Quantum mechanical predictions to elucidate the anisotropic elastic properties of zeolitic imidazolate frameworks: ZIF-4 vs. ZIF-zni

Abstract

We use ab initio density functional theory (DFT) to elucidate the mechanical properties of two topologically distinct zeolitic imidazolate framework (ZIF) materials: ZIF-4 and ZIF-zni, both of which have the same chemical composition of Zn(Im)₂ [Im = C₃H₃N₂⁻] and are constructed from an identical Zn—Im—Zn basic building block. The CRYSTAL code was used to compute the single-crystal elastic constants C_ij of the (orthorhombic) ZIF-4 and (tetragonal) ZIF-zni structures at the PBE level of theory. Through tensorial analysis of the C_ij, we reveal the three-dimensional representation surfaces of the Young's modulus, shear modulus, Poisson's ratio and linear compressibility, which enable us to describe the detailed elasticity behaviour and to pinpoint basic crystal structure–property correlations. Notably, we discover that ZIF-4 can potentially exhibit a negative Poisson's ratio, thereby representing the first example of an ‘auxetic-ZIF’ to be identified to date. Furthermore, we show that our DFT predictions are consistent with recently reported experimental measurements of the Young's and bulk moduli of such complex ZIF structures.