Stabilization of Ca₇Ge-type magnesium compounds by alloying of non-metal elements: A new family material for reversible hydrogen storage applications

A new family material of Mg_nTMX₂ consisted by alloying non-metal element X into the Ca₇Ge-type Mg_nTM (TM = V, Ti, or Nb; X = C, N, O, F, P, S, or Cl; n = 6 for TM = Nb and V or 7 for TM = Ti) to keep its structural stability during the de/hydrogenations was proposed. Formation energy of Mg_nTMX₂ alloys was calculated using density functional theory to clarify the possibility of synthesizing these compounds. Calculations illustrated that compounds with X = F, O, and S satisfy both the thermal stability with negative formation energies and mechanical stability at zero pressure. The hydrogen absorption is proceed in a manner of stepwise, and finally all tetrahedron interstices were filled by 56 hydrogen atoms in the unit cell forming formula hydrides as Mg_nTMX₂H₁₄. It is obvious that the interstice composition plays an important role in hydrogen adsorption performance. The tetrahedral interstices composed by three Mg atoms and one transition metal own more strong ability to capture H atoms than those constituted by four Mg atoms in the initial hydrogenation process Results indicate that new family compound Mg₆VO₂ could be possibly ideal hydrogen storage materials in terms of dehydrogenation temperature (170–246 K) and hydrogen storage capacity (∼5.81 mass %).