Diffusion path for an Al adatom on Al(001)

The barrier to self-diffusion on Al(001) by concerted displacement in a 〈100〉 or 〈010〉 direction is predicted to be smaller than 1/3 that for ordinary Al-adatom hopping over bridges between fourfold hollows. The transition state for the concerted motion corresponds to an Al ad-dimer symmetrically located above a surface site vacated by one of the dimer atoms. Covalent bonding in this configuration is favorable because each of the (trivalent) Al atoms above the surface bonds to the other, and to two surface atoms.