Phase Transitions in Zeolitic Imidazolate Framework 7: The Importance of Framework Flexibility and Guest-Induced Instability

Phase Transitions in Zeolitic Imidazolate Framework 7: The Importance of Framework Flexibility and Guest-Induced Instabilit

Phase Transitions in Zeolitic Imidazolate Framework 7: The Importance of Framework Flexibility and Guest-Induced Instability

Phase Transitions in Zeolitic Imidazolate Framework 7: The Importance of Framework Flexibility and Guest-Induced Instabilit

Crystal Structures of CaB₃N₃ at High Pressures

Using global structure searches, we have explored the structural stability of CaB₃N₃, a compound analogous to CaC₆, under pressure. There are two high-pressure phases with space groups <i>R</i>3<i>c</i> and <i>Amm</i>2 that were found to be stable between 29 and 42 GPa, and above 42 GPa, respectively. The two phases show different structural frameworks, analogous to graphitic CaC₆. Phonon calculations confirm that both structures are also dynamically stable at high pressures. The electronic structure calculations show that the <i>R</i>3<i>c</i> phase is a semiconductor with a band gap of 2.21 eV and that the <i>Amm</i>2 phase is a semimetal. These findings help advance our understanding of the Ca–B–N ternary system

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