Effect of Rare Earth Elements and Alloy Composition on Hydrogenation Properties and Crystal Structures of Hydrides in Mg2–xRExNi4

The effect of the rare earth elements and alloy composition on the hydrogenation properties and crystal structures of hydrides in Mg2–xRExNi4 (RE = La, Pr, Nd, Sm, and Gd; x = 0.6 and 1.0) was investigated. All Mg2–xRExNi4 alloys had a C15b Laves phase before hydrogenation. Mg1.4RE0.6Ni4 (RE = Pr, Sm, and Gd) alloys were hydrogenated through one plateau to form Mg1.4RE0.6Ni4H∼3.6 while maintaining the C15b structure. Mg1.0RE1.0Ni4 (RE = La, Pr, and Nd) alloys were hydrogenated to ∼1.0 H/M proceeding through two plateaus, and Mg1.0RE1.0Ni4 (RE = Sm and Gd) alloys were hydrogenated to 0.6–0.7 H/M through one plateau. Mg1.0RE1.0Ni4 alloys initially transformed into Mg1.0RE1.0Ni4H∼4 with an orthorhombic structure. In addition it was experimentally confirmed that Mg1.0RE1.0Ni4H∼4 with La, Pr, and Nd transformed into Mg1.0RE1.0Ni4H∼6 with a C15b structure, while no formation of Mg1.0RE1.0Ni4H∼6 (RE = Sm and Gd) was observed at 40 MPa at 250 K. Theoretical calculations suggest that Mg1.0RE1.0Ni4H∼4 with Sm and G...