Exceptionally Low Shear Modulus in a Prototypical Imidazole-Based Metal-Organic Framework

Using Brillouin scattering, we measured the single-crystal elastic constants (Cij’s) of a prototypical metal-organic framework (MOF): zeolitic imidazolate framework (ZIF)-8 [Zn 2-methylimidazolate 2], which adopts a zeolitic sodalite topology and exhibits large porosity. Its Cij’s under ambient conditions are (in GPa) C11 9:522 7 , C12 6:865 14 , and C44 0:967 4 . Tensorial analysis of the Cij’s reveals the complete picture of the anisotropic elasticity in cubic ZIF-8.We show that ZIF-8 has a remarkably low shear modulus Gmin & 1 GPa, which is the lowest yet reported for a single-crystalline extended solid. Using ab initio calculations, we demonstrate that ZIF-8’s Cij’s can be reliably predicted, and its elastic deformation mechanism is linked to the pliant ZnN4 tetrahedra. Our results shed new light on the role of elastic constants in establishing the structural stability of MOF materials and thus their suitability for practical applications