OXYGEN-ADSORPTION ON A SINGLE-GRAIN, QUASI-CRYSTAL SURFACE

Oxygen adsorbs on the five-fold surface of Al70Pd21Mn9. A chemisorbed phase, possibly in or below the surface plane, serves as a precursor to oxidation of Al. This chemisorbed phase destroys the quasiperiodicity of the surface. The major features of oxygen adsorption and oxidation are similar in the temperature range 105-500 K, but are different at 870 K, where Al segregates strongly to the surface. We postulate that Al segregation is driven by the exothermicity of its oxide, which is higher than that of the other constituents of this alloy. At all temperatures, the oxide layer is passivating (under the conditions of these experiments), is quite thin (less than or similar to 10 Angstrom) and is thermally stable (at least upon heating to 870 K). The oxidation characteristics of this quasicrystal are broadly similar to those of its major constituent, Al, with the possible exception of the oxygen sticking coefficient.