  Density-functional theory calculations based on conventional as well as
hybrid exchange-correlation functionals have been carried out to study the
properties of helium in various oxides (Al2O3, TiO2, Y2O3, YAP, YAG, YAM, MgO,
CaO, BaO, SrO) as well as at oxide-iron interfaces. Helium interstitials in
bulk oxides are shown to be energetically more favorable than substitutional
helium, yet helium binds to existing vacancies. The solubility of He in oxides
is systematically higher than in iron and scales with the free volume at the
interstitial site nearly independently of the chemical composition of the
oxide. In most oxides He migration is significantly slower and He--He binding
is much weaker than in iron. To quantify the solubility of helium at oxide-iron
interfaces two prototypical systems are considered (Fe|MgO, Fe|FeO|MgO). In
both cases the He solubility is markedly enhanced in the interface compared to
either of the bulk phases. The results of the calculations allow to construct a
schematic energy landscape for He interstitials in iron. The implications of
these results are discussed in the context of helium sequestration in oxide
dispersion strengthened steels, including the effects of interfaces and lattice
strain.
