Speaker
Description
Zeolitic imidazolate frameworks (ZIFs) represent kind of porous metal−organic frameworks (MOFs) in which all tetrahedrally coordinated atoms are transition metals, and all bridging ones are imidazolate (Im) units. In Co(mIm)$_{2}$ (ZIF-$67$) and Zn(mIm)$_{2}$ (ZIF-$8$) where HmIm = $2$-methylimidazole (C$_{4}$H$_{6}$N$_2$), the structures are based on nets of linked CoN$_4$ or ZnN$_4$ tetrahedra, where mIm bridges make an M–mIm–M angle, close to $145$°, which is coincident with the Si–O–Si angle in zeolites. It was also found that Co-Zn substitution preserves the ZIF-8 structure up to $100\%$ Co(II) concentration, resulting in the formation of ZIF-$67$. ZIF-$8$/ZIF-$67$ hybrids exhibit improved catalytic and electrochemical energy storage capabilities [1].
In the present work the synthesis of both compounds in nanocrystalline form was performed. The obtained products were characterized through powder X-ray diffraction, infrared and Raman spectroscopy, confirming the identity of both materials which share an identical crystal structure and vibrational spectra, with the only distinction being the substitution of Zn by Co in ZIF-$67$. The heat capacity of powdered samples was experimentally studied within a temperature range of $0.4$ to $300$ K in zero magnetic field. While at higher temperatures the data coincide as a result of the same crystal structure, deviations appear at lower temperatures due to excitation of magnetic subsystem in ZIF-$67$. To isolate the magnetic contribution, lattice subtraction was performed using ZIF-$8$ in the $0.4$–$60$ K range, where the data sets could be distinguished. Additionally, the heat capacity of ZIF-$67$ was measured under various nonzero magnetic fields in the temperature interval from $0.4$ to $10$ K. It was observed that the heat capacity increases with the applied field; however, the field influence remains relatively weak. The calculated magnetic entropy in the temperature region between $0.4$ and $60$ K represents $95$ $\%$ of the theoretical maximum entropy for a spin-$3/2$ system. Moreover, the magnetic susceptibility of the nanocrystalline ZIF-$67$ sample was investigated between $2$ K and $300$ K under applied fields of $10$ mT and $1$ T in both field-cooled (FC) and zero-field-cooled (ZFC) conditions. A divergence between ZFC and FC curves was observed around $15$ K at $10$ mT, shifting to $5$ K at $0.1$ T and vanishing entirely at higher fields. The role of surface spins in magnetic properties is discussed.
Acknowledgements
The work was supported by the project APVV-22-0172, APVV-18-0197, VEGA 1/0132/22 and VVGS-2023-3040.
References
[1] S. Bakhtavar et al., “Three-dimensional porous g-C3N4 nanosheet, CNT and ZIF-8@ZIF-67-derived carbon nanoarchitecture composite as oxygen reduction electrocatalyst,” Materials Chemistry and Physics, vol. 328. Elsevier BV, p. 129993, Dec. 2024. https://doi.org/10.1016/j.matchemphys.2024.129993
