DFT STUDY OF ELECTRONIC STRUCTURE AND MECHANICAL PROPERTIES OF CLAY MINERALS, AND USING LARGE-SCALE SUPERCELL MODELING FOR SOLVATED MONTMORILLONITE

Abstract

Clay mineral materials have attracted attention due to their many proper- ties and applications. The applications of clay minerals are closely linked to their structure and composition. Here, we studied the electronic structure properties of kaolinite, muscovite, and montmorillonite crystals, which are classified as clay minerals, by using DFT-based ab initio packages VASP and the OLCAO. This work aims to have a deep understanding of clay mineral materials, including electronic structure, bond strength, mechanical proper- ties, and optical properties. It is worth mentioning that understanding these properties may help continually result in new and innovative clay products in several applications, such as in pharmaceutical applications using kaolinite for their potential in cancer treatment, muscovite used as insulators in elec- trical appliances, and engineering applications that use montmorillonite as a sealant. In addition, our results show that the role played by hydrogen bonds in O-H bonds has an impact on the hydration in these crystals. Based on calculated total bond order density, it is concluded that kaolinite is slightly more cohesive than montmorillonite, which is consistent with the calculated mechanical properties. Montmorillonite clay (MMT) has been widely used in engineering and environmental applications as a landfill barrier and toxic waste repository due to its unique property as an expandable clay mineral that can absorb water easily. This absorption process rendered MMT to be highly exother- mic due to electrostatic interactions among molecules and hydrogen bonds between surface atoms. A detailed study of a large supercell model of struc- tural clay enables us to predict long-term nuclear waste storage. Herein, a large solvent MMT model with 4071 atoms is studied using ab initio den- sity functional theory. The DFT calculation and analysis clarify important issues, such as bond strength, solvation effect, elasticity, and seismic wave velocities. These results are compared to our previous study on crystalline MMT (dry). The solvated MMT has reduced shear modulus (G), bulk mod- ulus (K), and Young’s modulus (E). We observe that the conduction band (CB) in the density of states (DOS) of solvated MMT model has a single, conspicuous peak at -8.5 eV. Moreover, the atom-resolved partial density of states (PDOS) summarizes the roles played by each atom in the DOS. These findings illuminate numerous potential sophisticated applications of MMT clay.