long-range architecture of framework topology ubiquitous in metal-organic frameworks is therefore a dielectric degree of freedom beyond the density scaling of conventional solids.