Mg substitution effect on the hydrogenation behaviour, thermodynamic and structural properties of the La2Ni7–H(D)2 system
| Autor: | Roman V. Denys, Robert G. Delaplane, Masashi Sato, A.B. Riabov, Volodymyr A. Yartys |
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| Rok vydání: | 2008 |
| Předmět: |
Hydrogen
Neutron diffraction Intermetallic chemistry.chemical_element Crystal structure Condensed Matter Physics Electronic Optical and Magnetic Materials Inorganic Chemistry Trigonal bipyramidal molecular geometry Tetragonal crystal system Crystallography chemistry Octahedron X-ray crystallography Materials Chemistry Ceramics and Composites Physical and Theoretical Chemistry |
| Zdroj: | Journal of Solid State Chemistry. 181:812-821 |
| ISSN: | 0022-4596 |
| Popis: | 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 La 2 Ni 7 by Mg to form La 1.5 Mg 0.5 Ni 7 preserves the initial Ce 2 Ni 7 type of the hexagonal P 6 3 / mmc structure and leads to contraction of the unit cell. The system La 1.5 Mg 0.5 Ni 7 –H 2 (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 LaMgNi 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 2 Ni 7 , the unit cell of La 1.5 Mg 0.5 Ni 7 D 9.1 is formed by nearly equal hydrogen-induced expansions proceeding in the basal plane (Δ a / a =7.37%) and along [001] (Δ c / c =9.67%). In contrast with La 2 Ni 7 D 6.5 where only LaNi 2 layers absorb hydrogen atoms, in La 1.5 Mg 0.5 Ni 7 D 9.1 both LaNi 5 and LaMgNi 4 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 La 2 Ni 3 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 MgD 6 and a 16-vertex polyhedron LaD 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 Δ H and Δ S of hydrogenation/decomposition; Δ H H =−15.7±0.9 kJ (mol H ) −1 and Δ S H =−46.0±3.7 J (K mol H ) −1 for H 2 absorption, and Δ H H =16.8±0.4 kJ (mol H ) −1 and Δ S H =48.1±1.5 J (K mol H ) −1 for H 2 desorption. |
| Databáze: | OpenAIRE |
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