Structural investigation of the (010) surface of the Al13 Fe4 catalyst.

We have investigated the structure of the Al(13)Fe(4)(010) surface using both experimental and ab initio computational methods. The results indicate that the topmost surface layers correspond to incomplete puckered (P) planes present in the bulk crystal structure. The main building block of the corrugated termination consists of two adjacent pentagons of Al atoms, each centered by a protruding Fe atom. These motifs are interconnected via additional Al atoms referred to as "glue" atoms which partially desorb above 873 K. The surface structure of lower atomic density compared to the bulk P plane is explained by a strong Fe-Al-Fe covalent polar interaction that preserves intact clusters at the surface. The proposed surface model with identified Fe-containing atomic ensembles could explain the Al(13)Fe(4) catalytic properties recently reported in line with the site-isolation concept [M. Armbrüster et al., Nat. Mater. 11, 690 (2012)].

Nanodomains due to phason defects at a quasicrystal surface.

Among the three coexisting types of terraces found on the twofold surface of the d-Al-Cu-Co quasicrystal, nanodomains are essentially observed on the transition-metal rich ones, with a coherent interface boundary. Both clean surface and Ag growth analyses, demonstrate that nanodomain surfaces are structurally identical to one of the two other terraces, which contains 85 at. % Al. We provide evidence that the nanodomains are a manifestation of phason defects that extend downward toward the bulk, and state that nanodomains develop because the energetic cost of creating the phason is outweighed by the change in surface energy. Consequently, the formation of nanodomains involves more than just the surface layer, but is driven by surface energetics.

Model for the transformation of an icosahedral phase into a B2 crystalline phase.

A model for the transformation of an Al-Cu-Fe icosahedral quasicrystal into a crystal with a B2-type phase is proposed. The model is based on two assumptions: (1) the main building block for the quasicrystal structure is a hierarchical dodecahedron composed of two icosahedral clusters, coinciding with two different sections of the {3, 3, 5} polytope; (2) the transformation of the quasicrystal into a B2-type crystal phase can be described as the transition between 3D sections of two polytopes, namely {3, 3, 5} and {3, 4, 3}. In the framework of the proposed model, two experimental facts gain plausible explanations: the transformation of the Al-Cu-Fe quasicrystal into the BCC phase specifically and the orientational relationships observed between this BCC phase and the initial icosahedral quasicrystal.

Oxygen adsorption on the Al9Co2(001) surface: first-principles and STM study.

Atomic oxygen adsorption on a pure aluminum terminated Al9Co2(001) surface is studied by first-principle calculations coupled with STM measurements. Relative adsorption energies of oxygen atoms have been calculated on different surface sites along with the associated STM images. The local electronic structure of the most favourable adsorption site is described. The preferential adsorption site is identified as a 'bridge' type site between the cluster entities exposed at the (001) surface termination. The Al-O bonding between the adsorbate and the substrate presents a covalent character, with s-p hybridization occurring between the states of the adsorbed oxygen atom and the aluminum atoms of the surface. The simulated STM image of the preferential adsorption site is in agreement with experimental observations. This work shows that oxygen adsorption generates important atomic relaxations of the topmost surface layer and that sub-surface cobalt atoms strongly influence the values of the adsorption energies. The calculated Al-O distances are in agreement with those reported in Al2O and Al2O3 oxides and for oxygen adsorption on Al(111).

Pseudomorphy, surface alloys and the role of elementary clusters on the domain orientations in the Cu/Al13Co4(100) system.

We have used the pseudo-tenfold surface of the orthorhombic Al(13)Co(4) crystal as a template for the adsorption of Cu thin films of various thicknesses deposited at different temperatures. This study has been carried out by means of low energy electron diffraction (LEED), scanning tunnelling microscopy (STM), x-ray photoelectron spectroscopy (XPS) and x-ray photoelectron diffraction (XPD). From 300 to 573 K, Cu adatoms grow pseudomorphically up to one monolayer. At 300 K, the ß-Al(Cu, Co) phase appears for coverages greater than one monolayer. For higher temperature deposition, the ß-Al(Cu, Co) phase further transforms into the γ-Al(4)Cu(9) phase. Both ß and γ phases grow as two (110) domains rotated by 72° ± 1° from each other. Instead of following the substrate symmetry, it is the orientations of the bipentagonal motifs present on the clean Al(13)Co(4)(100) surface that dictate the growth orientation of these domains. The initial bulk composition and structural complexity of the substrate have a minor role in the formation of the γ-Al(4)Cu(9) phase as long as the amount of Al and the Cu film thickness reach a critical stoichiometry.