The barrier, to self-diffusion via concerted displacement in a [100]- or [010]-type direction on (001), was predicted to be smaller than 1/3 of that for ordinary Al adatom hopping over bridges between 4-fold hollows. The transition state for the concerted motion corresponded to an Al ad-dimer which was symmetrically located above a surface site that had been vacated by one of the dimer atoms. Covalent bonding in this configuration was favorable because each of the (trivalent) Al atoms above the surface bonded to the other, and to 2 surface atoms.

P.J.Feibelman: Physical Review Letters, 1990, 65[6], 729-32