Inverse change in positron lifetimes of vacancies in tungsten by binding of interstitial impurity atoms to a vacancy: A first-principles study

Abstract

First-principles calculations related to defect complexes formed from a monovacancy and multiple interstitial impurity atoms (hydrogen, carbon, nitrogen, and oxygen atoms) in tungsten were performed. The most stable atomic configurations, the electron density distributions, the binding energies of impurity atoms, and the positron lifetimes of each defect complex were calculated. In calculating positron lifetimes, slight deviations in the initial positions of the H atoms were found to be enhanced by positron localization, which affected the positron lifetimes of the vacancy-hydrogen complexes. In addition, the positron lifetimes of vacancy-nitrogen and vacancy-oxygen complexes were found to become longer in some cases with increasing numbers of impurity atoms that bound to the vacancy. Such longer positron lifetimes with increasing numbers of binding impurity atoms were attributed to the fact that the impurity atoms bind slightly further away from the vacancy, expanding the tungsten lattice.