Equilibration of stoichiometricallydistorted Fe1-xAlx(100) surfaces

Abstract

The correlation between segregation kinetics and the geometric andchemical structure of iron-rich Fe1-xAlx(100) surfaces wasinvestigated for four samples with different bulk stoichiometries(x = 0.03, 0.15, 0.30, 0.47) using quantitative low-energy electrondiffraction (LEED) and Auger electron spectroscopy (AES). Afterbeing largely depleted of Al through preferential sputtering, thesurfaces were annealed in a stepwise fashion. Auger signals as wellas LEED patterns and intensity spectra were recorded for thetransient phases which developed after each step, so monitoring thesurfaces' return to equilibrium by means of aluminium segregation.They were used as fingerprints for comparison to the correspondingdata of the equilibrium phases which had been analysedquantitatively in earlier investigations. It appears that transientphases of a sample with a certain bulk Al concentration can beidentified with equilibrium phases of samples with lower bulk Alcontent. In other words, the structure of the surface slabaccessible by means of LEED and AES is dictated by the actualintermediate slab stoichiometry in accordance with the bulk phasediagram and prevailing local quasi-equilibrium. The intermediatestructures are metastable in the sense that the full equilibrationof the crystal is kinetically limited by long-range mass transportfrom the bulk. In contrast, the top layer is always aluminium richin accordance with the full equilibrium with the surface slab. So,the kinetics observed is dominated by the equilibration between thesurface slab and the bulk, a feature possibly applying generally forstoichiometric equilibration in surfaces.