The entropic role of vacancy defects in governing glass formation within zeolitic imidazolate frameworks

Abstract

Vacancy defects between metal nodes and organic ligands significantly influence glassy formation of zeolitic imidazolate frameworks (ZIF), but their atomic-scale governing mechanisms remain unclear. In this work, we establish that defect-driven entropy gain underpins ZIF vitrification using metadynamics simulations and a probabilistic cellular automaton model. Thermally activated ligand detachment generates transient undercoordinated Zn nodes. Topology-dependent coordination loss shows ZIF-62 decreasing from 4.00 to 2.62 versus ZIF-8 dropping from 3.74 to 2.47 between 300 and 1200 K, explaining their divergent melt stability. In glassy ZIF-62, defect concentrations exceeding 1.8% destabilize imidazole rings and yield molten-state free energy landscapes. Crucially, we classify defects as transient (reversible bond-switching) or persistent (irreversible decomposition), with the latter inducing structural heterogeneity. Our model quantifies how defect evolution during cooling maximizes configurational entropy at critical temperature T_me, where persistent defects freeze-in to define the glassy state. This work establishes vacancy defects as fundamental directors of ZIF glass dynamics and provides a predictive framework for engineering melt-processable metal–organic glass.