FIRST-PRINCIPLE STUDY OF THE METAL-ORGANIC FRAMEWORK MIL-69 COMPRESSIBILITY

10.25712/ASTU.1811-1416.2023.04.003

Abstract

At present, metal-organic frameworks are being actively studied both experimentally and theoretically. This is due to the unique properties of these compounds, which are of interest for practical applications in various fields. The compressibility of Al(OH)(O₂C–C₁₀H₆–CO₂)·H₂O (MIL-69) metal-organic crystalline hydrate was studied on the basis of first-principle calculations. The calculation was performed within the framework of the density functional theory using the pseudopotential method and the plane wave basis. To account for van der Waals forces in the MIL-69 crystal, the DFT-D3(BJ) computational scheme was used. The calculated values of the lattice parameters of this crystal demonstrate good agreement with the available experimental data. For this framework crystal, the pressure dependences of the structural parameters, the full set of elastic constants and the linear compressibility dependences on the direction were calculated. It was found that under pressure, the behavior of parameters a and c of the MIL-69 crystal is similar. The lattice parameter b under pressure decreases much more strongly. The resulting complete sets of elastic constants made it possible to conclude that the MIL-69 crystal is mechanically stable. The negative linear compressibility of MIL-69 was found and its relationship with the features of the atomic structure of MIL-69 and their baric changes was shown. Based on the values of the elastic constants, the basic mechanical characteristics of this crystal were calculated. Young's modulus, shear modulus and Poisson's ratio in the Voigt-Reuss-Hill approximation are 40.99 GPa, 15.88 GPa and 0.29, respectively. It has been established that the bulk moduli of a single crystal and polycrystalline MIL-69 are significantly different. The shear modulus is almost two times less than the bulk modulus.