Uniaxial Negative Thermal Expansion and Mechanical Properties of a Zinc-Formate Framework

High pressure and elastic properties of a guanidinium-formate hybrid perovskite

The high pressure and elastic properties of a hybrid ABX₃-type perovskite, [C(NH₂)₃][Cd(HCOO)₃] (CdGF), based on the A-site guest molecules are revealed via combining the high-pressure synchrotron X-ray diffraction experiments with density functional theory (DFT) calculations. The experimental results indicate that the anisotropic axial compressibilities are K_a = 2.8(4) and K_c = 20.9(10) TPa^-1 within the pressure scope of 4.12 GPa, and calculations reveal the microscopic evolution of structure under pressure and demonstrate that this structural anisotropy is a consequence of the directional guest molecule and the cooperative effect of hydrogen bonding and octahedral tilting. In addition, full elastic constants of the perovskite were calculated to describe Young's moduli, shear moduli and Poisson's ratios. Notably, these discrete moduli are governed by the orientational guest molecule suggesting that the perovskite may display positive (negative) linear compression if it is flexible (rigid) along one principal axis.

Exploring Host-Guest Interactions in the α-Zn3(HCOO)6 Metal-Organic Framework

Metal-organic frameworks (MOFs) are promising gas adsorbents. Knowledge of the behavior of gas molecules adsorbed inside MOFs is crucial for advancing MOFs as gas capture materials. However, their behavior is not always well understood. In this work, carbon dioxide (CO₂) adsorption in the microporous α-Zn₃(HCOO)₆ MOF was investigated. The behavior of the CO₂ molecules inside the MOF was comprehensively studied by a combination of single-crystal X-ray diffraction (SCXRD) and multinuclear solid-state magnetic resonance spectroscopy. The locations of CO₂ molecules adsorbed inside the channels of the framework were accurately determined using SCXRD, and the framework hydrogens from the formate linkers were found to act as adsorption sites. ⁶⁷Zn solid-state NMR (SSNMR) results suggest that CO₂ adsorption does not significantly affect the metal center environment. Variable-temperature ¹³C SSNMR experiments were performed to quantitatively examine guest dynamics. The results indicate that CO₂ molecules adsorbed inside the MOF channel undergo two types of anisotropic motions: a localized rotation (or wobbling) upon the adsorption site and a twofold hopping between adjacent sites located along the MOF channel. Interestingly, ¹³C SSNMR spectroscopy targeting adsorbed CO₂ reveals negative thermal expansion (NTE) of the framework as the temperature rose past ca. 293 K. A comparative study shows that carbon monoxide (CO) adsorption does not induce framework shrinkage at high temperatures, suggesting that the NTE effect is guest-specific.