Impact of Hydrogen Cage Occupancy on the Mechanical Properties and Elastic Anisotropies of sII Hydrates

Abstract

In this study, we investigate the composition-dependent mechanical properties and elastic anisotropies of sII hydrogen hydrates using first-principles method. The evaluation of the elastic moduli and their direction dependency is achieved by computing the second-order elastic constants (SOECs) of the unit lattice. The various trends of elastic constants with the hydrogen composition of the cages introduces variations in the bonding strength, compressibility, stiffness, and shear properties of the structure which are captured by the Poisson's ratio, bulk, Young, and shear moduli, respectively. Elastic properties were found to be significantly influenced by the system's anisotropy, arising from the geometry of the cages and their unique arrangement within the lattice being affected by the increase in the hydrogen occupancy of the cages. The detailed analysis of elastic anisotropies revealed shifts in the strongest and weakest directions of the material with varying the hydrogen content of the cages. The Poisson's ratio captures the anisotropic bonding strengths within the crystal structure with the hydrogen composition of the lattice, explaining the reason behind the existence of strongest and weakest directions in terms of compression, tension, and shear forces. Taken together the established structure-property-composition relations will be useful in the design and optimization of hydrogen sII hydrates for energy storage applications.