THE STRUCTURE OF SODIUM ADSORPTION PHASES ON AL(111)

The structures formed by the adsorption of Na and Al(111) have been studied by normal incidence standing X-ray wavefield absorption using both the (111) and (111BAR) Bragg reflections in order to obtain not only the Na-Al layer spacings, but also the adsorption sites. In the case of the ordered (square-root 3 x square-root 3))R30-degrees-Na phase, at a nominal coverage of 0.33 ML, and at lower coverages down to approximately 0.12 ML, Na is found to adsorb in a coverage independent site which involves a substitution of some of the top layer Al atoms; if the substrate layer spacings remain unchanged by this reconstruction, the effective radius of the adsorbed Na species is found to be 1.67 angstrom. The higher coverage (2 x 2)-Na phase is found to involve (at least) two distinct Na adsorption sites, but each of these has a very similar layer spacing relative to the extended (111) substrate scatterer lattice planes. A specific model of this phase, involving two reconstructed layers each of stoichiometry NaAl2 is proposed, which accounts in a quantitative way for the present X-ray standing wave data and previous X-ray absorption data, and in a qualitative way for published high resolution soft X-ray photoelectron spectroscopy results. As Al and Na are essentially immiscible in the bulk, this two-layer structure appears to be an entirely new kind of surface phase.