While diffusion mechanisms of interstitial elements in fcc systems are generally well-known, especially in the case of H atoms, we show in this work that even in the case of a simple metallic system (aluminum), the diffusion of interstitials exhibits a wide variety of paths and mechanisms that depend on the specie. We used an approach based on first-principles calculations associated with kinetic Monte-Carlo simulations and a multi-state diffusion formalism to compute the diffusion coefficients of five interstitial elements: hydrogen, boron, carbon, nitrogen and oxygen. For instance, at the atomic scale, whilst we find that C atoms prefer to be located in octahedral sites (labeled $o$) rather than in tetrahedral positions (labeled $t$), we find one additional stable position in the lattice ($M$). The diffusion through these three stable positions are thus studied in detail. In the case of B atoms, for which the tetrahedral site is found unstable, the diffusion path is between $o$-$o$ sites. Similarly, in the case of oxygen, $t$ positions are found to be the only stable positions ($o$ are unstable) and the path of migration, along $t$-$t$ direction, is found through a twice degenerated asymmetric transition state. In the case of H and N atoms for which $t$ and $o$ sites are stable, we explain why the only path is along the $t$-$o$ direction. Finally, we discuss explicit formulas to compute coefficients of diffusion of interstitials in fcc structures. .

We present a discussion on the influence of high-order transition states on interstitial diffusion in fcc systems using first-principles calculations. In earlier works, only first-order transition states (1TS) were used to compute the diffusivity at the atomic-scale: the direct diffusion between tetrahedral (t) and octahedral (o) sites has been proposed to describe atomic-scale diffusion mechanisms. However, we show here that if this direct diffusion makes it possible to reproduce displacements remarkably well, neglecting higher-order transition states induces an underestimation of the diffusion coefficient at high temperature. We hereinafter revisit the diffusion coefficient of interstitial species in different fccsystems. The effect of these configurations on atom diffusion in Al, Co, Cu, Ni and Pd, whose only stable sites are the tetrahedral and octahedral positions (H and O atoms) is thus discussed here. We show that if the correction is low, taking into account higher-transition states can modify the diffusivity values at high temperature.