Investigation of modification of hydrogenation and structural properties of LaNi5 intermetallic compound induced by substitution of Ni by Pd

The hydrogenation properties of the LaNi₅ (CaCu₅ type, hP6, P6/mmm) and Pd substituted derivatives LaNi_5−xPd_x compounds have been studied in the whole homogeneity range of the solid solution (0.25≤x≤1.5). The pressure versus hydrogen content isotherms show several plateaus and an increase of the plateau pressure as a function of palladium concentration. The volume increase of the Pd substituted alloys should have resulted in a lowering, and not an increase, of the plateau pressure, according to the conventional models based on the size effect. In order to elucidate the origin of this anomalous behavior, both an experimental and a theoretical ab initio electronic structure investigation have been carried out. Experimentally, the nature and the structural properties of the hydrides have been studied by both in situ and ex situ neutron diffraction. The crystal structures of the three hydride phases are reported (LaNi_3.5Pd_1.5D_1.96, filled-up CaCu₅ type, P6/mmm; LaNi₄PdD_2.72, LaNi₂(Ni_0.75Pt_0.25)₃H_2.61 type, oI128, Ibam; LaNi_4.75Pd_0.25D_5.67, partly ordered CaCu₅ type, P6mm). In addition, the phase diagram of LaNi_5−xPd_x-H system has been investigated. The electronic and thermodynamic properties of both intermetallic compounds and the hydrides have been studied by ab initio electronic structure calculations. The theoretical results are in good agreement with our experimental data, and show that electronic interactions play a major role in the hydrogenation behavior of these Pd substituted intermetallic compounds, and that these effects cannot be accounted for by a simplistic model based on the size effect alone.