Equilibration of stoichiometrically distorted Fe1-xAlx(100) surfaces

The correlation between segregation kinetics and the geometric and chemical structure of iron-rich Fe1-xAlx(100) surfaces was investigated for four samples with different bulk stoichiometries (x = 0.03, 0.15, 0.30, 0.47) using quantitative low-energy electron diffraction (LEED) and Anger electron spectroscopy (AES). After being largely depleted of Al through preferential sputtering, the surfaces were annealed in a stepwise fashion. Auger signals as well as LEED patterns and intensity spectra were recorded for the transient phases which developed after each step, so monitoring the surfaces' return to equilibrium by means of aluminium segregation. They were used as fingerprints for comparison to the corresponding data of the equilibrium phases which had been analysed quantitatively in earlier investigations. It appears that transient phases of a sample with a certain bulk Al concentration can be identified with equilibrium phases of samples with lower bulk Al content. In other words, the structure of the surface slab accessible by means of LEED and AES is dictated by the actual intermediate slab stoichiometry in accordance with the bulk phase diagram and prevailing local quasi-equilibrium. The intermediate structures are metastable in the sense that the full equilibration of the crystal is kinetically limited by long-range mass transport from the bulk. In contrast, the top layer is always aluminium rich in accordance with the full equilibrium with the surface slab. So, the kinetics observed is dominated by the equilibration between the surface slab and the bulk, a feature possibly applying generally for stoichiometric equilibration in surfaces.