LaNi 5 -Assisted Hydrogenation of MgNi 2 in the Hybrid Structures of La 1.09 Mg 1.91 Ni 9 D 9.5 and La 0.91 Mg 2.09 Ni 9 D 9.4

This work focused on the high pressure PCT and in situ neutron powder diffraction studies of the LaMg 2 Ni 9 -H 2 (D 2 ) system at pressures up to 1,000 bar. LaMg 2 Ni 9 alloy was prepared by a powder metallurgy route from the LaNi 9 alloy precursor and Mg powder. Two La 3−x Mg x Ni 9 samples with slightly different La/Mg ratios were studied, La 1.1 Mg 1.9 Ni 9 (sample 1) and La 0.9 Mg 2.1 Ni 9 (sample 2). In situ neutron powder diffraction studies of the La 1.09 Mg 1.91 Ni 9 D 9.5 (1) and La 0.91 Mg 2.09 Ni 9 D 9.4 (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 LaNi 3 . The Mg-containing intermetallics reversibly form hydrides with DH des = 24.0 kJ/mol H2 and an equilibrium pressure of H 2 desorption of 18 bar at 20 °C (La 1.09 Mg 1.91 Ni 9 ). A pronounced hysteresis of H 2 absorption and desorption, ~100 bar, is observed. The studies showed that LaNi 5 -assisted hydrogenation of MgNi 2 in the LaMg 2 Ni 9 hybrid structure takes place. In the La 1.09 Mg 1.91 Ni 9 D 9.5 (1) and La 0.91 Mg 2.09 Ni 9 D 9.4 (2) (a = 5.263/5.212; c = 25.803/25.71 Å) D atoms are accommodated in both Laves and CaCu 5 -type slabs. In the LaNi 5 CaCu 5 -type layer, D atoms fill three types of interstices; a deformed octahedron [La 2 Ni 4 ], and [La(Mg) 2 Ni 2 ] and [Ni 4 ] tetrahedra. The overall chemical compositions can be presented as LaNi 5 H 5.6/5.0 + 2*MgNi 2 H 1.95/2.2 showing that the hydrogenation of the MgNi 2 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 MgNi 2 slab takes place, including [MgNi 3 ] and [Mg 2 Ni 2 ] tetrahedra.