Mechanical Properties Of Hybrid ABX₃ Type Inorganic-Organic Frameworks

There are two broad categories of hybrid inorganic-organic frameworks materials: porous and dense frameworks. Because of high surface area and porosity in combination with the tuneable architecture and functionality,porous framework materials have been widely applied to gas sorption and storage,molecular separation, drug delivery and catalysis. As close packing structures, dense frameworks have exhibited attractive dielectric, ferroelectric, and multiferroic properties traditionally associated only with purely inorganic materials. Now. although a lot of researches deal with the synthesis of framework structures and characterization of their functional properties, studies devoted to understanding the mechanical properties, as a key element of industrial process, are comparatively few. There have been serveral excellent reports on the mechanical properties of porous frameworks such as (Zn4O(1,4-benzenedicarboxylate)3. MOF-5; Zn(2-methylimidazolate)2. ZIF-8), but the fundamental mechanical properties of dense frameworks have not been addressed and understood.In this dissertation, systematical investigations on the mechanical properties of several typical ABX3-type dense frameworks are presented via a variety of characterization methods.(1) The high presure synchrotron X-ray powder diffraction experiment is conducted to explore the pressure-induced phase transition of hybrid perovskite CH3NH3PbI3. A phase transion has been observed by analyzing the cell pararmeters and volume as a function of pressure between 0-2 GPa. Furthermore, density functional theory (DFT) calculation shows a significant change of the variation with mechanical properties of two phases, such as Young's modulus, shear modulus and Poisson's ratios, and its impact on the application of the flexible solar cells, have been systematically studied via contrasting analysis.(2) Three dense frameworks [C(NH2)3][Mn(HCOO)3] (1), [(CH2)3NH2] [Mn(HCOO)3] (2) and [NH2CHNH2][Er(HCOO)4] (3) have been successfully synthesized. The evolution of crystal structure with rising temperature and lattice parameter as a function of temperature are investigated via variable-temperature single crystal and powder X-ray diffraction experiments. The significant negative thermal expansion phenomena are revealed. The average coefficients of thermal expansion of the frameworks 1,2 and 3 are ?c=-8.2 MK-1, ?c =-27.5 MK-1,?b=-7.1 MK-1. respectively. A hinge-strut like structure model is used to explain the negative thermal expansion mechanism of these three frameworks. This study suggests that the number and orientation of hydrogen bonds have a marked impact on the thermal expansion properties of the frameworks.(3) Two ABX3-type perovskite frameworks [CH3NH3][Mn(HCOO)3] and [TPrA][Ni(dca)3] (TPrA= (CH3CH2CH2)4N+), dca=N(CN)2-) have also been synthesized. Nano-indentation experiments disclose the mechanical properties of the frameworks. The Young's modulus of frameworks [CH3NH3][Mn(HCOO)3] and [TPrA][Ni(dca)3] is 13.3(2) and 9.7 (1) GPa. respectively, and the hardness is 0.91(4) and 0.39(1) GPa. respectively. After compared with the mechanical properties of different frameworks, such as hybrid perovskite materials and perovskite oxides, the great compliance of these two ABX3 perovskite frameworks is understandable in terms of the enhanced flexibility of the much larger and longer [HCOO-] and [dca-] in comparison to the single O2 anion and halogen anion.(4) An ABX3-type cubic dense framework [DABCOH22+][K(ClO4)3] (DABCOH22+= diazabicyclo[2.2.2]octane-1,4-diium) has been prepared. The high pressure elastic property has been systematically studied by powder and synchrotron X-ray powder diffraction and DFT calculation. The obtained bulk modulus is 30(1) GPa, and the corresponding axial compressibility is 7.6(4)×10-3 GPa-1. Moreover, the bulk modulus 30(1) GPa is in reasonable agreement with the result derived from the DFT calculation. These results indicate that the framework is less compressible than a great deal of porous materials (-5-30 GPa). Further extensive DFT calculations of elastic tensors give a full mapping of the Young's modulus, shear modulus and Poisson's ratios of [DABCOH22+][K(C1O4)3], which are 31.6-36.6 GPa,12.3-14.6 GPa and 0.2-0.32, respectively. The Young's and shear modulus of the framework are larger than those from MOF-5 and CH3NH3PbI3, and are an order of magnitude higher than those from ZIF-8. In addition, the range of Poisson's ratios is significantly narrower than those from MOF-5 (0.09-0.8) and ZIF-8 (0.33-0.57), implying its more isotropic feature in response to biaxial stretch and compression.