The effect of substitution of Ga for Ni on the hydrogen sorption properties of NdNi5

The alloys of the general composition NdNi5-xGax (x≤0.5) were prepared and studied by X-ray powder diffraction. It was found that the hexagonal structure of the prototype NdNi5 (CaCu5 type; space group P6/mmm) is preserved in all ternary alloys, but the unit cell parameters increase with increasing content of gallium. All alloys were exposed to hydrogen gas and the pressure composition desorption isotherms for the alloy-hydrogen systems were determined. The isotherms for the NdNi5-hydrogen system exhibit two well-separated plateau regions, whereby the plateau region observed at higher pressure is drastically altered by small amounts of gallium. This results in a final merge of both plateaus so that the alloys with x≥0.1 exhibit only a single plateau pressure. The equilibrium pressure and the hydrogen capacity of all alloy-hydrogen systems decrease with increasing content of gallium. The thermodynamic parameters of hydride formation, i.e. the entropy change, the enthalpy and the Gibbs free energy of formation have also been derived from desorption isotherms. The relevant parameters for the NdNi5-xGax-hydrogen system are compared with the corresponding values for the related NdNi5-xSnx-hydrogen system and briefly discussed