Structural and hydrogen sorption studies of NdNi 5− xAl x and GdNi 5− xAl x

Abstract

Crystallographic and hydrogen sorption studies of the NdNi 5− x Al x and GdNi 5− x Al x systems have been carried out. It is found that up to x<2 the prototype hexagonal crystal structure of the CaCu 5-type is preserved. However, at x≥2, a larger unit cell of the HoNi 2.6Ga 2.4-type is observed. In both structures, a site preference occupancy between Al and Ni atoms occurs. The structural transition with x is explained by considering steric effects: Al being bigger than Ni. All alloys were subjected to hydrogen sorption. The decrease of maximum hydrogen content and plateau pressure with increasing aluminium content (from 4.1 H/f.u. and 0.87 MPa for GdNi 4.5Al 0.5 to 2.6 H/f.u. and 0.01 MPa for GdNi 3.26Al 1.74) is explained from both the thermodynamic and crystallographic point of view. These results are consistent with: (i) previous observations on the LaNi 5− x Al x system and (ii) the `Westlake criterium' except when the HoNi 2.6Ga 2.4-type structure appears. The huge decrease of the hydrogen content observed when the superstructure appears is explained by the change of possible hydrogen tetrahedral sites from RE 2Ni 2 to RE 2Al 2 (RE=Nd, Gd).