Tailoring the Melting and Glass Transition Behavior of Zeolitic Imidazolate Frameworks via Ammonium Halide Salts.

Metal-organic framework (MOF) glasses represent a promising class of organic-inorganic hybrid materials, but most crystalline MOFs undergo thermal decomposition before melting and can thus not be quenched into a glassy state. To this end, recent work has introduced the concept of MOF modifiers, i.e., the mixing of a modifier agent with a MOF crystal to lower the MOF's melting temperature. While previous work has focused on hybrid modifiers, we here investigate the use of inorganic ammonium halide salts as modifiers for zeolitic imidazolate frameworks (ZIFs). We show that co-melting various ZIF crystals with the halide salt significantly lowers both melting and glass transition temperatures, enabling fabrication of centimeter-sized hybrid glasses and melt-processing of ZIF-8, which otherwise decomposes before melting. In situ X‑ray imaging is used to visualize that the molten halide salts infiltrate the ZIF crystals and ultimately promote the collapse of the crystalline domains into a homogeneous melt. X-ray pair distribution function analyses confirm the disruption of the Zn─N coordination network and formation of Zn─X (X = Cl, Br, and I) bonding, thus lowering the framework connectivity. Our findings broaden the scope of modified MOF glasses and offer a versatile route for tailoring their thermal processing and network structure.