Impeding effect of cerium on the growth of helium bubble in iron
  Our first-principles density functional theory calculations suggest that the rare earth element, Ce, has a stronger attraction (-1.31eV) to He than He-He (-1.18eV) in bcc Fe. Consequently, the mobile He atoms could be pinned to Ce, and hence a reduced merging of He clusters. Moreover, we find that the segregated Ce layer at the He bubble surface presents an energy barrier of 0.33 eV to the upcoming He atom and thus slows down the bubble growth. 
W. Hao and W. T. Geng
cond-mat.mtrl-sci