Electronic and bonding properties of MgH2–Nb containing vacancies

Luna, Carla Romina; Macchi, C.; Juan, Alfredo; Somoza, A.

doi:10.1016/j.ijhydene.2010.08.111

Cited by 14 publications

(15 citation statements)

References 51 publications

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“…The important role of hydrogen vacancies for dehydrogenation in hydrides was confirmed in studies on AlH 3 , Li amide (LiNH 2 ), Li imide (Li 2 NH), LiBH 4 and Li 4 BN 3 H 10 [10,11,12,13,14]. It was also shown that transition metal additives in MgH 2 lower the formation energy of native defects and increase their concentration resulting in higher hydrogen desorption rates [15,16,17,18]. Tao et al [19] identified diffusion paths for hydrogen defects in different MgH 2 structures.…”

Section: Introductionmentioning

confidence: 78%

Point-defect kinetics in α- and γ-MgH2

Sander

Ismer

Walle

2016

International Journal of Hydrogen Energy

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The kinetics of hydrogen desorption from storage materials in principle depend on the crystalline phase of the material. In MgH 2 , desorption rates may be higher in the crystalline γ phase compared to the equilibrium bulk α phase. It has been suggested [R. A. Varin, T. Czujko, Z. Wronski, Nanotechnol., 17 (15) (2006) 3856-3865] that this effect is responsible for enhanced desorption from ball-milled MgH 2 , since smaller particles contain a higher proportion of the metastable γ phase. We investigate hydrogen transport kinetics in these phases of MgH 2 by using first-principles calculations based on density functional theory. Imposing charge neutrality, we find that the formation energy of hydrogen vacancies in γ-MgH 2 is smaller by 0.032 eV compared to α-MgH 2. Our calculations of migration barriers show that the only relevant point defect for mass transport in both crystal structures is the positively charged hydrogen vacancy, and that its lowest migration barrier in γ-MgH 2 is 0.02 eV lower than in α-MgH 2. We conclude that hydrogen vacancies exist in higher concentrations and are also more mobile in the γ phase than in the α phase, thus explaining the faster dehydrogenation kinetics of γ-MgH 2 .

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Section: Introductionmentioning

confidence: 78%

Point-defect kinetics in α- and γ-MgH2

Sander

Ismer

Walle

2016

International Journal of Hydrogen Energy

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“…The lifetime components are defined as follows: τ A = 230 ± 1 ps (95.6%), τ B = 515 ± 20 ps (2.8%), τ C = 2.1 ± 0.1 ns (0.7%), τ F = 219 ± 1 ps. The short-lived component τ A = 230 ± 1 ps is slightly higher than the calculated values specific for magnesium hydride τ MgH 2 = 220 ± 1 ps [ 26 , 43 ], but much lower than the values typical of vacancy-type defects in this material τ V(MgH 2 ) = 245 ± 6 ps [ 26 ], which is apparently caused by a mixed state. The long-lived components τ B = 515 ± 20 ps and τ C = 2.1 ± 0.1 ns are also associated with positron annihilation on the powder surface and in nanopores, respectively.…”

Section: Resultsmentioning

confidence: 63%

“…Table 1 shows that all lifetime spectra are well decomposed into four exponential components related to the annihilation of positrons at a different state in magnesium powder. The component with the shortest lifetime τ F = 226 ± 1 ps corresponds to the positrons’ annihilation in a defect-free lattice of magnesium [ 26 , 27 , 28 , 29 ]. The second component with a lifetime of τ A = 232 ± 1 ps is significantly lower than the values typical for monovacancies (253 ps [ 27 , 30 ], 292 ps [ 26 ]), and dislocations (244 ± 4 ps [ 28 , 31 , 32 ]).…”

