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Title: Hydrogen atom distribution and hydrogen induced site depopulation for the La{sub 2-x}Mg{sub x}Ni{sub 7}-H system

Abstract

La{sub 2-x}Mg{sub x}Ni{sub 7} and its hydrides/deuterides were investigated by high resolution synchrotron powder X-ray and neutron diffraction. Upon deuteration the single phase sample of the intermetallic compound with the refined composition La{sub 1.63}Mg{sub 0.37}Ni{sub 7} (space group: P6{sub 3}/mmc) expands isotropically, in contrast to the Mg free phase. The hydrogen uptake, {approx}9 D/f.u., is higher than in La{sub 2}Ni{sub 7}D{sub 6.5}. The refined composition accounts for La{sub 1.63}Mg{sub 0.37}Ni{sub 7}D{sub 8.8} (beta-phase). Rietveld refinements using the neutron and synchrotron diffraction data suggest that deuterium atoms occupy 5 different interstitial sites within both AB{sub 2} and AB{sub 5} slabs, either in an ordered or a disordered way. All determined D sites have an occupancy >50% and the shortest D-D contact is 1.96(3) A. It is supposed that a competition between the tendency to form directional bonds and repulsive D-D (H-H) interactions is the most important factor that influences the distribution of deuterium atoms in this structure. A hitherto unknown second, alpha-phase with composition La{sub 1.63}Mg{sub 0.37}Ni{sub 7}D{sub 0.56}, crystallizing with the same hexagonal symmetry as La{sub 1.63}Mg{sub 0.37}Ni{sub 7}D{sub 8.8}, has been discovered. The unit cell parameters for this D-poor phase differ slightly from those of the intermetallic. Alpha-phase displaysmore » only one D site (4f, space group: P6{sub 3}/mmc) occupied >50%, which is not populated in the D-rich beta-phase. This hydrogen/deuterium induced site depopulation can be explained by repulsive D-D (H-H) interactions that are likely to influence non-occupancy of certain interstices in metal lattice when absorbing hydrogen. - Graphical abstract: The detailed D atoms arrangement in La{sub 1.63}Mg{sub 0.37}Ni{sub 7}D{sub 8.8} differs significantly from the previously reported La{sub 1.5}Mg{sub 0.5}Ni{sub 7}D{sub 8.9(9.1)}. The present model consists of only five deuterium sites as opposed to nine proposed for La{sub 1.5}Mg{sub 0.5}Ni{sub 7}D{sub 8.9(9.1)}. The reported four remaining deuterium atom positions in La{sub 1.5}Mg{sub 0.5}Ni{sub 7}D{sub 8.9(9.1)} were not found in the investigated La{sub 1.63}Mg{sub 0.37}Ni{sub 7}D{sub 8.8}. The five Ni atoms have deuterium among their nearest neighbors, which surround them in a way similar to configurations observed in some complex transition metal hydrides and already reported for metallic hydrides. In the presented deuterium-rich phase, deformed tetrahedron, rigid trigonal pyramids as well as disordered and deformed saddle-like configuration are observed. Highlights: Black-Right-Pointing-Pointer Alpha- and beta-phase for La{sub 2-x}Mg{sub x}Ni{sub 7}-H system have been characterized. Black-Right-Pointing-Pointer Five different interstitial sites are occupied by deuterium/hydrogen atoms in the beta-phase. Black-Right-Pointing-Pointer One D/H site has been determined in the alpha-phase. Black-Right-Pointing-Pointer Deuterium/hydrogen induced site depopulation during phase transformation is observed. Black-Right-Pointing-Pointer Ni atoms tend to have tetrahedral-like D/H atom coordination.« less

Authors:
 [1];  [2]
  1. Physics Department, Institute for Energy Technology, P.O. Box 40, NO-2027 Kjeller (Norway)
  2. MANEP, Physics Department, University of Geneva, 24 Quai Ernest Ansermet, CH-1211 Geneva (Switzerland)
Publication Date:
OSTI Identifier:
21612883
Resource Type:
Journal Article
Journal Name:
Journal of Solid State Chemistry
Additional Journal Information:
Journal Volume: 186; Other Information: DOI: 10.1016/j.jssc.2011.11.026; PII: S0022-4596(11)00632-3; Copyright (c) 2011 Elsevier Science B.V., Amsterdam, The Netherlands, All rights reserved.; Country of input: International Atomic Energy Agency (IAEA); Journal ID: ISSN 0022-4596
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; CONFIGURATION; DEUTERATION; DEUTERIDES; DEUTERIUM; DISTRIBUTION; HCP LATTICES; HYDRIDES; HYDROGEN; INTERACTIONS; INTERMETALLIC COMPOUNDS; NEUTRON DIFFRACTION; NEUTRONS; PHASE TRANSFORMATIONS; SLABS; SPACE GROUPS; SYMMETRY; SYNCHROTRONS; TRANSITION ELEMENTS; X-RAY DIFFRACTION; ACCELERATORS; ALLOYS; BARYONS; CHEMICAL REACTIONS; COHERENT SCATTERING; CRYSTAL LATTICES; CRYSTAL STRUCTURE; CYCLIC ACCELERATORS; DEUTERIUM COMPOUNDS; DIFFRACTION; ELEMENTARY PARTICLES; ELEMENTS; FERMIONS; HADRONS; HEXAGONAL LATTICES; HYDROGEN COMPOUNDS; HYDROGEN ISOTOPES; ISOTOPES; LIGHT NUCLEI; METALS; NONMETALS; NUCLEI; NUCLEONS; ODD-ODD NUCLEI; SCATTERING; STABLE ISOTOPES; SYMMETRY GROUPS

