Nanoporous metal-organic frameworks (MOFs) have emerged as a class of materials with unique mechanical responses, particularly exhibiting negative linear compressibility (NLC), where certain dimensions expand under isotropic compression. This study investigates zinc squarate tetrahydrate (ZnC₄O₄·4H₂O, ZnSqQh) and titanium oxalate trioxide dihydrate (Ti₂(C₂O₄)O₃·2H₂O, TiOxDh), two MOFs characterized by empty structural channels within their crystal lattices. Using first-principles solid-state calculations based on Density Functional Theory (DFT), the crystal structures and mechanical properties of these materials were analyzed comprehensively. The results confirm that both compounds display significant NLC effects over broad pressure ranges, accompanied by negative Poisson’s ratios (NPR), indicating auxetic behavior.
The structural analysis reveals that under isotropic compression, the width of the empty channels expands along the direction of minimum compressibility, leading to elongation in that axis—manifesting as negative linear compressibility. For ZnSqQh, this effect is observed along the [010] direction, while for TiOxDh, it occurs along the [100] direction. These directions correspond to one of the semiaxes in the perpendicular cross-section of the channels, which are nearly rhombic in ZnSqQh and elliptical in TiOxDh. The deformation mechanism is consistent with the “empty channel structural mechanism,” where the reduction in lateral constraints allows for channel widening under compression, resulting in counterintuitive expansion in specific directions.
Further investigation under uniaxial compression along the direction of minimum compressibility confirms the presence of anisotropic negative volumetric compressibility (ANVC).Collagen IV Antibody Data Sheet In both materials, the unit-cell volume increases with applied pressure in this direction, leading to negative volumetric compressibilities reaching up to −1022 TPa⁻¹ in ZnSqQh and −15.7 TPa⁻¹ in TiOxDh. This phenomenon arises from the simultaneous increase in channel height and length, driven by structural reorganization rather than phase transitions, as confirmed by bond-length and hydrogen-bond network analyses.
The influence of hydration on the NLC effect was also examined through the parent compound titanium oxalate trioxide trihydrate (Ti₂(C₂O₄)O₃·3H₂O, TiOxTh). Unlike the dihydrate, the additional water molecules are not located inside the channels but are integrated into the channel walls, effectively reinforcing them. As a result, the NLC effect is significantly reduced in TiOxTh, with much smaller negative compressibilities and only weak ANVC observed. This demonstrates that hydration can suppress the flexibility required for the empty channel mechanism to operate efficiently.Fibrinogen γ Antibody supplier
Mechanical stability assessments confirm all three materials are mechanically stable, as their elasticity tensors are positive definite.PMID:35200615 The universal anisotropy indices (AU) are high for ZnSqQh and TiOxDh, indicating strong anisotropic behavior correlated with the presence of NLC and NPR. In contrast, TiOxTh exhibits lower anisotropy, consistent with its diminished NLC response.
In conclusion, this work provides a clear mechanistic explanation for negative linear compressibility in nanoporous MOFs: the empty channel structural mechanism. The ability of such frameworks to expand in specific directions under compression opens new avenues for designing smart materials with applications in ultra-sensitive pressure sensors, actuators, sound dampers, and tunable filters. Furthermore, the findings suggest that screening known MOF structures with empty channels could yield numerous new NLC materials, and that artificial design of such architectures offers promising routes toward advanced metamaterials with tailored mechanical functionalities.MedChemExpress (MCE) offers a wide range of high-quality research chemicals and biochemicals (novel life-science reagents, reference compounds and natural compounds) for scientific use. We have professionally experienced and friendly staff to meet your needs. We are a competent and trustworthy partner for your research and scientific projects.Related websites: https://www.medchemexpress.com