Research On The Mechanical Properties Of Metal-Formate Frameworks

Metal-Formate Frameworks?MFFs?are constructed by formate ligands,metal ions and templating organic amine cations,and they can exhibit abundant and tunable physical properties.The metal ions with different types and valences,coordination modes of formate ligand,modes and strengths of the hydrogen bonding all have significant influences in the physical properties of MFFs.The properties of MFFs in magnetism,phase transition,fluorescence have been extensively investigated by researchers in the past decade.However,researchers recently have realized that mechanical stresses can cause the variations of MFFs'structures,and therefore dramatically influence in their mechanical properties.In this regard,it is worthy of further in-depth researching.In view of this,here we have selected several typical MFFs and studied their mechanical properties to uncover the effects of various chemical and structural factors on them.The main contents are detailed as the following three sections:?1?Jahn-Teller?JT?effect on the mechanical properties of the two analogous MFFs[C?NH₂?₃][Zn?HCOO?₃]?6?and[C?NH₂?₃][Cu?HCOO?₃]?7?has been studied.Single-crystal nanoindentation measurements show that the elastic moduli?E?and hardnesses?H?of 6 are greater up to⁵2%and?25%than those of 7 with JT effect.Temperature-dependent X-ray diffraction measurements indicate that the thermal expansion along the b-axis is switched from negative to positive by replacing Zn²⁺with Cu²⁺on the M-site,moreover,equivalent isotropic atomic displacement parameters(U_iso)as a function of temperature for copper atoms in framework 7 are greater⁵0%than those for zinc atoms in 6.High pressure powder X-ray diffraction experiments indicate that both of them experience phase transition during the hydrostatic pressure process,nonetheless,the pressure of phase transitions for framework 6?1.82-2.87 GPa?is much greater than that for7?0.66-0.82 GPa?.Furthermore,the bulk modulus?B?prior to phase transition fitted by the second order Birch-Murnaghan equation is 30.3?28?GPa and 25.6?29?GPa for framework6 and 7,respectively.These stark distinctions in mechanical properties of the two frameworks are primarily attributed to the?10%elongation of Cu-O bonds induced by JT effect and alterations in octahedral tilting and hydrogen-bonding.Our results demonstrate the prominent influences of JT effect on the mechanical properties of MFFs and highlight the possibilities of tuning materials'properties using orbital order.?2?Mechanical properties of a family of MFFs?[AmineH_n][M?HCOO?₃]_n,?n=1-4??with similar density in relation to their organic amine cation sizes,framework topologies,hydrogen bonding modes and metal cation radii have been systematically studied via single crystal nanoindentation experiments.Our findings demonstrate that the E and H of these MFFs are primarily dominated by their solvent accessible volumes?SAVs?,the network topology changes from uni-nodal?8,SAV=15.2%?to bi-nodal?10,SAV=30.3%?,?11,SAV=30.8%?,?9,SAV=33%?,and finally to uni-nodal?12,SAV=39.4%?,the E and H are decreasing.Additionally,the SAVs of these formates decrease when their network topologies evolve from the unimodal acs net?4⁹·6⁶?to bimodal?4⁹·6⁶?_n(4¹²·6³)?n=1-3?net,then to unimodal(4¹²·6³)net.The SAVs of 8 and HAZ-Zn with uni-nodal topology(4⁹·6⁶,4¹²·6³)are lower?15.2%,20.7%?,by contrast,frameworks 12(4¹²·6³)and 13(4¹²·6³)show considerably higher SAVs?39.4%and 32.3%?,hence implying the intimate correlations between their topologies and mechanical properties.In addition,we disclose that the framework stiffness of these formates can also be influenced by hydrogen-bonding and metal cation radii.Our study demonstrates the possibility that mechanical robustness of coordination polymers and metal-organic frameworks could be tuned via facile alteration of organic guests,hydrogen bonding fashions and metal ions.?3?By using density functional theory?DFT?calculation and high pressure powder X-ray diffraction,we have investigated the fundamental relationships between mechanical properties and chemical bonding in a lanthanide formate La?C₂O₄??HCO₂?.DFT calculations reveal that rigid formate ligands align exhibits the highest stiffness(E_max=85.1GPa)along c-axis.In contrast,the<310>direction was found to be the most compliant(E_min=55.7 GPa)along a-axis,this can be attributed to the La-O-La chains that zigzag down the a-axis and the fact that the 9-coordinated LaO₉ polyhedra are expected to be geometrically more compliant.By contrast,the E_maxax and E_min for framework La?C₂O₄??HCO₂?is slightly greater than that for another analogue Ce?C₂O₄??HCO₂?with E_max?78.2 GPa?and E_min?43 GPa?,respectively.Furthermore,the E and shear modulus?G?for framework La?C₂O₄??HCO₂?are greater than those for frameworks MOF-5 and CH₃NH₃PbI₃,and even an order of magnitude greater than that for framework ZIF-8.The Poisson's ratio?v?ranged from 0.17 to 0.45 for framework La?C₂O₄??HCO₂?is narrower than that for MOF-5?0.09-0.8?,which demonstrates that framework La?C₂O₄??HCO₂?can exhibit stronger resistance to stress when stretched or compressed along the axial direction.The significant differences in these mechanical properties for framework La?C₂O₄??HCO₂?compared with other compounds can be attributed to the organic linking chains of formate and oxalate which is perpendicular to each other and the inorganic zigzag chains of La-O-La.Moreover,the bulk modulus?B,54.9 GPa?for framework La?C₂O₄??HCO₂?obtained by DFT calculations,which is consistent with the experimental value?58.4?1?GPa?by using HP-PXRD.Our results present the evidence that the crystal orientation dominated by inorganic chains is not necessarily more rigid from the standpoint of mechanical properties.Rigid organic bridging ligands,on the other hand,can be used to produce higher stiffness and hardness properties in chosen crystallographic orientations.