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Johansson, Robert
Publications (10 of 13) Show all publications
Frisk, A., Ahlberg, M., Muscas, G., George, S., Johansson, R., Klysubun, W., . . . Andersson, G. (2019). Magnetic and structural characterization of CoFeZr thin films grown by combinatorial sputtering. Physical Review Materials, 3(7), Article ID 074403.
Open this publication in new window or tab >>Magnetic and structural characterization of CoFeZr thin films grown by combinatorial sputtering
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2019 (English)In: Physical Review Materials, E-ISSN 2475-9953, Vol. 3, no 7, article id 074403Article in journal (Refereed) Published
Abstract [en]

We report on a detailed investigation of structural and magnetic properties of thin CoFeZr films, produced by combinatorial cosputtering, with compositions in the ranges 30–85 at.% Co, 12–63 at.% Fe, and 4–8 at.% Zr. Extended x-ray absorption fine structure and x-ray diffraction measurements reveal that alloys with a Zr content below 5 at.% are polycrystalline with a bcc structure, while an amorphous morphology is stabilized at Zr contents above 6 at.%. All samples display a growth-induced in-plane uniaxial anisotropy, which is closely related to the Zr concentration gradients across the wafers. A model for the angular dependence of the reduced remanence, including a Gaussian distribution of easy/hard anisotropy axes, is presented and successfully used to fit the data for all samples. The magnetic moments of the polycrystalline films approximately follow the Slater-Pauling curve, and the magnetic moments of the amorphous films follow a similar trend, but with about 20 % lower values. X-ray magnetic circular dichroism measurements show, for the amorphous films, that the Co moments are virtually constant at 1.7(2)μB/atom.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-305520 (URN)10.1103/PhysRevMaterials.3.074403 (DOI)000474390900005 ()
Funder
Swedish Research Council, C0514401Swedish Foundation for Strategic Research , RIF-14-0053
Available from: 2016-10-22 Created: 2016-10-18 Last updated: 2019-08-21Bibliographically approved
Mooij, L., Huang, W., Droulias, S. A., Johansson, R., Hartmann, O., Xin, X., . . . Hjörvarsson, B. (2017). The influence of site occupancy on diffusion of hydrogen in vanadium. Physical Review B, 95(6), Article ID 064310.
Open this publication in new window or tab >>The influence of site occupancy on diffusion of hydrogen in vanadium
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2017 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 6, article id 064310Article, review/survey (Refereed) Published
Abstract [en]

We investigate the effect of site occupancy on the chemical diffusion of hydrogen in strained vanadium. The diffusion rate is found to decrease substantially, when hydrogen is occupying octahedral sites as compared to tetrahedral sites. Profound isotope effects are observed when comparing the diffusion rate of H and D. The changes in the diffusion rate are found to be strongly influenced by the changes in the potential energy landscape, as deduced from first-principles molecular dynamics calculations.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-275058 (URN)10.1103/PhysRevB.95.064310 (DOI)000395988800002 ()
Note

The manuscript version of this article is part of two PhD theses: http://uu.diva-portal.org/smash/record.jsf?pid=diva2:900624

http://uu.diva-portal.org/smash/record.jsf?pid=diva2:950756

Available from: 2016-01-28 Created: 2016-01-28 Last updated: 2018-05-14Bibliographically approved
Xin, X., Johansson, R., Wolff, M. & Hjörvarsson, B. (2016). Hydrogen in vanadium: Site occupancy and isotope effects. PHYSICAL REVIEW B, 93(13), Article ID 134107.
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, article id 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
Nedumkandathil, R., Kranak, V. F., Johansson, R., Ångström, J., Balmes, O., Andersson, M. S., . . . Haussermann, U. (2016). Hydrogenation induced structure and property changes in GdGa. Journal of Solid State Chemistry, 239, 184-191
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, p. 184-191Article 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.

Keywords
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: 2018-06-08Bibliographically approved
Ångström, J., Johansson, R., Sarkar, T., Sorby, M. H., Zlotea, C., Andersson, M. S., . . . Sahlberg, M. (2016). 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. Inorganic Chemistry, 55(1), 345-352
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, p. 345-352Article 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: 2018-06-08Bibliographically approved
Johansson, R. (2016). Metal Hydrogen Interaction and Structural Characterization of Amorphous Materials from first principles. (Doctoral dissertation). Uppsala: Acta Universitatis Upsaliensis
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. p. 72
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1396
Keywords
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
Johansson, R., Ahuja, R., Eriksson, O., Hjörvarsson, B. & Scheicher, R. H. (2015). Effect of uniaxial strain on the site occupancy of hydrogen in vanadium from density-functional calculations. Scientific Reports, 5, Article ID 10301.
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, E-ISSN 2045-2322, Vol. 5, article id 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: 2017-12-04Bibliographically approved
Ångström, J., Johansson, R., Rude, L. H., Gundlach, C., Scheicher, R. H., Ahuja, R., . . . Sahlberg, M. (2013). Hydrogen storage properties of the pseudo binary Laves phase (Sc1-xZrx)(Co1-yNiy)2 system. International journal of hydrogen energy, 38(23), 9772-9778
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, E-ISSN 1879-3487, Vol. 38, no 23, p. 9772-9778Article 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. 

Keywords
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: 2017-12-06
Johansson, R., Pálsson, G. K., Ahuja, R., Eriksson, O., Hjörvarsson, B. & Scheicher, R. H.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
Ångström, J., Johansson, R., Balmes, O., Sørby, M. H., Scheicher, R. H., Ulrich, H. & Sahlberg, M.Hydrogen Absorption in Rare-Earth-Gallide Zintl-Phases LnGa (Ln=Nd, Gd).
Open this publication in new window or tab >>Hydrogen Absorption in Rare-Earth-Gallide Zintl-Phases LnGa (Ln=Nd, Gd)
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(English)Manuscript (preprint) (Other academic)
Keywords
Zintl phase, Metal Hydride, Neutron Diffraction, in-situ diffraction, structure determination
National Category
Inorganic Chemistry
Research subject
Chemistry
Identifiers
urn:nbn:se:uu:diva-245042 (URN)
Funder
Swedish Research Council
Available from: 2015-02-24 Created: 2015-02-24 Last updated: 2015-09-07
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