Influence of Impurity Atoms on Hydrogen Diffusion into Ruthenium

Ruthenium is considered as a promising protection layer to prevent the penetration of small atoms such as hydrogen into the underlying material. However, hydrogen is known to be able to penetrate ruthenium films under certain conditions and it has been hypothesized that impurity atoms, such as Sn, could play a major role in this process, decreasing the barrier of diffusion into the material for incoming hydrogen. We demonstrate that this is not the case for low-to-medium coverages, that is, as long as the adsorbed species can be treated individually without significant adsorbate-adsorbate interactions. Within this coverage regime, the hydrogenation is destabilized in the vicinity of impurity atoms, such as X = Mg, P, S, Si, Sn, or Zn, due to the strong bonding of the adsorbates with the free ruthenium surface. As a result, the diffusion barriers are not significantly reduced in the vicinity of any of the tested impurity atoms. The most likely and energetically favorable process occurs at a sufficiently large distance from the impurity, with an energy barrier that is almost identical to that of the free surface. Thus, such impurities can be ruled out to enhance the hydrogen uptake of ruthenium layers for the considered coverages, and other structural properties such as grain boundaries and dislocations should be considered as a pathway for hydrogen penetration.