Hydrogen in La2MgNi9D13: The Role of Magnesium

Reversible hydrogen storage capacity of the La3-xMgxNi9 alloys, charged by gaseous hydrogen or by electro-chemical methods, reaches its maximum at composition La2MgNi9. As (La,Mg)Ni3-3.5 alloys are the materials used in advanced metal hydride electrodes in Ni-MH batteries, this raises interest in the study of the structure properties interrelation in the system La2MgNi9-H-2 (D-2). In the present work, this system has been investigated by use of in situ synchrotron X-ray and neutron powder diffraction in H-2/D-2 gas and by performing pressure composition temperature measurements. The saturated La2MgNi9D13.1 hydride forms via an isotropic expansion and crystallizes with a trigonal unit cell (space group R (3) over barm (No.166); a = 5.4151(1) angstrom; c = 26.584(2) angstrom; V= 675.10(6) angstrom(3)). The studied hybrid structure is composed of a stacking of two layers resembling existing intermetallic compounds LaNi5 (CaCu5 type) and LaMgNi4 (Laves type). These are occupied by D to form LaNi5D5.2 and LaMgNi4D7.9. The LaNi5D5.2 slab has a typical structure observed for all reported LaNi5-containing hybrid structures of the AB(5) + Laves phase types. However, the Laves type slab LaMgNi4D7.9 is different from the characterized individual LaMgNi4D4.85 hydride. This results from the filling of a greater variety of interstitial sites in the La2MgNi9D13/ LaMgNi4D7,9, including MgNi2, Ni-4, (La/Mg)(2)Ni-2, and (La/Mg)Ni-3, in contrast with individual LaMgNi4D4.85 where only La2MgNi2 and Ni-4 interstitials are occupied. Despite a random distribution of La and Mg in the structure, a local hydrogen ordering takes place with H atoms favoring occupation of two Mg-surrounded sites, triangles MgNi2 and tetrahedra LaMgNi2. A directional bonding between Ni, Mg, and hydrogen is observed and is manifested by a formation of the NiH4 tetrahedra and MgH6 octahedra, which are connected to each other by sharing H vertexes to form a spatial framework.