Interfacial Alloy Hydride Destabilization in $\mathrm{Mg}/\mathrm{Pd}$ Thin Films

Recently, a large increase in the equilibrium hydrogen pressure has been reported for MG thin films capped with a Pd layer. We show that this increase is due to intermixing of Mg and Pd, as opposed to a strain effect as previously claimed. Transmission electron microscopy and depth profiling x-ray photoemission spectroscopy are used to directly measure interfacial intermixing between Mg and Pd, and we find that intermixing and equilibrium hydrogen pressure both increase with annealing. We present a thermodynamic model of the effect of alloying on equilibrium pressure, and find that the observed equilibrium pressure increase is consistent with the observed thickness of the intermixed region, which is of the order of a few nm. We also show that stress measured during hydrogenation corresponds to a negligible increase in equilibrium pressure.

Figure 1

(a) Force per width during hydrogenation at 1 atm ${\mathrm{H}}_2$ and dehydrogenation in vacuum of a $\mathrm{Pd}/\mathrm{Mg}/\mathrm{Pd}$ layered thin film. (b) Symmetric reflection x-ray diffraction results for a Pd-capped, 50 nm thick Mg film, before and after hydrogenation at $T=363\mathrm{K}$ under 1.6 atm ${\mathrm{H}}_2$ gas.

Figure 2

Pressure-transmission-isotherm (PTI) curves of Mg-based thin film samples: as-deposited $\mathrm{Pd}/\mathrm{Mg}/\mathrm{Pd}$, annealed $\mathrm{Pd}/\mathrm{Mg}/\mathrm{Pd}$, and $\mathrm{Ti}/\mathrm{Mg}/\mathrm{Pd}$.

Figure 3

(a) Cross-section TEM image of $\mathrm{Ti}/\mathrm{Mg}/\mathrm{Pd}$ multilayer samples. (b) HRTEM image of the $\mathrm{Ti}/\mathrm{Mg}$ interface in (a). (c) HRTEM image of the $\mathrm{Mg}/\mathrm{Pd}$ interface in(a). (d) Cross-section TEM image and HRTEM of annealed $\mathrm{Ti}/\mathrm{Mg}/\mathrm{Pd}$ sample. The dashed area highlights the intermixing region.

Figure 4

Pd $3d$ XPS signals at different position in the palladium layer of the $\mathrm{Ti}/\mathrm{Mg}/\mathrm{Pd}$ multilayer.

Figure 5

Mg layer thickness dependence of the equilibrium hydrogen pressure from Eq. (4) fit to the data using the interface width $\sigma$ and bulk equilibrium pressure as fitting parameters. The data of Baldi et al.[5] is also shown fitted to the same expression. The interface widths extracted from these fits is 1.5 and 1.9 nm, respectively, for our data and the data of Baldi et al..