Trapping of positrons at vacancies in magnesium

Hautojärvi, P.; Johansson, J.; Vehanen, A.; Yli‐Kauppila, J.; Hillairet, J.; Tzanétakis, P.

doi:10.1007/bf01197546

Cited by 56 publications

(39 citation statements)

References 18 publications

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“…In the case of positron trapping at vacancies in Mg, the calculated positron lifetime increase is from 219 to 289 ps, whereas the much smaller increase from 225 to 255 ps has been measured between a well-annealed sample and a sample with thermally generated vacancies. 43 The calculated large lifetime increase is also reflected in a rather small W rel parameter of 0.56 for V Mg . However, the calculated positron trapping energy of 0.34 eV is in agreement with the estimate of 0.3-0.4 eV by Hautojärvi et al 43 One possible source of the theory-experiment disagreement could be difficulties in extracting bulk and vacancy lifetimes for Mg in measurements.…”

Section: Discussion and Comparison With Experimentsmentioning

confidence: 94%

“…43 The calculated large lifetime increase is also reflected in a rather small W rel parameter of 0.56 for V Mg . However, the calculated positron trapping energy of 0.34 eV is in agreement with the estimate of 0.3-0.4 eV by Hautojärvi et al 43 One possible source of the theory-experiment disagreement could be difficulties in extracting bulk and vacancy lifetimes for Mg in measurements.…”

Section: Discussion and Comparison With Experimentsmentioning

confidence: 94%

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Energetics of positron states trapped at vacancies in solids

Makkonen

Puska

2007

Phys. Rev. B

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We report a computational first-principles study of positron trapping at vacancy defects in metals and semiconductors. The main emphasis is on the energetics of the trapping process including the interplay between the positron state and the defect's ionic structure and on the ensuing annihilation characteristics of the trapped state. For vacancies in covalent semiconductors the ion relaxation is a crucial part of the positron trapping process enabling the localization of the positron state. However, positron trapping does not strongly affect the characteristic features of the electronic structure, e.g., the ionization levels change only moderately. Also, in the case of metal vacancies, the positron-induced ion relaxation has a noticeable effect on the calculated positron lifetime and momentum distribution of annihilating electron-positron pairs.

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Section: Discussion and Comparison With Experimentsmentioning

confidence: 94%

Section: Discussion and Comparison With Experimentsmentioning

confidence: 94%

Energetics of positron states trapped at vacancies in solids

Makkonen

Puska

2007

Phys. Rev. B

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“…This can be understood since positron studies by Hautojaervi et al 20 and Segers et al 19 using defect quenching and annealing of Mg samples demonstrated that Mg vacancies are highly mobile ͑the migration energy is ϳ0.5 eV͒, and a complete annealing of vacancies and small vacancy clusters occurs well below room temperature. Appreciable defect concentrations on the order of 10 −6 or higher are reached only at temperatures above about 700 K in thermodynamic equilibrium, 19,20 and positron trapping ͓the positron trapping energy is ϳ0.34 eV ͑Ref. 21͔͒ in these thermally generated vacancies was detected only at such high temperatures.…”

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confidence: 99%

Positron depth profiling of the structural and electronic structure transformations of hydrogenated Mg-based thin films

Eijt

Kind

Singh

et al. 2009

Journal of Applied Physics

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We report positron depth-profiling studies on the hydrogen sorption behavior and phase evolution of Mg-based thin films. We show that the main changes in the depth profiles resulting from the hydrogenation to the respective metal hydrides are related to a clear broadening in the observed electron momentum densities in both Mg and Mg 2 Ni films. This shows that positron annihilation methods are capable of monitoring these metal-to-insulator transitions, which form the basis for important applications of these types of films in switchable mirror devices and hydrogen sensors in a depth-sensitive manner. Besides, some of the positrons trap at the boundaries of columnar grains in the otherwise nearly vacancy-free Mg films. The combination of positron annihilation and x-ray diffraction further shows that hydrogen loading at elevated temperatures, in the range of 480-600 K, leads to a clear Pd-Mg alloy formation of the Pd catalyst cap layer. At the highest temperatures, the hydrogenation induces a partial delamination of the ϳ5 nm thin capping layer, as sensitively monitored by positron depth profiling of the fraction of ortho-positronium formed at interface with the cap layer. The delamination effectively blocks the hydrogen cycling. In Mg-Si bilayers, we investigated the reactivity upon hydrogen loading and heat treatments near 480 K, which shows that Mg 2 Si formation is fast relative to MgH 2 . The combination of positron depth profiling and transmission electron microscopy shows that hydrogenation promotes a complete conversion to Mg 2 Si for this destabilized metal hydride system, while a partially unreacted, Mg-rich amorphous prelayer remains on top of Mg 2 Si after a single heat treatment in an inert gas environment. Thin film studies indicate that the difficulty of rehydrogenation of Mg 2 Si is not primarily the result from slow hydrogen dissociation at surfaces, but is likely hindered by the presence of a barrier for removal of Mg from the readily formed Mg 2 Si.

