Mg substitution effect on the hydrogenation behaviour, thermodynamic and structural properties of the La{sub 2}Ni{sub 7}-H(D){sub 2} system
- Institute for Energy Technology, P.O. Box 40, Kjeller NO 2027 (Norway)
- Department of Applied Chemistry, School of Engineering, Tokai University, 1117 Kita-Kaname, Hiratsuka, Kanagawa 259-1292 (Japan)
The present work is focused on studies of the influence of magnesium on the hydrogenation behaviour of the (La,Mg){sub 2}Ni{sub 7} alloys. Substitution of La in La{sub 2}Ni{sub 7} by Mg to form La{sub 1.5}Mg{sub 0.5}Ni{sub 7} preserves the initial Ce{sub 2}Ni{sub 7} type of the hexagonal P6{sub 3}/mmc structure and leads to contraction of the unit cell. The system La{sub 1.5}Mg{sub 0.5}Ni{sub 7}-H{sub 2} (D{sub 2}) was studied using in situ synchrotron X-ray and neutron powder diffraction in H{sub 2}/D{sub 2} 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 LaMgNi{sub 4} 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 La{sub 2}Ni{sub 7}, the unit cell of La{sub 1.5}Mg{sub 0.5}Ni{sub 7}D{sub 9.1} is formed by nearly equal hydrogen-induced expansions proceeding in the basal plane ({delta}a/a=7.37%) and along [001] ({delta}c/c=9.67%). In contrast with La{sub 2}Ni{sub 7}D{sub 6.5} where only LaNi{sub 2} layers absorb hydrogen atoms, in La{sub 1.5}Mg{sub 0.5}Ni{sub 7}D{sub 9.1} both LaNi{sub 5} and LaMgNi{sub 4} layers become occupied. Nine types of sites were found to be filled by D in total, including tetrahedral (La,Mg){sub 2}Ni{sub 2}, (La,Mg)Ni{sub 3}, Ni{sub 4}, tetragonal pyramidal La{sub 2}Ni{sub 3} and trigonal bipyramidal (La,Mg){sub 3}Ni{sub 2} interstices. The hydrogen sublattice around the La/Mg site shows formation of two co-ordination spheres of D atoms: an octahedron MgD{sub 6} and a 16-vertex polyhedron LaD{sub 16} 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 A). All D-D distances exceed 1.9 A. Thermodynamic PCT studies yielded the following values for the {delta}H and {delta}S of hydrogenation/decomposition; {delta}H{sub H}=-15.7{+-}0.9 kJ (mol{sub H}){sup -1} and {delta}S{sub H}=-46.0{+-}3.7 J (K mol{sub H}){sup -1} for H{sub 2} absorption, and {delta}H{sub H}=16.8{+-}0.4 kJ (mol{sub H}){sup -1} and {delta}S{sub H}=48.1{+-}1.5 J (K mol{sub H}){sup -1} for H{sub 2} desorption. - Graphical abstract: Substitution of La by Mg in La{sub 2}Ni{sub 7} occurs only in the Laves-type parts of the crystal structure to form LaMgNi{sub 4} slabs within the La{sub 1.5}Mg{sub 0.5}Ni{sub 7} crystal. Subsequent hydrogenation forms La{sub 1.5}Mg{sub 0.5}Ni{sub 7}D{sub 9.1} 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 La{sub 2}Ni{sub 7}. Contrary to La{sub 2}Ni{sub 7}D{sub 6.5} where only LaNi{sub 2} layers absorb hydrogen atoms leaving LaNi{sub 5} slabs unmodified, in La{sub 1.5}Mg{sub 0.5}Ni{sub 7}D{sub 9.1} both LaNi{sub 5} and LaMgNi{sub 4} layers become occupied; the effect of Mg substitution is the increase of the reversible hydrogenation storage capacity. Within the hydrogen sublattice inside the LaMgNi{sub 4}D{sub 7.56} slab local ordering of Mg and La in the layer gives a packing of the MgD{sub 6} octahedra and LaD{sub 16} polyhedra as shown.
- OSTI ID:
- 21128251
- Journal Information:
- Journal of Solid State Chemistry, Vol. 181, Issue 4; Other Information: DOI: 10.1016/j.jssc.2007.12.041; PII: S0022-4596(08)00007-8; Copyright (c) 2008 Elsevier Science B.V., Amsterdam, The Netherlands, All rights reserved; Country of input: International Atomic Energy Agency (IAEA); ISSN 0022-4596
- Country of Publication:
- United States
- Language:
- English
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