Investigation of the structural, electronic, mechanical and thermodynamic properties of XBH4 X(BH)3 (X=Li, Na) hydrogen storage materials with ab-initio

In this study, XBH4 materials in the F4 ̅3m space group and X(BH)3 (X=Li, Na) materials in the Fm3 ̅m space group were investigated. Hydrogen storage, structural, electronic and mechanical properties of these hydride compounds were calculated by generalized gradient approximation based on density functional theory. Within the scope of structural properties, atomic positions at equilibrium, lattice constants and energies of formation were obtained. Formation energies were found to be negative in all of these materials and they were found to be synthesisable. Electronic band structures, total and partial densities of states were calculated and drawn. It was observed that these hydride compounds have a GAP between their electronic bands around the Fermi level and have insulating properties. Within the scope of mechanical properties, second order elastic constants, Bulk modulus, Young's modulus, Shear modulus, B/G ratio and Poisson's ratio of these hydride compounds were obtained. In addition, the variation of lattice constants, elastic constants and elastic moduli with pressure between 0 kbar - 200 kbar were calculated and drawn. From the elastic constants, it was determined that they fulfil the Born stability conditions and are mechanically stable. The gravimetric hydrogen density and desorption temperatures of these hydride compounds were calculated within the scope of hydrogen storage properties. Gravimetric hydrogen density was obtained as 15.61 wt% for LiBH4, 6.65 wt% for Li(BH)3, 9.63 wt% for NaBH4 and 4.91 wt% for Na(BH)3. Keywords: Hydrogen storage, hydride compounds, mechanical properties