Ab initio calculations on energetics and electronic structures of cubic Mg3MNi2 (M = Al, Ti, Mn) hydrogen storage alloys

Mg3MNi2 (M = Al, Ti, Mn) ternary intermetallic compounds with cubic structure are a new type of potential hydrogen storage alloys. Using ab initio density functional theory (DFT) calculations, the energetics and electronic structures of Mg3MNi2 (M = Al, Ti, Mn) compounds are systematically investigated. The optimized structural parameters including lattice constants and internal atomic positions are close to experimental data determined from X-ray powder diffraction. The calculated results of formation enthalpy ΔHform show that the stabilities of cubic Mg3MNi2 (M = Al, Ti, Mn) compound, compared with hexagonal Mg2Ni, increase in the order of Mg3MnNi2, Mg2Ni, Mg3TiNi2 and Mg3AlNi2, whereas the stabilities of their saturated Mg3MNi2H3 (M = Al, Ti, Mn) hydrides, compared with monoclinic Mg2NiH4, decrease in the order of Mg2NiH4, Mg3AlNi2H3, Mg3TiNi2H3 and Mg3MnNi2H3. Further calculations of hydrogen desorption enthalpy ΔHdes indicate that these cubic Mg3MNi2 (M = Al, Ti, Mn) compounds possess promising dehydrogenation properties for their relatively lower ΔHdes values. Among of them, the dehydrogenation ability of Mg3TiNi2 is the most pronounced. Analysis of electronic structures suggests that the strong covalent bonding interactions between Ni and M within cubic Mg3MNi2 (M = Al, Ti, Mn) are dominant and directly control the structural stabilities of these compounds.