Monovacancy-enhanced trapping physical mechanism for interstitial oxygen in molybdenum and tungsten

Oxygen is one of the most common impurity elements in molybdenum (Mo) and tungsten (W), where its interaction with irradiation-induced vacancies can strongly influence the microstructural evolution and service performance of plasma-facing materials. In this work, first-principles calculations were performed to systematically investigate the solution behavior, monovacancy trapping, diffusion kinetics, and binding characteristics of oxygen in bcc Mo and W, with the aim of revealing the atomic-scale mechanism governing monovacancy-enhanced oxygen trapping. The results show that interstitial O preferentially occupies tetrahedral interstitial sites in both metals, while its solution energy is markedly lower in Mo than in W owing to the stronger Mo-O electronic interaction. Monovacancies exhibit a strong trapping capability for O atoms, with trapping energies of −2.66 and −3.28 eV for the first O atom in Mo and W, respectively. The trapped O atoms preferentially occupy off-center positions adjacent to the monovacancy rather than the monovacancy center, and up to four O atoms can be stably accommodated within the monovacancy trapping region. Crucially, the local electronic redistribution and lattice distortion induced by the resulting 𝑂 4 ⁢ 𝑉 complex generate additional energetic traps in adjacent interstitial sites, expanding the capturing zone to facilitate continuous O accumulation. CI-NEB calculations demonstrate that a monovacancy significantly reduces the migration barriers for nearby interstitial O atoms, even leading to spontaneous barrier-free migration along specific pathways in Mo, while imposing high barriers for O detrapping. This systematic stabilization underscores a strong thermodynamic driving force for monovacancy-assisted local O enrichment, providing atomic-scale insights into oxide precipitation pathways in fusion materials.