Bulk and surface structure of the clean and adsorbate-covered decagonal Al-Co-Ni quasicrystal

Abstract

We review our Al adsorption experiments on the tenfold-symmetry surface of the decagonal Al-Co-Ni quasicrystal and present computational simulations of adsorption on a structural model based on a fundamental Al-Co cluster with 20 \AA diameter, symmetry $\bar{10}2m$, and 8 \AA periodicity. This cluster is the building unit of $\tau ^2$-Al$_{13}$Co$_4$, from which, by a sequence of minor changes, the structures of the phases in the stability region of decagonal Al-Co-Ni can be derived. The model used for the decagonal Al$_{70}$Co$_{15}$Ni$_{15}$ is an idealized model with a two-layer periodicity (4 \AA) and no chemical or structural disorder. We find that the bulk and surface properties of this model are in good agreement with experiments. Our molecular-dynamics simulations of Al adsorption reproduce the experimental results and show that by varying the thermal relaxation rates of the adsorbed layer, a variety of different surface morphologies can be achieved. We also present our recent experiments on dissociative adsorption of oxygen on the decagonal surface.