(Nd_1.5Mg_0.5)Ni₇-Based Compounds: Structural and Hydrogen Storage Properties

Abstract

The structural and hydrogen storage properties of (Nd(1.5)Mg(0.5))Ni(7)-based alloys (i.e., A(2)B(7)-type) with a coexistence of two structures (hexagonal 2H and rhombohedral 3R) are investigated in this study. In both 2H- and 3R-type A(2)B(7) structures, Mg atoms occupy Nd sites of Laves-type AB(2) subunits rather than those of AB(5) subunits because Mg substitution for Nd in the AB(2) subunits more significantly strengthens the ionic bond in the system. An increase in the A-atomic radius or the B-atomic radius stabilizes the 2H structure, but a decrease in the A-atomic radius or the B-atomic radius is favorable for formation of the 3R structure. The 2H-A(2)B(7) and 3R-A(2)B(7) phases in each alloy have quite similar equilibrium pressures upon hydrogen absorption and desorption, which show a linear relationship with the average subunit volume. The hydriding enthalpy for the (Nd(1.5)Mg(0.5))Ni(7) compound is about -29.4 kJ/mol H(2) and becomes more negative with partial substitution of La for Nd and Co/Cu for Ni but less negative with partial substitution of Y for Nd.

Electrochemical performance and capacity degradation mechanism of single-phase La–Mg–Ni-based hydrogen storage alloys

“…5(d) between equilibrium pressure and average subunit volume is observed. Similar correlation has also been reported for (Nd,Mg) 2 Ni 7 -type alloys with different foreign elements substituting for Nd [29]. Therefore, we conclude that the linear correlation between equilibrium pressure and average subunit volume is applicable for practical selection of superlattice alloys with different phases but same phase constituent subunits.…”

Effect of Magnesium on the Crystal Transformation and Electrochemical Properties of A2B7-Type Metal Hydride Alloys

“…The rising mass of the 3R-type phase indicates that the increasing partial substitution of Mg for La is beneficial for the construction of the 3R-type phase and complies with the fact that the layered structure of an La-Ni alloy is size-dependent, with the priority of a 3R-type structure for smaller atomic radii and a 2H-type structure for larger R atomic radii. 17,18 Structural stability of MgZn 2 -type Laves slabs is higher than that of MgCu 2 -type Laves slabs. 19 The increasing Mg atoms prefer to intrude into MgZn 2 -type Laves slabs of 3R-type structure, thereby slowing down the energy of the structure.…”

Hydrogen absorption–desorption features and degradation mechanism of ErNi3 compound

“…Hence, adding Mg to ErNi 3 compounds, like other RÀMgÀNi compounds [26][27][28][29][30][31][32][33][34][35][36][37][38][39], is also necessary for hydrogen storage applications.…”