Study of mechanical, optical, electrical and structural properties of magnesium-based double perovskites Mg2XH6 (X= V, Cr) for hydrogen storage applications using DFT

A first-principles study was conducted to explore advanced hydrogen storage materials by systematically investigating the mechanical, electronic, optical, and thermodynamic properties of ${\text{Mg}}_{2}X{H}_6$ (X = V, Cr). These latters are investigated using the CASTEP software in which the calculus are based on the approximations GGA-PBE and HSE06 hybrid functional. Both hydrides exhibit thermodynamic, mechanical and dynamic stability, as confirmed by their negative formation enthalpies, Born mechanical stability criteria, phonon dispersion curves, and thermodynamic properties analysis. Our first-principles calculations reveal that ${\text{Mg}}_{2}V{H}_6$ has a larger lattice constant (6.75 Å) compared to ${\text{Mg}}_2\text{Cr}{H}_6$ (6.60 Å). Electronic structure analysis reveals a zero-band gap, indicating metallic behavior. Optical properties analysis reveals that both hydrides exhibit a strong response in the ultraviolet region. Furthermore, both compounds exhibit high hydrogen storage capacities, with gravimetric capacities ($C_{\text{wt}}\%$) of 5.73 wt% for ${\text{Mg}}_{2}V{H}_6$ and 5.67 wt% for ${\text{Mg}}_2\text{Cr}{H}_6$. The formation enthalpies are (${\Delta H}_f$ = −2.48 eV/atom) for ${\text{Mg}}_{2}V{H}_6$ and (${\Delta H}_f=$-2.40 eV/atom) for ${\text{Mg}}_2\text{Cr}{H}_6$. According to thermodynamic analysis, ${\text{Mg}}_2\text{Cr}{H}_6$ has a lower desorption temperature (590.51 K) than ${\text{Mg}}_{2}V{H}_6$ (610.25 K).

In our study of the double perovskite hydride of ${\text{Mg}}_{2}X{H}_6$ (X = V, Cr), we show that these latters are promising candidates for hydrogen storage and fuel cell applications, due to their favorable hydrogen storage capacity, excellent stability and also their appropriate electronic and optical properties.