Synergistic interplay between H and He in molybdenum: A first-principles study

The synergistic interplay between the hydrogen (H) and the helium (He) in molybdenum (Mo) has been investigated by using the first-principles. In perfect Mo, both H and He prefer to occupy the tetrahedral interstitial sites (t-sites) where the equilibrium distance between H and He is 2.74 Å. Both energetics and kinetics show that an interstitial He can bind up to three H atoms to form one H3-He cluster. With the presence of vacancy in Mo, He will occupy the vacancy center to form a He-vacancy (He-V) cluster; while a large amount of H atoms stay around He in vacancy. If the concentration of H is much higher than that of vacancies, one He-V cluster can capture as many as 12 H atoms, which is much larger than the H atoms trapped by an empty vacancy. The reason can be attributed to the electronic density redistribution of vacancy vicinity induced by He, which make capturing surface area of H expand outward from 19.00 Å2 (without He) to 32.96 Å2 (with He). The increasing multiple of H trapping area is 1.73, in nearly agreement with 1.71 that is the increasing multiples of H number captured by vacancy from 7 H to 12 H.