Localized Structural and Electronic Perturbations Induced by Mono-Vacancy in MgH2: A Comprehensive First-Principles Investigation

In the pursuit of sustainable energy, magnesium hydride (MgH2) stands out as a promising candidate for hydrogen storage due to its high capacity. Nevertheless, its high thermodynamic stability necessitates elevated operating temperatures, thereby hindering practical applications. To mitigate this limitation, our study employs a defect engineering approach by introducing a mono-vacancy to decrease its thermodynamic stability. Utilizing first-principles density functional theory calculations, we investigate the influence of a mono-vacancy on the structural and electronic properties of MgH2 crystal. Introducing the defect results in a 0.57% contraction of the a/b lattice parameters and a 1.03% expansion along the c-axis, causing lattice distortion. Electronically, the band gap narrows by 0.67 eV, indicating an increase in metallic character. We observe a distinct vacancy-affected zone, characterized by substantial alterations in electron density within a 26.505 Å3 volume and modifications to the potential energy distribution encompassing a 19.514 Å3 volume. The mono-vacancy enhances the polarity of the Mg-H bonds and maximally decreases the bond energy by 0.065 eV. A localized high-energy region of 0.354 eV emerges, functioning as an energy barrier to atomic diffusion. This energy barrier is encompassed by low-energy pathways, potentially facilitating H atom migration within the MgH2 crystal.