Pressure Promoted Low-Temperature Melting of Metal-Organic Frameworks

Metal-organic frameworks (MOFs) are microporous materials with huge potential as host structures for chemical processes, including retention, catalytic reaction, or separation of guest molecules. Structural collapse at high-pressure, and unusual behaviours at elevated temperatures, such as melting and transitions to liquid states, have recently been observed in the family. Here, we show that the effect of the application of simultaneous high-pressure and -temperature on a MOF can be understood in terms of silicate analogues, with crystalline, amorphous and liquid states occurring across the pressure - temperature phase diagram. The response of ZIF-62, the MOF on which we focus, to simultaneous pressure and temperature reveals a complex behaviour with distinct high- and low- density amorphous phases occurring over different regions of the pressure-temperature space. In-situ powder X-ray diffraction, Raman spectroscopy and optical microscopy reveal that the stability of the liquid MOF-state expands significantly towards lower temperatures at intermediate, industrially achievable pressures. Our results imply a novel route to the synthesis of functional MOF glasses at low temperatures, avoiding decomposition upon heating at ambient pressure.