Vacancy mechanism of oxygen diffusivity in bcc Fe: A first-principles study

•We propose a vacancy mechanism to describe diffusivity of oxygen in bcc Fe.•Oxygen in octahedral interstice of bcc Fe is always energetically favorable.•Vacancy possesses an extremely high affinity for oxygen in bcc Fe.•First-principles diffusivity of oxygen in bcc Fe agrees well with measurements. Diffusivity of interstitial oxygen (O) in bcc iron (Fe) with and without the effect of vacancy has been investigated in terms of first-principles calculations within the framework of transition state theory. Examination of migration pathway and phonon results indicates that O in octahedral interstice is always energetically favorable (minimum energy) with and without vacancy. It is found that vacancy possesses an extremely high affinity for O in bcc Fe, increasing dramatically the energy barrier (∼80%) for O migration, and in turn, making the predicted diffusion coefficient of O in bcc Fe in favorable accord with experiments.