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 similar to 3.6 while maintaining the C15b structure. Mg1.0RE1.0Ni4 (RE = La, Pr, and Nd) alloys were hydrogenated to similar 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 similar to 4 with an orthorhombic structure. In addition it was experimentally confirmed that Mg1.0RE1.0Ni4H similar to 4 with La, Pr, and Nd transformed into Mg1.0RE1.0Ni4H similar to 6 with a C15b structure, while no formation of Mg1.0RE1.0Ni4H similar to 6 (RE = Sm and Gd) was observed at 40 MPa at 250 K. Theoretical calculations suggest that Mg1.0RE1.0Ni4H similar to(4) with Sm and Gd also transform to Mg1.0RE1.0Ni4H similar to 6 at higher pressures than those used in our experiments (264 MPa for Mg1.0Sm1.0Ni4 and 8.5 GPa for Mg1.0Gd1.0Ni4 at 253 K). It was found that the hydrogenation properties and crystal structure of the hydrides in Mg2-xRExNi4 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.