Crystal structure and hydride formation of the DyNi5−xGax alloys

Abstract

Alloys of the composition DyNi5−xGax were prepared and studied by X-ray powder diffraction. It was found that in the DyNi5−xGax system a single phase region exists up to the composition DyNi2Ga3. The hexagonal structure of the prototype compound DyNi5 (CaCu5-type, space group P6/mmm) is preserved in all alloys for x≤1.5 regardless to heat treatment applied, but only in the as-cast alloys for x=2 and 2.5. After annealing, the alloys of the later composition crystallize also in the hexagonal symmetry, but with a larger unit cell (YCo3Ga2-type, space group P6/mmm). The as-cast and annealed single phase alloy with the highest gallium content (DyNi2Ga3) crystallizes only with the YCo3Ga2 structure type. All single phase alloys have been exposed to hydrogen gas. Binary DyNi5, ternary alloys with the YCo3Ga2 type of structure and ternary alloys with the CaCu5 type of structure for 0.5≥x≥2.5 do not show any significant hydrogen absorption up to 10 MPa of hydrogen. However, all other ternary alloys having the CaCu5 structure (0.75≤x≤2) react readily with hydrogen. The equilibrium pressure and the hydrogen capacity decrease with increasing gallium content. The pressure–composition desorption isotherms were measured. The entropy, the enthalpy and the Gibbs free energy of formation have been extracted for the alloy–hydrogen systems that exhibit an equilibrium pressure plateau.