First-principles calculations of the diffusion of atomic oxygen in nickel: thermal expansion contribution.

Within the framework of density functional theory using the projector augmented-wave (PAW) method, we present some energetic properties of atomic oxygen interstitials in crystalline Ni, i.e. the insertion and activation energies of the O diffusion. Concerning the activation energy, two pathways for the migration process are studied. The charge transfer process between atomic oxygen and nickel atoms is analysed in the interstitial sites. We find that the interstitial octahedral site (O site) is lower in energy than the tetrahedral site (T site). The most favourable pathway for the migration between two octahedral sites corresponds to an intermediate metastable state located in a tetrahedral site. Concerning the charge transfers we find that the atomic oxygen ionizes as O(-) and that the electron migrates essentially from the Ni nearest neighbours of atomic oxygen. In addition, the thermal expansion contribution through the dilatation of the solid is studied. When the thermal expansion is introduced, we show that the insertion process is stabilized and that the tetrahedral insertion energy becomes nearly equal to the octahedral ones. However, the activation energy decreases with the dilatation. Taking into account the thermal expansion effects, our results are consistent with the more reliable experimental data.