Crystal structure, thermal expansion and hydrogen sorption properties of the GdNi5-xGax alloys

Alloys of the composition GdNi5-xGax were prepared and their structural and hydrogen sorption properties were studied. It was found that nickel in GdNi5 can be replaced by gallium up to the composition GdNi2Ga3. The crystal structure of the alloys depends on the heat treatment. The hexagonal structure of the prototype compound (CaCu5 type; space group P6/mmm) is preserved in all as-cast alloys. After annealing the alloys of the compositions GdNi3Ga2 and GdNi2.5Ga2.5 crystallize with a larger unit cell of the YCo3Ga2 type, but closely related to the structure of GdNi5. The linear coefficients of thermal expansion were determined between room temperature and 873 K by x-ray powder diffraction. All alloys exhibit expansion anisotropy; the expansion along the a axis being larger than along the c axis. Alloys have been exposed to hydrogen and were found to absorb up to 4.3 hydrogen atoms per alloy formula unit at room temperature. The hydrogen uptake depends on the structure and the composition of the alloy. Binary GdNi5 and alloys with the larger unit cell are inert to hydrogen at pressures up to 20 MPa. The hydrogen equilibrium pressure and the hydrogen capacity decrease as the content of gallium increases. The entropy and the enthalpy have been calculated for the alloy-hydrogen systems that exhibit a desorption equilibrium plateau. (author)