Ab initio study of He trapping, diffusion and clustering in Y₂O₃

Ab initio calculations have been performed to study the formation and migration energies of helium atoms and the stability of helium-vacancy clusters in a Y₂O₃ crystal. The calculated formation energies show that a helium atom is preferred to occupy an yttrium vacancy site with a large volume and low electron density. The migration energy of the helium atom by an interstitial mechanism is 0.31 eV. Calculations of the binding energies of an extra helium atom to the helium-vacancy clusters vary with the number of helium atoms in the clusters with a typical value of 0.4–0.7 eV. This turns negative when the He atoms reach saturation; that indicates that vacancy clusters can attract a limited number of helium atoms to form small stable helium-vacancy clusters. Our calculations suggest that the use of Y₂O₃ in oxide dispersion strengthened ferritic steels may reduce He gas bubble formation as it may act as sink for trapping helium atoms.