Citation Formats

Guzik, Matylda N., E-mail: Matylda.Guzik@ife.no, Physics Department, Institute for Energy Technology, P.O. Box 40, NO-2027 Kjeller, Hauback, Bjorn C, and Yvon, Klaus. Hydrogen atom distribution and hydrogen induced site depopulation for the La{sub 2-x}Mg{sub x}Ni{sub 7}-H system. United States: N. p., 2012. Web. doi:10.1016/j.jssc.2011.11.026.
Guzik, Matylda N., E-mail: Matylda.Guzik@ife.no, Physics Department, Institute for Energy Technology, P.O. Box 40, NO-2027 Kjeller, Hauback, Bjorn C, & Yvon, Klaus. Hydrogen atom distribution and hydrogen induced site depopulation for the La{sub 2-x}Mg{sub x}Ni{sub 7}-H system. United States. https://doi.org/10.1016/j.jssc.2011.11.026
Guzik, Matylda N., E-mail: Matylda.Guzik@ife.no, Physics Department, Institute for Energy Technology, P.O. Box 40, NO-2027 Kjeller, Hauback, Bjorn C, and Yvon, Klaus. Wed . "Hydrogen atom distribution and hydrogen induced site depopulation for the La{sub 2-x}Mg{sub x}Ni{sub 7}-H system". United States. https://doi.org/10.1016/j.jssc.2011.11.026.
@article{osti_21612883,
title = {Hydrogen atom distribution and hydrogen induced site depopulation for the La{sub 2-x}Mg{sub x}Ni{sub 7}-H system},
author = {Guzik, Matylda N., E-mail: Matylda.Guzik@ife.no and Physics Department, Institute for Energy Technology, P.O. Box 40, NO-2027 Kjeller and Hauback, Bjorn C and Yvon, Klaus},
abstractNote = {La{sub 2-x}Mg{sub x}Ni{sub 7} and its hydrides/deuterides were investigated by high resolution synchrotron powder X-ray and neutron diffraction. Upon deuteration the single phase sample of the intermetallic compound with the refined composition La{sub 1.63}Mg{sub 0.37}Ni{sub 7} (space group: P6{sub 3}/mmc) expands isotropically, in contrast to the Mg free phase. The hydrogen uptake, {approx}9 D/f.u., is higher than in La{sub 2}Ni{sub 7}D{sub 6.5}. The refined composition accounts for La{sub 1.63}Mg{sub 0.37}Ni{sub 7}D{sub 8.8} (beta-phase). Rietveld refinements using the neutron and synchrotron diffraction data suggest that deuterium atoms occupy 5 different interstitial sites within both AB{sub 2} and AB{sub 5} slabs, either in an ordered or a disordered way. All determined D sites have an occupancy >50% and the shortest D-D contact is 1.96(3) A. It is supposed that a competition between the tendency to form directional bonds and repulsive D-D (H-H) interactions is the most important factor that influences the distribution of deuterium atoms in this structure. A hitherto unknown second, alpha-phase with composition La{sub 1.63}Mg{sub 0.37}Ni{sub 7}D{sub 0.56}, crystallizing with the same hexagonal symmetry as La{sub 1.63}Mg{sub 0.37}Ni{sub 7}D{sub 8.8}, has been discovered. The unit cell parameters for this D-poor phase differ slightly from those of the intermetallic. Alpha-phase displays only one D site (4f, space group: P6{sub 3}/mmc) occupied >50%, which is not populated in the D-rich beta-phase. This hydrogen/deuterium induced site depopulation can be explained by repulsive D-D (H-H) interactions that are likely to influence non-occupancy of certain interstices in metal lattice when absorbing hydrogen. - Graphical abstract: The detailed D atoms arrangement in La{sub 1.63}Mg{sub 0.37}Ni{sub 7}D{sub 8.8} differs significantly from the previously reported La{sub 1.5}Mg{sub 0.5}Ni{sub 7}D{sub 8.9(9.1)}. The present model consists of only five deuterium sites as opposed to nine proposed for La{sub 1.5}Mg{sub 0.5}Ni{sub 7}D{sub 8.9(9.1)}. The reported four remaining deuterium atom positions in La{sub 1.5}Mg{sub 0.5}Ni{sub 7}D{sub 8.9(9.1)} were not found in the investigated La{sub 1.63}Mg{sub 0.37}Ni{sub 7}D{sub 8.8}. The five Ni atoms have deuterium among their nearest neighbors, which surround them in a way similar to configurations observed in some complex transition metal hydrides and already reported for metallic hydrides. In the presented deuterium-rich phase, deformed tetrahedron, rigid trigonal pyramids as well as disordered and deformed saddle-like configuration are observed. Highlights: Black-Right-Pointing-Pointer Alpha- and beta-phase for La{sub 2-x}Mg{sub x}Ni{sub 7}-H system have been characterized. Black-Right-Pointing-Pointer Five different interstitial sites are occupied by deuterium/hydrogen atoms in the beta-phase. Black-Right-Pointing-Pointer One D/H site has been determined in the alpha-phase. Black-Right-Pointing-Pointer Deuterium/hydrogen induced site depopulation during phase transformation is observed. Black-Right-Pointing-Pointer Ni atoms tend to have tetrahedral-like D/H atom coordination.},
doi = {10.1016/j.jssc.2011.11.026},
url = {https://www.osti.gov/biblio/21612883}, journal = {Journal of Solid State Chemistry},
issn = {0022-4596},
number = ,
volume = 186,
place = {United States},
year = {2012},
month = {2}
}