Theoretical Insights into the Mechanical Properties of DUT-8(Ni) MOF in the Open and Closed Pore Phases

Insights into the mechanical properties of DUT-8(Ni) MOF crystals capable of structural phase transitions are presented. The open and closed pore phases of DUT-8(Ni) are drastically different in their mechanical behavior. For the open-pore phase, a huge anisotropy of compressibility and shear modulus is observed, caused by the presence of two orthogonal motives formed by aromatic linkers bound with Ni atoms via Ni–O bonds. The possibility of easy shifting of linkers with breaking and formation of new Ni–O and Ni–N bonds at the application of shear stress, which is at the same time more stable at uniaxial stress, is shown on the basis of the stiffness tensor analysis. This fact assumes the flexibility of the open-pore phase crystals and their ability for structural transformation. The less prominent anisotropy of elastic moduli and the increased values of these moduli for the closed-pore phase crystals indicate the predominantly hardness and absence of flexibility of this phase.