Atomic Structure of the Passive Oxide Film Formed on Iron

We have used x-ray scattering to measure the structure of the passive oxide film formed at high anodic potentials on Fe(110) and Fe(001). The crystalline film has a small crystallite size ( $\approx 50Å$) and is oriented with the substrate. The film structure is based on ${\mathrm{Fe}}_3{\mathrm{O}}_4$, but with cation vacancies on octahedral and tetrahedral sites (80% and 66% occupancies, respectively) and with cations occupying octahedral interstitial sites (12% occupancy). These results resolve the long-standing controversy surrounding the film structure and provide a basis for understanding and modeling film properties important for corrosion resistance.