Structural, electronic and thermodynamic properties of Al- and Si-doped α-, γ-, and β-MgH2: Density functional and hybrid density functional calculations

In this work, we present a detailed study of Al- and Si-doped α-, γ-, and β-MgH2 phases using the gradient corrected density functional GGA-PBE and the hybrid Hartree–Fock density functionals PBE0 and HSE06 within the framework of generalized Kohn–Sham density functional theory (DFT) using a plane-wave basis set. We investigate the structural, electronic, and thermodynamical properties of these compounds with regard to their hydrogen storage effectiveness. PBE0 and HSE06 predict cell parameters and bond lengths that are in good agreement with the GGA-PBE calculations and previously known experimental results. As expected smaller band gaps (Egs) are predicted by GGA-PBE for the pure magnesium hydride phases. PBE0 overcomes the deficiencies of DFT in treating these materials better than HSE06 and yields Egs that compare even better with previous GW calculations. Both the hybrid functionals increase the Egs of the Al-doped magnesium hydrides by much less magnitudes than of the Si-doped phases. This difference is interpreted in terms of charge density distributions. Best H2 adsorption energies (ΔHads) are computed by HSE06 while GGA-PBE significantly overestimates them. Si-doped α- and β-MgH2 exhibited the least negative ΔHads in close proximity to the H2 binding energy range of −0.21 to −0.41 eV ideal for practical H2 storage transportation applications.