Pore closure in zeolitic imidazolate frameworks under mechanical pressure.

Area of Science:

• Materials Science
• Chemistry
• Physics

Background:

• Zeolitic imidazolate frameworks (ZIFs) are porous materials with tunable properties.
• Understanding the mechanical behavior of ZIFs under pressure is crucial for their application.
• Previous studies have explored ZIFs' response to various stimuli, but mechanical pressure-induced transitions are less understood.

Purpose of the Study:

• To investigate the pressure-dependent mechanical behavior of ZIF-4 with different metal cations (Co2+ and Zn2+).
• To characterize the phase transition from an open pore (op) to a closed pore (cp) phase under mechanical pressure.
• To elucidate the role of metal cations in the transition's threshold pressure, reversibility, and repeatability.

Main Methods:

• High-pressure synchrotron powder X-ray diffraction.
• Mercury intrusion porosimetry.
• Mechanical compression experiments.

Main Results:

• A displacive phase transition from a highly compressible open pore (op) to a closed pore (cp) phase was observed in ZIF-4 under mechanical pressure.
• Both ZIF-4(Zn) and ZIF-4(Co) exhibited a significant volume contraction (∼20%) during the op-cp transition.
• ZIF-4(Zn) transitioned at a lower pressure (28 MPa) than ZIF-4(Co) (50 MPa), and ZIF-4(Co) showed full reversibility, unlike ZIF-4(Zn).

Conclusions:

• The observed mechanical pressure-induced op-cp phase transitions (breathing transitions) in ZIFs are metal-cation dependent.
• Differences in transition behavior are attributed to the distinct electron configurations of Zn2+ (3d10) and Co2+ (3d7).
• These findings suggest potential applications for ZIF-4 materials as shock absorbers, nanodampers, or in mechanocalorics.