Stability and dissolution of helium–vacancy complexes in vanadium solid

We report the energetics, stability, and diffusion behavior of helium (He), vacancies (V), and helium–vacancy complex clusters HenVm (n, m = 0–4) in vanadium solid from first-principles calculations. For He, vacancy site is more energetically favorable than tetrahedral interstitial by ∼0.74 eV, while hydrogen always prefers to stay in tetrahedral sites in vanadium. He exhibits a low migration energy (0.06 eV) and can be easily trapped in vacancy. A nearly linear relationship between formation energy and the number of He or vacancy is obtained for He or vacancy clusters, and the weak binding energies of He clusters indicate that He clusters themselves are unstable. The binding energies and dissociation energies of He and vacancy to helium–vacancy complex clusters are computed and compared well with the experimental observation from helium desorption spectra. The cluster stability depends on He content. Finally, He diffusion coefficients are predicted to be (1.07–1.27) × 10−8 m2 s−1 at typical temperatures of 600–800 K. We thus propose that He aggregation via vacancy trapping should be the main mechanism for He bubble formation.