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Hydrogenation induced structure and property changes in GdGa
Stockholm Univ, Dept Mat & Environm Chem, SE-10691 Stockholm, Sweden..
Stockholm Univ, Dept Mat & Environm Chem, SE-10691 Stockholm, Sweden..
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström.
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2016 (English)In: Journal of Solid State Chemistry, ISSN 0022-4596, E-ISSN 1095-726X, Vol. 239, 184-191 p.Article in journal (Refereed) PublishedText
Abstract [en]

Hydrides GdGaH were obtained by exposing the Zintl phase GdGa with the CrB structure to a hydrogen atmosphere at pressures from 1.5 to 50 bar and temperatures from 50 to 500 degrees C. Structural analysis by powder X-ray diffraction suggests that conditions with hydrogen pressures in a range between 15 and 50 bar and temperatures below 500 degrees C afford a uniform hydride phase with the NdGaH1.66 structure (Cmcm, a=3.9867(7) angstrom, b=12.024(2) angstrom, c=4.1009(6) angstrom) which hosts H in two distinct positions, H1 and H2. H1 is coordinated in a tetrahedral fashion by Gd atoms, whereas H2 atoms are inserted between Ga atoms. The assignment of the NdGaH1.66 structure is corroborated by first principles DFT calculations. Modeling of phase and structure stability as a function of composition resulted in excellent agreement with experimental lattice parameters when x=1.66 and revealed the presence of five-atom moieties Ga-H2-Ga-H2-Ga in GdGaH1.66. From in situ powder X-ray diffraction using synchrotron radiation it was established that hydrogenation at temperatures above 200 degrees C affords a hydride with x approximate to 1.3, which is stable up to 500 degrees C, and that additional H absorption, yielding GdGaH1.66, takes place at lower temperatures. Consequently, GdGaH1.66 desorbs H above T=200 degrees C. Without the presence of hydrogen, hydrides GdGaHx decompose at temperatures above 300 degrees C into GdH2 and an unidentified Gd-Ga intermetallics. Thus the hydrogenation of GdGa is not reversible. From magnetic measurements the Curie Weiss constant and effective magnetic moment of GdGaH1.66 were obtained. The former indicates antiferromagnetic interactions, the latter attains a value of similar to 8 mu B which is typical for compounds containing Gd3+ ions.

Place, publisher, year, edition, pages
2016. Vol. 239, 184-191 p.
Keyword [en]
Metal hydrides, Zintl phases, in situ powder diffraction
National Category
Inorganic Chemistry Engineering and Technology
Identifiers
URN: urn:nbn:se:uu:diva-299032DOI: 10.1016/j.jssc.2016.04.028ISI: 000377422000027OAI: oai:DiVA.org:uu-299032DiVA: diva2:948866
Funder
Swedish Research Council
Available from: 2016-07-14 Created: 2016-07-13 Last updated: 2016-08-26Bibliographically approved
In thesis
1. Metal Hydrogen Interaction and Structural Characterization of Amorphous Materials from first principles
Open this publication in new window or tab >>Metal Hydrogen Interaction and Structural Characterization of Amorphous Materials from first principles
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis, first-principles calculations based on density functional theory have been employed to investigate metal hydrogen interaction in transition, p-block and rare earth metals. Furthermore, the accuracy of the stochastic quenching method was tested in describing the structure of amorphous Fe(1-x)Zrx.

The investigated systems of transition metal hydrides are V-H and ScZr(CoNi)2-H. For V-H, the main focus of the studies is the effect that strain has on the potential energy landscape which governs the metal hydrogen interactions. The investigation has focused on how the properties of hydrogen occupancy in the interstitial sites changes with strain and also how the hydrogen atoms themselves exert strain on the vanadium structure to lower the energy. Results on diffusion, induced strain and zero-point energy are presented which all reveal the considerable difference between tetrahedral and octahedral site occupancy. Diffusion was studied by employing ab initio molecular dynamics simulations to obtain diffusion coefficients and to map the movement of the hydrogen atom. A description of hydrogen in vanadium is provided from a fundamental basis that is expected to be applicable to any lattice gas system. For ScZr(CoNi)2-H, the difference of hydrogen occupancy in various interstitial sites and the hydrogen-induced strain was also investigated through calculations of the change in total volume as a function of hydrogen concentration.

The fundamental properties of metal hydrogen bonding were investigated by studying the Zintl phase hydrides that are constituted of the electropositive metal of Nd or Gd and the electronegative metal Ga. Mixing metals of very different electronegativity gives rise to an intricate potential energy landscape in which the incorporation of hydrogen will have a big effect on both the electronic and atomic structure. From the theoretical side of the investigation, structural parameters are presented along with the density of states and Bader charge analysis to describe the hydrogen induced changes to the atomic and electronic structures.

Finally, the accuracy of the stochastic quenching method in describing amorphous Fe(1-x)Zrx was evaluated by comparing simulated and measured EXAFS spectra. Once the structural agreement had been established the simulated structures were characterized through radial distribution functions and an analysis of the short-range order from Voronoi tessellation. The structural changes with respect to the composition parameter x were also evaluated.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2016. 72 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1396
Keyword
hydrogen, vanadium, zintl, laves, strain, diffusion, amorphous, dft, molecular dynamics, md
National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-299940 (URN)978-91-554-9635-7 (ISBN)
External cooperation:
Public defence
2016-09-28, Å80127, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:15 (English)
Opponent
Supervisors
Available from: 2016-08-31 Created: 2016-07-29 Last updated: 2016-09-05

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Johansson, RobertAndersson, Mikael S.Nordblad, PerScheicher, Ralph H.Sahlberg, Martin
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