Oxygen induced transformations of the δ-Pu(111) surface

The oxides that form on metal surfaces are critical to the preservation of the material under oxidizing conditions. For the case of plutonium, oxide film properties may affect a variety of kinetic phenomena related to the corrosion of the material. To investigate the behavior of the metal–oxygen interactions critical to the formation of such passive films, electronic structure calculations were performed on the interactions between O and the δ-Pu(111) surface. By probing the relationship between structure (coordination), charge transfer (valence) and thermodynamics (reactivity), it was found that oxidation is initially a local phenomenon, but, at 1.0 ML oxygen coverage, significant reconstructions that alter the phase structure of the near-surface region are induced. At this high-surface coverage the surface metal atoms are disbonded and the PuO coordination becomes four-fold as opposed to three fold. Even at this high coverage, however, the charge state more closely resembles Pu2O3 rather than PuO2. The ability for surface Pu atoms to adapt to a variety of changes in charge state and coordination provides a fundamental basis for understanding the multilayer structure of passive films formed on clean Pu surfaces. An analysis using first-principles thermodynamics also suggests that these films should develop uniformly, and not via an island or columnar growth mechanism.