Synthesis, phase transitions and vitrification of the zeolitic imidazolate framework: ZIF-4

To produce bulk MOF glasses with high quality and yield, it is important to synthesize their parent crystals with less defects, high homogeneity and low solvent inclusions. To understand the phase transition and polyamorphic transition in ZIFs, it is crucial to know the nature of the parent crystals. For these two purposes, we perform a systematic study by varying the solvothermal synthesis condition of ZIF-4 (Zn[C₃N₂H₃]₂) crystals. We found that pure ZIF-4 crystals could be obtained at synthetic temperatures (T_s) below 383 K for 72 h, or for durations (t_s) shorter than 96 h at 393 K. An increase in T_s shifts T_g to higher values. Only under a specific synthetic condition (e.g., at 393 K for 72 h), where pure ZIF-4 crystals form (i.e., without ZIF-zni formation), can the low-density amorphous phase occur. The origins of the above-mentioned phenomena are discussed.