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

Abstract

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.

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

“…This is in a sharp contrast to Mgbased alloys such as Mg 2 Pd and Mg 2 Si, which show large shifts toward S-W points intermediate of those of Mg and Pd or Si, respectively. 10 Evidently, in our case, positrons do not see an Mg-Ti alloy with an atomically mixed composition but rather detect the presence of pure Mg, as revealed from the position of the S-W cluster points. This selective detection requires the presence of chemically segregated Mg areas, into which the positron will trap during its diffusional motion which follows on the implantation and subsequent thermalization process.…”

“…10 Here, we provide direct evidence for chemical segregation in Mg-Ti films using the positron Doppler broadening depth profiling method. Furthermore, our positron annihilation lifetime spectroscopy ͑PALS͒ study shows that divacancies are present in the metal sublattice.…”

“…The depth-profiles were analyzed using the VEPFIT program; 14 the fitted curves are characterized by straight lines in the S-W diagram for as-deposited and for hydrogenated Mg 70 Ti 30 , reflecting the single layer behavior of the metal ͑metal hydride͒ layer. 10 The S-W cluster point characteristic for the metal Mg 70 Ti 30 phase shows a clear and large shift to the S-W point for the fluorite Mg 70 Ti 30 H x phase upon hydrogenation of the layer. Our previous studies 10,15 demonstrated that such a large reduction in S-parameter is a direct result of the metalinsulator transition occurring during the transition from the Mg to the MgH 2 phase.…”

“…10 The S-W cluster point characteristic for the metal Mg 70 Ti 30 phase shows a clear and large shift to the S-W point for the fluorite Mg 70 Ti 30 H x phase upon hydrogenation of the layer. Our previous studies 10,15 demonstrated that such a large reduction in S-parameter is a direct result of the metalinsulator transition occurring during the transition from the Mg to the MgH 2 phase. The transition from the free-electronlike Mg metal to the insulator metal hydride phase leads to a substantial broadening of the electron momentum distribution.…”

“…15 A remarkable feature of the S-W diagram is that the cluster points for Mg 70 Ti 30 and Mg 90 Ti 10 lie very close to those of well-annealed ͑defect-free͒ Mg and of sputterdeposited Mg layers. 10 Despite the high Ti-fractions of up to 30%, the cluster points remain surprisingly far from the S-W point of well-annealed Ti. This is in a sharp contrast to Mgbased alloys such as Mg 2 Pd and Mg 2 Si, which show large shifts toward S-W points intermediate of those of Mg and Pd or Si, respectively.…”