| 1. |
- Droulias, S. A., et al.
(författare)
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Influence of deuterium-induced volume changes on optical transmission in Fe/V (001) and Cr/V (001) superlattices
- 2022
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 105:19
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Tidskriftsartikel (refereegranskat)abstract
- The deuterium-induced changes of the optical transmission in Fe/V (001) and Cr/V (001) superlattices are found experimentally to be dominated by the volume changes of the vanadium layers and thus indirectly linked to concentration. The deuterium-induced expansion is 67% larger in Cr/V 2/14 monolayers (ML) as compared to Fe/V 2/14 ML. This large difference can be explained by a difference in the site of deuterium from tetrahedral in Fe/V to octahedral in Cr/V. First-principles calculations based on this assumption give quantitative agreement with both the measured optical transmission and the deuterium-induced expansion coefficient. Placing hydrogen in the middle of the vanadium layers results in total energies at 0 K that favor tetrahedral occupancy at low concentrations, although the energy difference is of the order of the thermal energy available in the experiments. Hence small changes in strain, defect concentration, and/or vibrational spectrum of the superlattices may tip the balance to octahedral occupancy at low concentrations. Given this link to concentration and the linear scaling, optical transmission can, therefore, be used in a straightforward way to obtain pressure-composition isotherms also in thin metal films that do not undergo metal-insulator transitions upon hydrogenation.
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| 2. |
- Droulias, Sotirios A., et al.
(författare)
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Tailoring thermodynamic properties of metal hydrides with interfaces
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- We report profound changes in the enthalpy of solution and critical temperature of nano-sizedmetal hydrides in proximity to dierent non-absorbing metals. We use Cr/V and Fe/V (001) superlatticesas model systems to map out these eects since the superlattices can be made singlecrystalline,have identical strain states and are fully reversible. The thermodynamic propertiesare determined using isothermal pressure and optical transmission measurements. We determinethe mechanism underlying the changes in the thermodynamic properties as well as the ecacyof optical transmission by utilizing simultaneous neutron reectometry and optical transmissionmeasurements, combined with rst principle calculations. The underlying mechanism, which issupported by quantitative agreement between theory and experiment is found to be rooted in aradically dierent volume expansion in the two systems. The commonly used optical transmissionmethod is found to be linear with concentration in both types of superlattices and is attributed tochanges in electron density, rather than a direct inuence from a redistribution of electronic states.The conclusions are generalized to other metal hydride systems and can open up for exploration ofproximity induced eects in metal hydrides.
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| 3. |
- Droulias, Sotirios A., et al.
(författare)
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Using interfaces to influence thermodynamic properties of metal hydrides
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- We report profound proximity effects on the enthalpy of solution and critical temperature ofnano-sized vanadium hydrides. We use single crystalline Cr/V and Fe/V (001) superlattices forthese studies, in which the V layers are under close to identical strain. The thermodynamic propertiesare determined using isothermal pressure and optical transmission measurements, utilisingthe fully reversibel hydrogen uptake and release.The underlying mechanism is argued to be rootedin a radically different volume expansion in the two systems, arising from different site occupancyof hydrogen in Fe/V(001) and Cr/V(001). The optical transmission is found to be linear withconcentration in both types of superlattices, while exhibiting different slopes. The differences areattributed to changes in electron density arising from the volume expansion, rather than a directinfluence from a redistribution of electronic states.
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| 4. |
- Komander, Kristina, et al.
(författare)
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Accurate measurement of hydrogen concentration in transition metal hydrides utilizing electronic excitations by MeV ions
- 2024
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Ingår i: International journal of hydrogen energy. - : Elsevier. - 0360-3199 .- 1879-3487. ; 57, s. 583-588
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Tidskriftsartikel (refereegranskat)abstract
- This study focuses on enhancing the accuracy of hydrogen content verification in hydrogen-rich ultrathin materials relevant for sustainable energy applications. Ion beams are used for distinctive real-space detection leveraging elastic and inelastic interactions with hydrogen atoms. However, the lack of experimental reference data on electronic interactions poses a challenge to the accuracy of analytical techniques. We investigate the effect of absorbed hydrogen on the electronic energy deposition of 15N-ions in amorphous transition metal compounds, specifically V and Zr, covering concentrations >1H/M. Employing resonant nuclear reactions and Rutherford backscattering, the energy loss is found to increase considerably with hydrogen content, in line with Bragg's additivity. The electronic energy loss cross section for 15N-ions at 6.5 MeV measured (64.55±3.38) eV cm2/1015 atoms. Results are compared to semi-empirical and theoretical models. The findings improve hydrogen profiling accuracy using 15N-nuclear reaction analysis and enable unprecedented methods for hydrogen quantification by other, commonly available ion beams.
