Interaction of hydrogen with the LaNi4.9In0.1, LaNi4.8In0.2 and LaNi4.8 alloys and their Nd analogues

The single phase nature of the alloys LaNi4.9In0.1, LaNi4.8In0.2, NdNi4.9In0.1, NdNi4.8In0.2 of the systems LaNi5−xInx and NdNi5−xInx was confirmed by means of X-ray powder diffractometry. Nonstoichiometric alloys LaNi4.8 and NdNi4.8 were prepared and were also found to be good single phase materials. All these alloys crystallize with the same hexagonal structure of the CaCu5 type (space group P6/mmm) as do their prototypes LaNi5 and NdNi5. In order to determine the interaction with hydrogen the alloys were exposed to hydrogen gas and the pressure composition desorption isotherms were measured. It was found that all alloys react readily and reversibly absorb large amounts of up to 6.54 hydrogen atoms per alloy formula unit. Generally the equilibrium pressure and the hydrogen capacity decrease with the decreasing nickel content. Presence of indium in the alloy acts in favour of these trends. Furthermore, the increasing content of indium in the alloy system drastically alters the slope and the pressure of the plateau observed at higher pressure of the two isotherm plateaux of the NdNi5–hydrogen system. The final result is a merge of both plateaux into a single one for the hydrogen desorption isotherms of NdNi4.8In0.2. However, the isotherms of nonstoichiometric NdNi4.8 still exhibit two separated pressure plateau regions. The thermodynamic parameters of hydride formation, i.e., the entropy change, the enthalpy and the Gibbs free energy of formation have also been extracted for all alloy–hydrogen systems.