The phase stability, structural transition, and hydrogen storage characteristics of the polymorphic La4MgNi19 compound are systematically investigated in the present work. The La4MgNi19 compound has two variants in the temperature range of 840−960 °C with a preference for a hexagonal structure (2H) at a higher temperature and a rhombohedral structure (3R) at a lower temperature. Because the structure and composition are almost identical, the polymorphic transition between the 2H and 3R phases may proceed through short-range displacements of their subunit layers. Detailed analyses on XRD patterns for the hydrided or dehydrided samples indicate that the equilibrium pressures of hydrogen absorption and desorption for the 2H phase are lower than the pressures for the 3R phase, but the pressure gap between the two phases is too small to be identified from the measured pressure−composition (P−C) isotherms. The average hydriding and dehydriding enthalpies are determined to be −32.1 and 31.5 kJ/mol H2, respectively. As hydrogen atoms enter into the lattices, the strains in both the 2H and the 3R phases increase first in the region of the dilute H-dissolved solid solutions and then dramatically decrease once the dilute solid solutions transform into the corresponding hydrides.

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.