Density functional theory studies of Yb-, Ca- and Sr-substituted Mg2NiH4 hydrides

The role of rare-earth and alkaline-earth metals in determining the properties of MMgNiH4 (M = Yb, Ca and Sr) has been systematically investigated using density functional theory (DFT) and DFT + U methods. The calculated lattice parameters, cohesive energies and elastic parameters are in good agreement with available values. A detailed study of electronic structures shows clearly the ionic interactions between M (M = Yb, Ca and Sr) atoms and NiH4 units, and the main covalent interactions between Ni and H atoms in NiH4 units. Analysis of the bader charge suggests that the degree of ionization of Ca, Sr and Yb atoms decreases in following order: Ca > Sr > Yb. The H site energy (ɛH) is adopted to evaluate the stabilizing effect of rare-earth and alkaline-earth metals. The calculated (ɛH) in H1 (4a) and H2 (12b) sites are 4.7552 and 4.1176 eV/H for YbMgNiH4, which are smaller than the corresponding values of CaMgNiH4 and SrMgNiH4. We also have studied two possible decomposition reactions of MMgNiH4 (M = Yb, Ca and Sr). The results suggest that the feasible decomposition reaction is MMgNiH 4 → MH 2 + 1 2 MgNi 2 + 1 2 Mg + H 2 (M = Yb, Ca and Sr). The vibration properties show that these compounds are kinetically stable.