Energetics and clustering properties of boron in tungsten and molybdenum: A comparative analysis from first-principles study

Using first-principles calculations, we have systematically explored the geometric structures, electronic properties, diffusion behavior, and clustering with vacancies for impurity B (boron) in tungsten (W) and molybdenum (Mo). A single B atom prefers to occupy octerhedral interstitial position (oip) rather than tetrahedral interstitial position (tip) in bulk metals. B atoms can be easily captured by vacancies, and a single B atom prefers to occupy an oip next to vacancy center with a capturing energies of −2.67 and −2.51 eV in W and Mo, respectively. As the trapping progresses, at least 6B atoms can be captured by one vacancy, which is therefore regarded as the capturing center of B atoms to form B_nV clusters in both metals. For interstitial B and mono-vacancy, the most favorable diffusion pathways are oip → tip → oip and the <111> direction, respectively. At the same temperature, the diffusion coefficients of interstitial B and mono-vacancy in W are about 2–9 orders of magnitude lower than those in Mo, indicating that both interstitial B and mono-vacancy migrate much slower in W than in Mo. On the other hand, since the diffusion coefficient of interstitial B is significantly greater than that of mono-vacancy in both metals, the interstitial B migration is much easier than that of mono-vacancy. We therefore conclude that the B_nV formation mechanism can be attributed to that the relatively stable vacancies capture these relatively mobile interstitial B atoms in both metals.