Extreme Flexibility in a Zeolitic Imidazolate Framework: Porous to Dense Phase Transition in Desolvated ZIF‐4

Abstract

Desolvated zeolitic imidazolate framework ZIF-4(Zn) undergoes a discontinuous porous to dense phase transition on cooling through 140 K, with a 23 % contraction in unit cell volume. The structure of the non-porous, low temperature phase was determined from synchrotron X-ray powder diffraction data and its density was found to be slightly less than that of the densest ZIF phase, ZIF-zni. The mechanism of the phase transition involves a cooperative rotation of imidazolate linkers resulting in isotropic framework contraction and pore space minimization. DFT calculations established the energy of the new structure relative to those of the room temperature phase and ZIF-zni, while DSC measurements indicate the entropic stabilization of the porous room temperature phase at temperatures above 140 K.

The Zeolitic Imidazolate Framework ZIF‐4 under Low Hydrostatic Pressures

“…The profile fit of ZIF ‐ 4 ( Zn )‐ cp was performed in P 2 1 / c , as suggested in the literature , . The evolution of volume and lattice parameters as obtained from the Pawley profile fit analysis is shown in Figures b and c. In agreement with previous findings,, a volume contraction of approximately 20 % is observed, when considering the volume of the unit cell at the last pressure point at which phase pure ZIF ‐ 4 ( Zn )‐ op is present compared to the volume at the first pressure where only ZIF ‐ 4 ( Zn )‐ cp can be observed. The bulk moduli of ZIF ‐ 4 ( Zn )‐ op and ZIF ‐ 4 ( Zn )‐ cp were obtained by fitting a 2 nd order Birch‐Murnaghan (BM) equation of state to the V ( p ) data between p = ambient – 0.045 GPa for ZIF ‐ 4 ( Zn )‐ op and p = 0.125 GPa – 0.4 GPa for ZIF ‐ 4 ( Zn )‐ cp using EoSFit‐7c (see Supporting Information for fits of the BM equation of state and stress‐strain plots).…”

Significant Enhancement of C2H2/C2H4 Separation by a Photochromic Diarylethene Unit: A Temperature‐ and Light‐Responsive Separation Switch

“…In contrast to the above mentioned strategies that demand precise MOF design, the approach followed in the current work is to utilize external stimuli (for example, heat, light, magnetic) to modulate adsorption selectivity; this is demonstrated to be easy to implement. Such a smart material can be constructed with some specific elements on MOFs that are responsive to external stimuli, and accordingly shows dynamical alteration in their physical properties . In view of this, controlling CO 2 uptake in MOFs has been achieved by heat, or light, or a magnet .…”

An Unusual Phase Transition Driven by Vibrational Entropy Changes in a Hybrid Organic–Inorganic Perovskite

“…[37,38] Taking advantage of the abundant chemical diversity of HOIPs,t he present study reveals an ew aspect of their extensive PT landscape and highlights some fundamentally different features of these transitions at the atomic level, especially highlighting the possibility of achieving new desirable properties by manipulating collective modes.F urthermore,c onsidering the fact that many important properties of perovskites are consequences of PTs,t he cooperative contributions of all A, B, and Xsites for driving PTs in HOIPs can give rise to novel functionalities that conventional perovskites are unable to exhibit. Large entropic effects arising primarily from vibrational entropy across atransition are rare in molecular systems since entropy changes normally originate from configurational alterations related to different molecular ordering states.H owever, such effects have been seen in the open-pore to closed-pore transitions that are found in as mall number of metal-organic frameworks.…”

“…Large entropic effects arising primarily from vibrational entropy across atransition are rare in molecular systems since entropy changes normally originate from configurational alterations related to different molecular ordering states.H owever, such effects have been seen in the open-pore to closed-pore transitions that are found in as mall number of metal-organic frameworks. [37,38] Taking advantage of the abundant chemical diversity of HOIPs,t he present study reveals an ew aspect of their extensive PT landscape and highlights some fundamentally different features of these transitions at the atomic level, especially highlighting the possibility of achieving new desirable properties by manipulating collective modes.F urthermore,c onsidering the fact that many important properties of perovskites are consequences of PTs,t he cooperative contributions of all A, B, and Xsites for driving PTs in HOIPs can give rise to novel functionalities that conventional perovskites are unable to exhibit. Fore xample,f erroelectric phase transitions driven by vibrational entropy changes offer anew avenue for designing ferroelectric materials and related smart devices beyond the traditional routes.Another example is that the hybrid improper mechanism for creating ferroelectricity endowed by the synergistic involvement of all sites in HOIPs offers much more tunability owing to the presence of organic degree of freedoms not available in perovskite oxides.…”