Section: Resultsmentioning

confidence: 99%

“…The component with the shortest lifetime τ F = 226 ± 1 ps corresponds to the positrons’ annihilation in a defect-free lattice of magnesium [ 26 , 27 , 28 , 29 ]. The second component with a lifetime of τ A = 232 ± 1 ps is significantly lower than the values typical for monovacancies (253 ps [ 27 , 30 ], 292 ps [ 26 ]), and dislocations (244 ± 4 ps [ 28 , 31 , 32 ]). This component seems to be related to deformation defects (dislocations associated with twin boundaries [ 32 ]), which are typical for milled powders.…”

Section: Resultsmentioning

confidence: 99%

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Positron Annihilation Spectroscopy Complex for Structural Defect Analysis in Metal–Hydrogen Systems

et al. 2022

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The current work is devoted to developing a system for the complex research of metal–hydrogen systems, including in an in situ mode. The system consists of a controlled gas reactor with a unique reaction chamber, a radioisotope positron source, and a positron annihilation spectroscopy complex. The use of the system enables in situ investigation of the defect structure of solids in hydrogen sorption–desorption processes at temperatures up to 900 °C and pressures up to 50 bar. Experimental investigations of magnesium and magnesium hydride during thermal annealing were carried out to approve the possibilities of the developed complex. It was shown that one cycle of magnesium hydrogenation–dehydrogenation resulted in the accumulation of irreversible hydrogen-induced defects. The defect structure investigation of the magnesium–hydrogen system by positron annihilation techniques was supplemented with a comprehensive study by scanning electron microscopy, X-ray diffraction analysis, and hydrogen sorption–desorption studies.

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“…In the literature have been reported several studies about the microscopic mechanisms operating in the hydrogenation and dehydrogenation process in magnesium hydride, which are related to the formation and diffusivity of vacancies in bulk MgH 2 . − For example, Hao et al used density functional theory (DFT) calculation to study the influence of charged defects on the diffusion of H in MgH 2 and NaMgH 3 . These authors found that the physically relevant defects are charged and that H diffusion is dominated by the mobility of negatively charged H interstitials.…”

Section: Introductionmentioning

confidence: 99%

Geometric, Electronic, and Magnetic Properties of MgH₂: Influence of Charged Defects

et al. 2016

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A study on the effect of the presence of charged vacancy on the electronic and magnetic properties of perfect magnesium hydride is presented. To this aim spin polarized ab initio calculations for the MgH 2 structure containing a H vacancy or a Mg vacancy or a H−Mg divacancy were used. For each case three possible charge states (q = +1, 0, and −1) were taken into account. The calculated parameters were the vacancy formation energy, band gap, magnetic moment, Fermi level position with respect to the top of the valence band, and the bottom of the conduction band and density of states curves. From the calculations, it was found that positive and negative charged H vacancies and the negative charged divacancy are the most probable formed defects. Besides, the presence of a negative or neutral H vacancy produces an important reduction of the band gap, which should improve the semiconductor behavior of the material. Furthermore, the charged H vacancies provoke an important local rearrangement in the structure of the hydride. However, the positive charged Mg vacancy induces the highest magnetic moment.

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Electronic and bonding properties of MgH2–Nb containing vacancies

Cited by 14 publications

References 51 publications

Point-defect kinetics in α- and γ-MgH2

Point-defect kinetics in α- and γ-MgH2

Positron Annihilation Spectroscopy Complex for Structural Defect Analysis in Metal–Hydrogen Systems

Geometric, Electronic, and Magnetic Properties of MgH₂: Influence of Charged Defects

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Electronic and bonding properties of MgH2–Nb containing vacancies

Cited by 14 publications

References 51 publications

Point-defect kinetics in α- and γ-MgH2

Point-defect kinetics in α- and γ-MgH2

Positron Annihilation Spectroscopy Complex for Structural Defect Analysis in Metal–Hydrogen Systems

Geometric, Electronic, and Magnetic Properties of MgH2: Influence of Charged Defects

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Geometric, Electronic, and Magnetic Properties of MgH₂: Influence of Charged Defects