Hydrogen in La₂MgNi₉D₁₃: The Role of Magnesium

Reversible hydrogen storage capacity of the La_3–<i>x</i>Mg_<i>x</i>Ni₉ alloys, charged by gaseous hydrogen or by electrochemical methods, reaches its maximum at composition La₂MgNi₉. As (La,Mg)­Ni_3–3.5 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–H₂ (D₂). In the present work, this system has been investigated by use of <i>in situ</i> synchrotron X-ray and neutron powder diffraction in H₂/D₂ gas and by performing pressure–composition–temperature measurements. The saturated La₂MgNi₉D_13.1 hydride forms via an isotropic expansion and crystallizes with a trigonal unit cell (space group <i>R</i>3̅<i>m</i> (No.166); <i>a</i> = 5.4151(1) Å; <i>c</i> = 26.584(2) Å; <i>V</i> = 675.10(6) ų). The studied hybrid structure is composed of a stacking of two layers resembling existing intermetallic compounds LaNi₅ (CaCu_<i>5</i> type) and LaMgNi₄ (Laves type). These are occupied by D to form LaNi₅D_5.2 and LaMgNi₄D_7.9. The LaNi₅D_5.2 slab has a typical structure observed for all reported LaNi₅-containing hybrid structures of the AB₅ + Laves phase types. However, the Laves type slab LaMgNi₄D_7.9 is different from the characterized individual LaMgNi₄D_4.85 hydride. This results from the filling of a greater variety of interstitial sites in the La₂MgNi₉D₁₃/LaMgNi₄D_7.9, including MgNi₂, Ni₄, (La/Mg)₂Ni₂, and (La/Mg)­Ni₃, in contrast with individual LaMgNi₄D_4.85 where only La₂MgNi₂ and Ni₄ 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.