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Metal Hydrogen Interaction and Structural Characterization of Amorphous Materials from first principles
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
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 [en]
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: urn:nbn:se:uu:diva-299940ISBN: 978-91-554-9635-7OAI: oai:DiVA.org:uu-299940DiVA: diva2:950756
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
List of papers
1. Effect of uniaxial strain on the site occupancy of hydrogen in vanadium from density-functional calculations
Open this publication in new window or tab >>Effect of uniaxial strain on the site occupancy of hydrogen in vanadium from density-functional calculations
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2015 (English)In: Scientific Reports, ISSN 2045-2322, Vol. 5, 10301Article in journal (Refereed) Published
Abstract [en]

We investigate the influence of uniaxial strain on the site occupancy of hydrogen in vanadium, using density functional theory. The site occupancy is found to be strongly influenced by the strain state of the lattice. The results provide the conceptual framework for the atomistic description of the observed hysteresis in the alpha to beta phase transition in bulk, as well as the preferred octahedral occupancy of hydrogen in strained V layers.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-256841 (URN)10.1038/srep10301 (DOI)000355393500001 ()25993248 (PubMedID)
Available from: 2015-06-26 Created: 2015-06-26 Last updated: 2016-08-26Bibliographically approved
2. Effect of tetragonal distortion on the diffusion of hydrogen in vanadium studied with ab initio molecular dynamics
Open this publication in new window or tab >>Effect of tetragonal distortion on the diffusion of hydrogen in vanadium studied with ab initio molecular dynamics
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(English)Manuscript (preprint) (Other academic)
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-299491 (URN)
Available from: 2016-07-20 Created: 2016-07-20 Last updated: 2016-08-26Bibliographically approved
3. Hydrogen in vanadium: Site occupancy and isotope effects
Open this publication in new window or tab >>Hydrogen in vanadium: Site occupancy and isotope effects
2016 (English)In: PHYSICAL REVIEW B, ISSN 2469-9950, Vol. 93, no 13, 134107Article in journal (Refereed) Published
Abstract [en]

We discuss the influence of site occupancy on the absorption of the hydrogen isotopes H and D in thin V(001) layers. By growing V(001) under biaxial compressive strain in Fe/V(001) superlattices, the hydrogen (H as well as D) is forced to reside exclusively in octahedral (O-z) sites, even at the lowest concentrations. A weakening of the isotope effects is observed when hydrogen resides in octahedral as compared to tetrahedral sites.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-297117 (URN)10.1103/PhysRevB.93.134107 (DOI)000374298300001 ()
Funder
Swedish Research Council
Available from: 2016-06-22 Created: 2016-06-21 Last updated: 2016-08-26Bibliographically approved
4. The influence of site occupancy on diffusion of hydrogen in vanadium
Open this publication in new window or tab >>The influence of site occupancy on diffusion of hydrogen in vanadium
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(English)Manuscript (preprint) (Other academic)
National Category
Natural Sciences Physical Sciences
Identifiers
urn:nbn:se:uu:diva-275058 (URN)
Available from: 2016-01-28 Created: 2016-01-28 Last updated: 2016-08-26Bibliographically approved
5. Hydrogen storage properties of the pseudo binary Laves phase (Sc1-xZrx)(Co1-yNiy)2 system
Open this publication in new window or tab >>Hydrogen storage properties of the pseudo binary Laves phase (Sc1-xZrx)(Co1-yNiy)2 system
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2013 (English)In: International journal of hydrogen energy, ISSN 0360-3199, Vol. 38, no 23, 9772-9778 p.Article in journal (Refereed) Published
Abstract [en]

The (Sc1-xZrx)(Co1-yNiy)(2)-H-z system has been studied using both experimental techniques and ab initio calculations. The material was synthesised through high temperature synthesis and characterised using powder XRD. Hydrogen absorption and desorption was studied in-situ using synchrotron radiation. Maximal storage capacity increased when Co replaced Ni and substitution of Sc for Zr increased the equilibrium pressure. Density functional based calculations reproduce the experimental trends in terms of cell parameters both for the non-hydrogenated systems as well as for the hydrogenated systems, and helped to quantitatively understand the observed hydrogen uptake properties. 