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| 5. |
- Komander, Kristina, et al.
(författare)
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Hydrogen induced lattice expansion and site occupation analyzed by ion beam methods
- 2021
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Ingår i: Nuclear Instruments and Methods in Physics Research Section B. - : Elsevier. - 0168-583X .- 1872-9584. ; 486, s. 63-67
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Tidskriftsartikel (refereegranskat)abstract
- We study the absorption of H in ultrathin crystalline Fe/V stacks using elastic recoil detection (ERD) and resonant nuclear reaction analysis (NRA) combined with Rutherford backscattering spectrometry (RBS). Probing with the resonant H-1 (N-15,alpha gamma) C-12 nuclear reaction allows us to profile the hydrogen concentration with high depth resolution, while ERD permits us to quantify the integral hydrogen inventory while minimizing possible ionbeam induced hydrogen desorption. We perform angular scans in order to directly identify the interstitial site occupancy. A detailed analysis of channeling patterns permits us the deduction of crystal strain in as-deposited films as well as H-loaded targets. We observe tetragonal strain formation resulting in c/a = 1.06 and find qualitative indications for tetrahedral site occupation.
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| 6. |
- Komander, Kristina, et al.
(författare)
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Hydrogen site location in ultrathin vanadium layers by N-15 nuclear reaction analysis
- 2019
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Ingår i: Nuclear Instruments and Methods in Physics Research Section B. - : ELSEVIER. - 0168-583X .- 1872-9584. ; 455, s. 57-60
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Tidskriftsartikel (refereegranskat)abstract
- We present a method using resonant nuclear reaction analysis combined with optical transmission and heavy-ion Rutherford backscattering spectrometry to study the absorption of hydrogen in single crystalline thin vanadium films. Probing with the resonant H-1(N-15,alpha gamma)C-12 reaction allows for highly resolved hydrogen depth profiling, while measurements along the crystal axes render possible the direct identification of the interstitial site occupancy. First experiments were performed on thin vanadium hydrides in Fe(Cr)/V superlattices revealing differences in site occupancy.
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| 7. |
- Komander, Kristina, et al.
(författare)
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Interstitial Hydrogen in Fe/V Superstructures : Lattice Site Location and Thermal Vibration
- 2021
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Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 127:13
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Tidskriftsartikel (refereegranskat)abstract
- We report real space location of hydrogen in single crystalline Fe/V superstructures. Anisotropic strain is quantified versus hydrogen concentration by using the yield of backscattered primary 2 MeV He-4 ions for incidence in different crystallographic directions. From a comparison of ion channeling in combination with H-1(N-15, alpha gamma)C-12 nuclear reaction analysis and Monte Carlo simulations we show that hydrogen is located in octahedral z sites and quantify its vibrational amplitude of 0.2 angstrom.
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| 8. |
- Komander, Kristina, et al.
(författare)
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Lattice site location in metal hydride thin films: Ion channeling and phononic properties
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- We investigate the relationship between lattice vibrations and ion channeling in hydrogenated strained thin V films within Fe/V- and Cr/V-superlattices, performing 15N-ion channeling experiments and Monte-Carlo simulations with the program FLUX7. Comparing simulated 15N-nuclear encounter probabilities (NEP) to Fe/V-atoms to the yield of backscattered 15N-ions demonstrates a correlation between the influence of beam divergence and Debye temperatures and provides insights into distinguishing them quantitatively from planar channeling patterns. Simulations of 15N-NEP to interstitial hydrogen and comparison to 1H(15N, αγ)12C nuclear reaction yield highlights the importance of analyzing multiple crystal symmetries to determine vibrational motions and impurity site locations accurately. The analysis of thermal vibrational motions in hydrogenated Fe/V- and CrV-superlattices reveal a reduction in Debye temperatures for Fe/V- and CrV-superlattices, emphasizing the influence of defect, surface, and interface effects on lattice vibrations.
