Mg substitution effect on the hydrogenation behaviour, thermodynamic and structural properties of the La2Ni7–H(D)2 system

The present work is focused on studies of the influence of magnesium on the hydrogenation behaviour of the (La,Mg)2Ni7 alloys. Substitution of La in La2Ni7 by Mg to form La1.5Mg0.5Ni7 preserves the initial Ce2Ni7 type of the hexagonal P63/mmc structure and leads to contraction of the unit cell. The system La1.5Mg0.5Ni7–H2 (D2) was studied using in situ synchrotron X-ray and neutron powder diffraction in H2/D2 gas and pressure–composition–temperature measurements. La replacement by Mg was found to proceed in an ordered way, only within the Laves-type parts of the hybrid crystal structure, yielding formation of LaMgNi4 slabs with statistic and equal occupation of one site by La and Mg atoms. Mg alters structural features of the hydrogenation process. Instead of a strong unilateral anisotropic expansion which takes place on hydrogenation of La2Ni7, the unit cell of La1.5Mg0.5Ni7D9.1 is formed by nearly equal hydrogen-induced expansions proceeding in the basal plane (Δa/a=7.37%) and along [001] (Δc/c=9.67%). In contrast with La2Ni7D6.5 where only LaNi2 layers absorb hydrogen atoms, in La1.5Mg0.5Ni7D9.1 both LaNi5 and LaMgNi4 layers become occupied. Nine types of sites were found to be filled by D in total, including tetrahedral (La,Mg)2Ni2, (La,Mg)Ni3, Ni4, tetragonal pyramidal La2Ni3 and trigonal bipyramidal (La,Mg)3Ni2 interstices. The hydrogen sublattice around the La/Mg site shows formation of two co-ordination spheres of D atoms: an octahedron MgD6 and a 16-vertex polyhedron LaD16 around La. The interatomic distances are in the following ranges: La–D (2.28–2.71), Mg–D (2.02–2.08), Ni–D (1.48–1.86 Å). All D–D distances exceed 1.9 Å. Thermodynamic PCT studies yielded the following values for the ΔH and ΔS of hydrogenation/decomposition; ΔHH=−15.7±0.9 kJ (molH)−1 and ΔSH=−46.0±3.7 J (K molH)−1 for H2 absorption, and ΔHH=16.8±0.4 kJ (molH)−1 and ΔSH=48.1±1.5 J (K molH)−1 for H2 desorption.

Substitution of La by Mg in La2Ni7 occurs only in the Laves-type parts of the crystal structure to form LaMgNi4 slabs within the La1.5Mg0.5Ni7 crystal. Subsequent hydrogenation forms La1.5Mg0.5Ni7D9.1 with a nearly isotropic expansion of the unit cell that is in contrast to the large unilateral anisotropic expansion that takes place on hydrogenation of La2Ni7. Contrary to La2Ni7D6.5 where only LaNi2 layers absorb hydrogen atoms leaving LaNi5 slabs unmodified, in La1.5Mg0.5Ni7D9.1 both LaNi5 and LaMgNi4 layers become occupied; the effect of Mg substitution is the increase of the reversible hydrogenation storage capacity. Within the hydrogen sublattice inside the LaMgNi4D7.56 slab local ordering of Mg and La in the layer gives a packing of the MgD6 octahedra and LaD16 polyhedra as shown.Figure optionsDownload as PowerPoint slide