Hybrid glasses connect emerging fields of metal-organic frameworks (MOFs) with the glass-formation, amorphization, and melting processes of these structurally diverse and chemically versatile systems. Most zeolites, including MOFs, amorphize around the glass transition, devitrifying and then melting at much higher temperatures. The relationship between the two processes has so far not been investigated. Herein we show how heating first results in a low density perfect glass, following an order-order transition, leading to a super-strong liquid of low fragility that dynamically controls MOF collapse. A subsequent order-disorder transition creates a high density liquid of greater fragility. After crystallization and melting, subsequent cooling results in a stable glass virtually identical to the high density phase. Furthermore, the wide-ranging melting temperatures of different MOFs suggest these can be differentiated by topology. Our research provides new insight into the stability and functionality of these novel ductile crystalline materials, including the possibility of melt-casting MOFs.
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