LaNi5-Assisted Hydrogenation of MgNi2 in the Hybrid Structures of La1.09Mg1.91Ni9D9.5 and La0.91Mg2.09Ni9D9.4

This work focused on the high pressure PCT and in situ neutron powder diffraction studies of the LaMg2Ni9-H-2 (D-2) system at pressures up to 1,000 bar. LaMg2Ni9 alloy was prepared by a powder metallurgy route from the LaNi9 alloy precursor and Mg powder. Two La3-xMgxNi9 samples with slightly different La/Mg ratios were studied, La1.1Mg1.9Ni9 (sample 1) and La0.9Mg2.1Ni9 (sample 2). In situ neutron powder diffraction studies of the La1.09Mg1.91Ni9D9.5 (1) and La0.91Mg2.09Ni9D9.4 (2) deuterides were performed at 25 bar D-2 (1) and 918 bar D-2 (2). The hydrogenation properties of the (1) and (2) are dramatically different from those for LaNi3. The Mg-containing intermetallics reversibly form hydrides with Delta H-des = 24.0 kJ/mol(H2) and an equilibrium pressure of H-2 desorption of 18 bar at 20 degrees C (La1.09Mg1.91Ni9). A pronounced hysteresis of H-2 absorption and desorption, ~100 bar, is observed. The studies showed that LaNi5-assisted hydrogenation of MgNi2 in the LaMg2Ni9 hybrid structure takes place. In the La1.09Mg1.91Ni9D9.5 (1) and La0.91Mg2.09Ni9D9.4 (2) (a = 5.263/5.212; c = 25.803/25.71 angstrom) D atoms are accommodated in both Laves and CaCu5-type slabs. In the LaNi5 CaCu5-type layer, D atoms fill three types of interstices; a deformed octahedron [La2Ni4], and [La(Mg)(2)Ni-2] and [Ni-4] tetrahedra. The overall chemical compositions can be presented as LaNi5H5.6/5.0 + 2*MgNi2H1.95/2.2 showing that the hydrogenation of the MgNi2 slab proceeds at mild H-2/D-2 pressure of just 20 bar. A partial filling by D of the four types of the tetrahedral interstices in the MgNi2 slab takes place, including [MgNi3] and [Mg2Ni2] tetrahedra.