Porous hybrid inorganic-organic framework materials have been studied intensively in chemistry, physics, and material science due to their potential applications in carbon capture, gas separation, catalysis, drug delivery and sensoring. Recently, considerable efforts have been dedicated to investigating dense inorganic-organic frameworks since they show fascinating physical properties, such as semiconductivity, ferromagnetism, ferroelectricity and multiferroicity, which are dominated by traditional inorganic materials. For example, a family of dense hybrid perovskites, [AmineH⁺][MX₃] (Amine = methylammonium or formadinium, M = Sn or Pb, X = Cl, Br or I), have been reported to function as light harvesters in solid state sensitised solar cell devices with impressive efficiency values about 20%. Another dense framework, [NH₄][Zn(HCOO)₃] has been shown to be a striking multifunctional material with combined ferroelectricity, negative thermal expansion and negative linear compressibility. However, there is a major lack of knowledge about their fundamental mechanical properties, which apparently could delay the development of driving these promising materials towards practical applications. In contrast, the mechanical properties of many porous frameworks (i.e. MOF-5, ZIF-8) have been extensively studied, and significant information about their elasticity, plasticity, and structural stability upon stress have been reported. To attract more attention on dense hybrid frameworks and motivate more efforts dedicated to this important area, herein we focus our interest in the mechanical properties of a dense framework, [DABCOH₂²⁺][K(ClO₄)₃] .

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 high pressure 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(ClO₄)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. 

This study has given one more example about the better understanding of mechanical properties of dense hybrid framework materials. And the high mechanical strength of these frameworks indicates that their more robust nature against stress could be an advantage in the future industrial manufacturing and processing.

Figure. 3D and 2D representations of Young’s modulus E (a) (b), Poisson‘s ratio ν (c) (d) and shear modulus G (e) (f) for the framework [DABCOH₂²⁺][K(ClO₄)₃]. Color scheme: maximum (blue), minimum positive (green), average positive (purple red).

Article: Guoqiang Feng, Xingxing Jiang, Wenjuan Wei, Pifu Gong, Lei Kang, Zhihua Li, Yanchun Li, Xiaodong Li, Xiang Wu, Zheshuai Lin,* Wei Li,* Peixiang Lu. High pressure behaviour and elastic properties of a dense inorganic–organic framework. Dalton Trans., 2016, 45, 4303-4308.