Pseudomorphic quasiperiodic alkali metal monolayers on an i-Al-Pd-Mn surface

The structure and stability of quasiperiodic alkali metal monolayers and multilayer films formed on a fivefold surface of an icosahedral AlPdMn quasicrystal have been investigated using ab initio density-functional methods. A structural model of the adsorbed monolayer has been constructed on the basis of a mapping of the potential-energy landscape of a single adatom on the fivefold surface of i-Al-Pd-Mn. This model consists of a dense decoration of the P1 tiling describing the atomic arrangement on a fivefold surface of i-Al-Pd-Mn. Upon relaxation under action of ab initio calculated interatomic forces, a dense Na monolayer spontaneously transforms to another ordered quasiperiodic structure. The quasiperiodic ordering can be described by a decagonal DHBS tiling of decagons (D), hexagons (H), boats (B), and pentagonal stars (S). The internal decoration of these tiles, however, reduces the overall symmetry from decagonal to pentagonal. The saturation coverage of the adsorbed monolayer is Theta=0.50. K monolayers can adopt the same structure. Alkali layers with a lower coverage adopt a semiregular structure with part of the atoms forming a regular grid imposed by the strong binding of the alkali atoms to the quasiperiodically distributed charge-density minima. A stable quasiperiodic bilayer film can be formed by placing alkali atoms into the hollows of the adlayer. The coverage in the second layer is reduced to Theta=0.39. Films with three of more monolayers do not form a regular quasiperiodic pattern, upon relaxation they transform to a disordered amorphous arrangement.