Using Brillouin scattering, we measured the single-crystal elastic constants (C ij 's) of a prototypical metal-organic framework (MOF): zeolitic imidazolate framework (ZIF)-8 [Znð2-methylimidazolateÞ 2 ], which adopts a zeolitic sodalite topology and exhibits large porosity. Its C ij 's under ambient conditions are (in GPa) C 11 ¼ 9:522ð7Þ, C 12 ¼ 6:865ð14Þ, and C 44 ¼ 0:967ð4Þ. Tensorial analysis of the C ij 's reveals the complete picture of the anisotropic elasticity in cubic ZIF-8. We show that ZIF-8 has a remarkably low shear modulus G min & 1 GPa, which is the lowest yet reported for a single-crystalline extended solid. Using ab initio calculations, we demonstrate that ZIF-8's C ij 's can be reliably predicted, and its elastic deformation mechanism is linked to the pliant ZnN 4 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.
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