Effect of Rare Earth Elements and Alloy Composition on Hydrogenation Properties and Crystal Structures of Hydrides in Mg_2–xRE_xNi₄

The effect of the rare earth elements and alloy composition on the hydrogenation properties and crystal structures of hydrides in Mg 2– x RE x Ni 4 ( RE = La, Pr, Nd, Sm, and Gd; x = 0.6 and 1.0) was investigated. All Mg 2– x RE x Ni 4 alloys had a C15b Laves phase before hydrogenation. Mg 1.4 RE 0.6 Ni 4 ( RE = Pr, Sm, and Gd) alloys were hydrogenated through one plateau to form Mg 1.4 RE 0.6 Ni 4 H ∼3.6 while maintaining the C15b structure. Mg 1.0 RE 1.0 Ni 4 ( RE = La, Pr, and Nd) alloys were hydrogenated to ∼1.0 H/M proceeding through two plateaus, and Mg 1.0 RE 1.0 Ni 4 ( RE = Sm and Gd) alloys were hydrogenated to 0.6–0.7 H/M through one plateau. Mg 1.0 RE 1.0 Ni 4 alloys initially transformed into Mg 1.0 RE 1.0 Ni 4 H ∼4 with an orthorhombic structure. In addition it was experimentally confirmed that Mg 1.0 RE 1.0 Ni 4 H ∼4 with La, Pr, and Nd transformed into Mg 1.0 RE 1.0 Ni 4 H ∼6 with a C15b structure, while no formation of Mg 1.0 RE 1.0 Ni 4 H ∼6 ( RE = Sm and Gd) was observed at 40 MPa at 250 K. Theoretical calculations suggest that Mg 1.0 RE 1.0 Ni 4 H ∼4 with Sm and Gd also transform to Mg 1.0 RE 1.0 Ni 4 H ∼6 at higher pressures than those used in our experiments (264 MPa for Mg 1.0 Sm 1.0 Ni 4 and 8.5 GPa for Mg 1.0 Gd 1.0 Ni 4 at 253 K). It was found that the hydrogenation properties and crystal structure of the hydrides in Mg 2– x RE x Ni 4 are dependent on the alloy composition, i.e., the ratio of Mg to RE in the alloy phase, but independent of the choice of rare earth element.