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

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.