First-Principles insights into the physical properties of double hydrides for hydrogen storage applications

This study focuses on the structural, elastic, electronic properties with Fermi surfaces, optical, thermodynamic and hydrogen storage properties of ternary hydrides Li${}_2$XH${}_6$ (X = V, Cr) using density functional theory based calculations. The structural stability is governed by negative formation energies and positive Born-Huang criteria on elastic constants. The lattice constants of Li${}_2$VH${}_6$, and Li${}_2$CrH${}_6$ are 6.8752 (Å) and 6.6316 (Å), respectively. The calculated formation energies are −0.650 eV/atom and −0.538 eV/atom of Li${}_2$VH${}_6$ and Li${}_2$CrH${}_6$ respectively. The Li${}_2$VH${}_6$ has the higher bulk modulus, Young’s modulus and shear modulus as compared with Li${}_2$CrH${}_6$ anticipating its greater mechanical strength. The high values of density of states at the Fermi level make the electronic properties interesting. The metallic nature for both materials is confirmed with zero band gap and the three-dimensional representation of the bands cutting the Fermi level known to be the Fermi surfaces. The important optical parameters such as absorption, reflection and dielectric function are studied for Li${}_2$VH${}_6$ and Li${}_2$CrH${}_6$ under the application of incident photonic energies. The impact of the change in pressure and temperature on the thermodynamic parameters of Li${}_2$XH${}_6$ (X = V, Cr) is calculated. The higher gravimetric hydrogen capacity of 7.88 and 7.77% for Li${}_2$VH${}_6$, and Li${}_2$CrH${}_6$ is recorded respectively.