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| 9. |
- Komander, Kristina
(författare)
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Nanoscale Metal Hydrides from MeV-Ion Perspectives: A Unique View
- 2024
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis work set out to investigate interstitial hydrogen in material systems of reduced dimensionality or high loading capacities. Experimental methodologies utilizing MeV-ions were developed addressing metal hydride formation in various transition metal alloys and in the presence of interfaces and lattice strain. Enhanced concentration measurement accuracy by 15N-resonant nuclear reaction analysis (NRA) in hydrogen-rich transition metal films was achieved by examining the influence of hydrogen on electronic excitations by 15N-ions. A non-iterative method combining NRA with Rutherford backscattering spectrometry (RBS) revealed the energy deposition of 15N-ions in various hydrogenated V/Zr-alloys, promoting reliable energy loss predictions. The findings imply the potential for indirect hydrogen concentration measurements via RBS, increasing accessibility across multiple facilities as a broader range of ion species can be utilized.To identify locations of interstitial hydrogen within thin single-crystalline films, the applicability of different MeV-ions species was explored, performing channeling experiments on Fe/V-superlattices as model systems. 4He-ions reveal hydrogen-induced anisotropic lattice expansion by RBS, while dechanneling effects and hydrogen recoils provide insights into lattice site locations. However, challenges for unambiguous site-location extraction were identified concerning deflections from channeling trajectories and limitations of simulations confined to backscattering geometry. Employing 15N-ions for channeling NRA and RBS, combined with Monte-Carlo simulations, enables quantitative real-space investigation of subsurface hydrogen site locations and thermal vibrational motion in nanosized transition metals. The 15N-channeling method was applied to study the influence of different adjacent metals on vanadium hydrides, comparing hydrogenated Fe/V- to Cr/V-superlattices. In combination with in-situ measurements of electrical resistivity and optical transmission – which serve as indirect measures for hydrogen order and concentration, respectively – the investigation revealed significant proximity effects. Specifically, Fe/V absorbs smaller integral amounts of hydrogen, and the critical temperature is lower than for Cr/V. Thereby, hydrogen atoms occupy octahedral z-sites with identical thermal vibrational amplitudes of 0.20-0.25 Å in both samples over an extensive range of hydrogen concentrations. The findings are consistent with the effects of a size reduction induced by hydrogen-depleted layers at interfaces, larger toward Fe than to Cr.The developed methodologies offer unique insights into alloying and boundary effects on metal hydride formation, which can control phase transitions and enhance the loading capacity, providing avenues to optimize material properties for sustainable energy solutions.
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| 10. |
- Komander, Kristina, et al.
(författare)
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Proximity and dimensionality effects on interstitial hydrogen
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- In this study, we investigate the proximity effects on hydrogen distribution in ultrathin single crystalline vanadium films, by comparing the absorption in Fe/V- and Cr/V-superlattices. Pressure concentration isotherms show higher absolute solubility of hydrogen in a Cr/V-superlattice. Isotherms of excess resistivity are used to locate phase boundaries which yielded a higher critical temperature and concentration in Cr/V. Measurements of hydrogen site location and thermal vibrations using 15N-nuclear reaction analysis demonstrate identical site occupation of 0Z at room temperature with a vibrational amplitude of 0.20‑0.25 Å over a wide range of hydrogen concentrations in both superlattices. The analysis excludes changes in site occupancy as the primary cause for variations in thermodynamics across different temperatures due to proximity. Our findings are consistent with a more extended region of hydrogen depletion in the vicinity of Fe and resolve previously puzzling differences in the hydrogen loading of Fe/V- and Cr/V-superlattices, implying an inverse spillover effect.
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