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

Abstract

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...

Layer-resolved study of the Mg to MgH2 transformation in Mg–Ti films with short-range chemical order

“…Upon hydrogenation, the S-parameter of the Mg 0.70 Ti 0.30 layer reduces significantly in the range of 1-5 keV in positron implantation energy, for which a substantial fraction of positrons annihilates in the metal (hydride) layer. The change in S-parameter is similar in magnitude as for the Mg to MgH 2 transformation in Mg [5][6][7] and Mg 0.90 Ti 0.10 films [7]. Our previous studies [5,15] demonstrated that such a large reduction in S-parameter is a direct result of the metal-insulator transition upon hydrogenation to the MgH 2 phase.…”

“…[7], we showed that the extracted positron lifetime of 312 ± 4 ps for the Mg film indicates that the positron trapping defect is the di-vacancy. The observed saturation trapping also accounted for the very short positron diffusion lengths of less than 5 nm for all Mg 1−y Ti y and Mg 1−y Ti y H x layers, as deduced by the VEPFIT analysis.…”

“…[7,8] it was shown that the Mg-Ti films consist of Mg regions which are chemically segregated from Ti domains. The positron detects the presence of Mg domains only, because of the large difference in positron affinity for Mg metal compared with Ti [7], leading to trapping in the Mg regions before annihilation. This enabled us to monitor the metalto-insulator transition in the Mg regions of the Mg 1−y Ti y films exclusively.…”

“…While X-ray diffraction studies indicate that codeposition of Mg and Ti by magnetron sputtering leads to Mg-Ti films with a coherent crystal structure, positron annihilation and EXAFS studies strongly indicate that atomically mixed alloys are not formed. Instead, partial chemical segregation occurs into Mg and Ti domains on a length scale of less than 10 nm [7,8].…”

Mg-Based Thin Films as Model Systems in the Search for Optimal Hydrogen Storage Materials

“…Three regimes where identified: (1) single fluorite phase for y < 0.87; (2) single phase rutile for y > 0.9 and (3) two phase coexistence for 0.87 < y < 0.90. The presence of a chemically partially segregated but structurally coherent metastable phase in Mg-Ti-H thin films was further confirmed by Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy and by the positron Doppler broadening depth profiling method [94,95]. Positron depth-profiling was also applied to monitor the effects of hydrogenation on thin films [96].…”