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“…19 Finally, the PALS studies showed that saturation trapping and annihilation in vacancy-related defects occurs inside the Mg areas of the Mg-Ti͑-H͒ films. POSWIN 21 The comparison with experimental and theoretical positron lifetimes for the monovacancy 22,23 and divacancy 21 in Mg strongly indicates that trapping occurs in divacancies. Clearly, the detected positron lifetime in the Mg 1−y Ti y metal phase scales proportional to the volume per metal atom 3,4 ͑Fig.…”

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confidence: 99%

Divacancies and the hydrogenation of Mg-Ti films with short range chemical order

Leegwater

Schüt

Egger

et al. 2010

Applied Physics Letters

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We obtained evidence for the partial chemical segregation of as-deposited and hydrogenated Mg 1−y Ti y films ͑0 Յ y Յ 0.30͒ into nanoscale Ti and Mg domains using positron Doppler-broadening. We exclusively monitor the hydrogenation of Mg domains, owing to the large difference in positron affinity for Mg and Ti. The electron momentum distribution broadens significantly upon transformation to the MgH 2 phase over the whole compositional range. This reveals the similarity of the metal-insulator transition for rutile and fluorite MgH 2 . Positron lifetime studies show the presence of divacancies in the as-deposited and hydrogenated Mg-Ti metal films. In conjunction with the relatively large local lattice relaxations we deduce to be present in fluorite MgH 2 , these may be responsible for the fast hydrogen sorption kinetics in this MgH 2 phase. © 2010 American Institute of Physics. ͓doi:10.1063/1.3368698͔Mg-Ti alloys are promising materials for application as hydrogen storage media, metal hydride rechargeable batteries, hydrogen sensors, and smart solar collectors. 1,2 Hydrogenation of Mg-Ti films with Ti-concentrations larger than ϳ15% ͑Mg 0.85 Ti 0.15 ͒ leads to the formation of a fluorite Mg 1−y Ti y H x phase with substantially faster hydrogenation kinetics than the common rutile MgH 2 phase. At the same time, a high hydrogen storage capacity of up to 6.5 wt % capacity is reached for Mg 0.80 Ti 0.20 . 1 The Mg-Ti͑-H͒ films have an intriguing microstructure. While Ti and Mg are immiscible metals on a macroscopic scale, codeposition of Mg and Ti by magnetron sputtering leads to Mg-Ti films with a coherent structure, as indicated by x-ray and electron diffraction studies, 3-5 and a very high degree of intermixing of the Mg and Ti. Extended x-ray absorption fine structure ͑EXAFS͒ studies, in contrast, indicated that atomically mixed alloys are not formed. Instead, a partial chemical segregation into Mg and Ti domains on a length scale of the order of less than 10 nm was suggested. 4 The stabilization of the cubic fluorite phase is thought to occur via elastic coupling of MgH 2 to the nanoscale TiH 2 domains. This coupling remains effective during the full ͑de-͒hydrogenation cycle because of the similarity of the unit cell volumes of cubic TiH 2 and hexagonal closed packed Mg in the intermediate state. 4 A direct observation of these phase segregated domains, however, is currently lacking.Furthermore, the fast hydrogen kinetics of these types of films-especially in the cubic hydride phase formed at Ti concentrations larger than 15%-is not well understood. The metal hydride lattice contracts upon increased incorporation of Ti, leading to reduced space for hydrogen mobility. 3,4 It was suggested that the unoccupied octahedral positions in the fluorite MgH 2 structure could aid the hydrogen mobility. 6 Ab initio studies indicate that vacancies form a key factor in the hydrogen diffusion of both the rutile and fluorite MgH 2 phases. [7][8][9] Further insight into the occurrence of vacancies and their possible role in t...

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Trapping of positrons at vacancies in magnesium

Cited by 56 publications

References 18 publications

Energetics of positron states trapped at vacancies in solids

Energetics of positron states trapped at vacancies in solids

Positron depth profiling of the structural and electronic structure transformations of hydrogenated Mg-based thin films

Divacancies and the hydrogenation of Mg-Ti films with short range chemical order

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