Equilibration of stoichiometrically distorted Fe_1-xAl_x(100) surfaces

Abstract

The correlation between segregation kinetics and the geometric and chemical structure of iron-rich Fe 1−x Al x (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 Auger 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.

Segregation in Strongly Ordering Compounds: A Key Role of Constitutional Defects

“…At a first glance, it might appear that this is simply an artificial nonequilibrium residuum of the initial sputtering process in which such antisite atoms were created. However, the comparison to a FeAl(100) surface, which had undergone the same treatment with respect to sputtering and subsequent annealing, tells a different story [2,23]: There, the initially created defects disappeared at much lower annealing temperatures, obviously dissolving into an extended region below the surface (or the bulk). The very surface consisted of a pure Al layer even though the bulk itself was significantly Fe enriched (6% Fe antisites).…”

“…Moreover, Refs. [2,23] show that there is always local equilibrium between the very top layer and the near-surface region below during the annealing process of FeAl. In other words, the transition-metal (TM) enriched surface slab is structurally well ordered long before full compositional equilibrium with the bulk is attained, with its structure strictly dictated by the phase diagram for its momentary average stoichiometry.…”

Al-rich Fe0.85Al0.15(1 0 0), (1 1 0) and (1 1 1) surface structures

“…Whatever the orientation, the near surface composition obtained is close to the B 2 Fe 0.5 Al 0.5 with a typical affected depth of around 25Å, except on the (110) surface for which those figures are slightly different (Fe 0.6 Al 0.4 ; ∼ 35Å) [17]. Although suspected in the diffraction study of Kottcke et al [19] on FeAl(100) and in the Auger measurements of Hammer et al [20] on the open FeAl(111) surface, such a composition gradient in the Fe 1−x Al x system was never evidenced before, probably because experiments [21,22,23,19,24,20,25,14,26,23,27] have focused mostly on ordered FeAl alloys with higher Al contents.…”

“…The c(2 × 2) structure was previously assigned to an ordered top layer involving equal fractions of Al and Fe, although vacancies could not be excluded [14]. However, much sharper c(2 × 2) LEED patterns were obtained on crystals of rather similar composition, namely Fe 0.97 Al 0.03 (100) and Fe 0.70 Al 0.30 (100) [23,14,27,22]. Such a sensitivity of the surface structure to the alloy bulk composition stems from the fact that c(2×2) reconstructions only appear for given surface Al/Fe relative concentrations [23,27].…”

Surface alloys and alloy surfaces: the platinum-tin system

“…A thermal equilibrium can be even completely out of reach if the sublimation energies differ largely. The latter was observed with Fe-Al alloy surfaces where at the temperature that is necessary to restore the surface composition severe evaporation of Al takes place [80]. With Pt-Sn surfaces no significant evidence of Sn for sublimation has observed: However, on the Pt¢ Sn(110) surface mobile monolayer-deep depressions have been observed at Sn-positions in the topography which are most likely vacancies left after sublimation of Sn atoms.…”