| 181. |
- Denys, R. V., et al.
(författare)
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Hydrogen storage properties and structure of La1-xMgx(Ni1-yMny)(3) intermetallics and their hydrides
- 2007
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Ingår i: Journal of Alloys and Compounds. - : Elsevier BV. - 0925-8388 .- 1873-4669. ; 446, s. 166-172
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Tidskriftsartikel (refereegranskat)abstract
- 'Hybrid' RNi3 (R =rare earth metal) crystal structures are built of the slabs of simpler types, CaCu5 and MgZn2. Different affinities of these slabs to hydrogen result in unusual "anisotropic" expansion of the RNi3 and R2Ni7 (R = La, Cc) structures upon hydrogenation. This work focuses on studies of the hydrogenation behaviour of LaNi3 and on the properties of the hydrides of the modified by Mg and Mn La-Ni alloys. The crystal structure of LaNi3D2.8 and the crystal structure and hydrogen storage behaviours of the La1.5Mg0.5Ni7 and La(Ni-1-,Mn,)3 (x=0; 0.067-, 0.133; 0.2: 0.267; 0.3; 0.333; 0.4) alloys were in focus. The deuteration of LaNi3 with PuNi3 type of structure leads to the formation of LaNi3D2.8 and is accompanied by a deformation of the metal matrix causing a change of the initial rhombohedral symmetry (space group R3m) to a nionoclinic one (space group C2/m; a = 8.6408(7) angstrom, b=4.928](4) angstrom, c= 32.774(3) A; = 90.850(8)degrees; V= 1395.5(2) angstrom(3)). Similar to the earlier studied CeNi3D2.8, preferential occupation by deuterium atoms of the AB, layers takes place, leading to the "anisotropic" expansion of the unit cell along [0 0 1] (Delta c/c = 30.6%). 14 occupied D crystallographic sites have 4 chemically different types of metal-atom surroundings, including Ni-4 (2), La2Ni2 (2), La3Ni (6), and La3Ni3 (4). Modification of the La-Ni alloys by magnesium and manganese leads to the formation of intermetallic compounds crystallising with the PuNi3, CeNi3, and Ce2Ni7-type structures. An ordered substitution of La by Mg in the MgZn2-type slabs was observed, causing a complete alteration of the hydrogenation behaviour of the original LaNi3 alloy. La1.5Mg0.5Ni7D9 isotropically expands upon its formation and leads to a substantial increase of the stability against hydrogenation-induced aniorphisation. On the other hand, replacement of Ni by Mn leads to the change in crystal-structure type from PuNi3 to CeNi3 in the LaNi3-xMn. alloys (x>0.1). An ordered substitution of Ni by Mn proceeds inside the RNi5 slabs only. This decreases the stability of the initial alloy against amorphisation on hydrogenation.
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| 182. |
- Denys, R. V., et al.
(författare)
-
Mg substitution effect on the hydrogenation behaviour, thermodynamic and structural properties of the La2Ni7-H(D)(2) system
- 2008
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Ingår i: Journal of Solid State Chemistry. - : Elsevier BV. - 0022-4596 .- 1095-726X. ; 181:4, s. 812-821
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Tidskriftsartikel (refereegranskat)abstract
- The present work is focused on studies of the influence of magnesium on the hydrogenation behaviour of the (La,Mg)(2)Ni-7 alloys. Substitution of La in La2Ni7 by Mg to form La1.5Mg0.5Ni7 preserves the initial Ce2Ni7 type of the hexagonal P6(3)/mmc structure and leads to contraction of the unit cell. The system La1.5Mg0.5Ni7-H2 (D-2) was studied using in situ synchrotron X-ray and neutron powder diffraction in H-2/D-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 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 (Delta a/a = 7.37%) and along [001] (Delta c/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)(2)Ni-2, (La,Mg)Ni-3, Ni-4, tetragonal pyramidal La2Ni3 and trigonal bipyramidal (La,Mg)(3)Ni-2 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 angstrom). All D-D distances exceed 1.9 angstrom. Thermodynamic PCT studies yielded the following values for the Delta H and Delta S of hydrogenation/decomposition; Delta H-H = -15.7 +/- 0.9 kJ (mol(H))(-1) and Delta S-H = -46.0 +/- 3.7 J (K mol(H))(-1) for H-2 absorption, and Delta H-H = 16.8 +/- 0.4 kJ (mol(H))(-1) and Delta S-H = 48.1 +/- 1.5 J (K mol(H))(-1) for H-2 desorption.
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