Keyword
Laves phase, Metal hydride, Hydrogen storage, In-situ diffraction, Ab initio
National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-206981 (URN)10.1016/j.ijhydene.2013.05.053 (DOI)000322500800018 ()
Available from: 2013-09-09 Created: 2013-09-09 Last updated: 2016-08-26
6. Hydrogenation-Induced Structure and Property Changes in the Rare-Earth Metal Gallide NdGa: Evolution of a [GaH](2-) Polyanion Containing Peierls-like Ga-H Chains
Open this publication in new window or tab >>Hydrogenation-Induced Structure and Property Changes in the Rare-Earth Metal Gallide NdGa: Evolution of a [GaH](2-) Polyanion Containing Peierls-like Ga-H Chains
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2016 (English)In: Inorganic Chemistry, ISSN 0020-1669, E-ISSN 1520-510X, Vol. 55, no 1, 345-352 p.Article in journal (Refereed) Published
Abstract [en]

The hydride NdGaH1+x (x approximate to 0.66) and its deuterized analogue were obtained by sintering the Zintl phase NdGa with the CrB structure in a hydrogen atmosphere at pressures of 10-20 bar and temperatures near 300 degrees C. The system NdGa/NdGaH1+x exhibits reversible H storage capability. H uptake and release were investigated by kinetic absorption measurements and thermal desorption mass spectroscopy, which showed a maximum H concentration corresponding to "NdGaH2" (0.93 wt % H) and a two-step desorption process, respectively. The crystal structure of NdGaH1+x was characterized by neutron diffraction (P2(1)/m, a = 4.1103(7), b = 4.1662(7), c = 6.464(1) angstrom, beta = 108.61(1)degrees Z = 2). H incorporates in NdGa by occupying two distinct positions, H1 and H2. HI is coordinated in a tetrahedral fashion by Nd atoms. The H2 position displays flexible occupancy, and H2 atoms attain a trigonal bipyramidal coordination by centering a triangle of Nd atoms and bridging two Ga atoms. The phase stability and electronic structure of NdGaH1+x, were analyzed by first-principles DFT calculations. NdGaH1H2 (NdGaH2) may be expressed as Nd3+(H1(-)[GaH2](2-). The two-dimensional polyanion [GaH](2-) features linear -H-Ga-H-Ga- chains with alternating short (1.8 A) and long (2.4 angstrom) Ga-H distances, which resembles a Peierls distortion. H2 deficiency (x < 1) results in the fragmentation of chains. For x = 0.66 arrangements with five-atom moieties, Ga-H-Ga-H-Ga are energetically most favorable. From magnetic measurements, the Curie-Weiss constant and effective magnetic moment of NdGaH1.66 were obtained. The former indicates antiferromagnetic interactions, and the latter attains a value of similar to 3.6 mu(B), which is typical for compounds containing Nd3 ions.

National Category
Inorganic Chemistry Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-275549 (URN)10.1021/acs.inorgchem.5b02485 (DOI)000367706100042 ()
Funder
EU, FP7, Seventh Framework Programme, 284522Swedish Research Council
Available from: 2016-02-04 Created: 2016-02-04 Last updated: 2016-08-26Bibliographically approved
7. Hydrogenation induced structure and property changes in GdGa
Open this publication in new window or tab >>Hydrogenation induced structure and property changes in GdGa
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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) Published
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.

Keyword
Metal hydrides, Zintl phases, in situ powder diffraction
National Category
Inorganic Chemistry Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-299032 (URN)10.1016/j.jssc.2016.04.028 (DOI)000377422000027 ()
Funder
Swedish Research Council
Available from: 2016-07-14 Created: 2016-07-13 Last updated: 2016-08-26Bibliographically approved
8. Structural characterization of amorphous Fe(1-x)Zrx
Open this publication in new window or tab >>Structural characterization of amorphous Fe(1-x)Zrx
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(English)Manuscript (preprint) (Other academic)
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-299490 (URN)
Available from: 2016-07-20 Created: 2016-07-20 Last updated: 2016-08-26Bibliographically approved

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