Effect of vacancy on the dissolution and diffusion properties of hydrogen and helium in molybdenum

We present first-principles study of the stability and diffusion properties of H and He in Mo. The results show that diatomic H–H and He–He prefer 〈1 0 0〉 and 〈1 1 1〉 dumbbell configurations in single vacancy, respectively. The occupation and migration of H and He nearby a vacancy (or He-vacancy complex) are very different from that in bulk: both H and He are more preferable to occupy the tetrahedral interstitial sites closer to the vacancy (or He-vacancy complex), and the diffusion barriers of H and He into the vacancy (or He-vacancy complex) are slightly reduced but the diffusion barriers out of the defect are severely increased, especially for He. Besides, the presence of single He at tetrahedral interstitial site reduces the energy required for its nearby vacancy formation more considerably than that of H (the produced vacancy traps the H or He), contributing to the accumulation and different disposition depth of H and He in Mo.

► Both H and He are more favorable to occupy the tetrahedral interstitial sites nearby a vacancy (or helium-vacancy complex) rather than that far away from the vacancy or complex.
► The diatomic hydrogen–hydrogen and helium–helium prefer 〈1 0 0〉 and 〈1 1 1〉 dumbbell configurations in single vacancy, respectively.
► The diffusion-out barriers of hydrogen and helium from a vacancy and helium-vacancy complex are much larger than diffusion-in barriers, especially for helium.
► Interstitial helium can reduce the nearby vacancy formation energy more significantly than hydrogen.