Vincent Fournée

Surfaces of Al-based complex metallic alloys: atomic structure, thin film growth and reactivity

Abstract We present a review on recent work performed on periodic complex metallic alloy (CMA) surfaces. The electronic and crystallographic structures of clean pseudo-tenfold, pseudo-twofold, sixfold surfaces will be presented along with the recent findings on CMA of lower structural complexity, i.e. with a smaller unit cell. The use of CMA surfaces as templates for thin film growth and the formation of surface alloy will also be introduced. The reactivity of these complex surfaces and their impact in the field of heterogeneous catalysis will be discussed. Finally, common trends among these systems will be highlighted when possible and future challenges will be examined.

Study of Al-Cu Hume-Rothery alloys and their relationship to the electronic properties of quasicrystals

The stability of quasicrystals is related to the Hume-Rothery mechanism known to take place in many intermetallics, especially Al-Cu alloys. In order to assess to what extent the electronic density of states in aperiodic Al-Cu-Fe compounds is sensitive to Hume-Rothery stabilization, we present experimental measurements by soft x-ray emission and absorption spectroscopies of the aluminium electronic states in a series of Al-Cu crystalline compounds. Some of them, like icosahedral Al-Cu-Fe, are characterized by nearly spherical Brillouin zones. We observe the opening of a narrow but definite pseudo-gap in such Al-Cu compounds. We point out a strong interaction between Cu d states and the Al states in the middle of the valence band which is well described by a Fano-like effect. Some other characteristic features of our spectra are interpreted on the basis of band structure calculations. A comparison to previous data published for quasicrystalline and approximant phases stresses the importance for the stability of these compounds of sp-d hybridization at the Fermi level. It also leads to the conclusion that the effect of Hume-Rothery interaction is dramatically enhanced by the hierarchical structure of quasicrystals rather than by the almost spherical shape of the Brillouin zone.

Al 3p occupied states in Al-Cu-Fe intermetallics and enhanced stability of the icosahedral quasicrystal

This paper concentrates on a comparison of Al 3p occupied densities of states in crystalline and quasicrystalline Al-Cu-Fe intermetallics. This comparison is made quantitative by computing the two first moments of this partial distribution, which correspond to the partial contribution of Al atoms to the cohesive energy of the material and to the orbital overlap modulated by the coordination number, respectively. From these data, we conclude that the icosahedral quasicrystal and its approximants represent a region of specifically enhanced stability in the Al-Cu-Fe phase diagram. We assign this effect to the specific role played by Al 3p states in the Hume-Rothery mechanism. We confirm the occurrence of a stronger hybridization between Al 3p and Fe 3d states in the icosahedral structure whereas we point out a weaker interaction with Cu 3d states in the icosahedral compounds as compared to stable approximants.

About the Al 3p density of states in Al–Cu–Fe compounds and its relation to the compound stability and apparent surface energy of quasicrystals

Abstract We report the characteristic parameters of the Al 3p densities of states gained from a thorough study of many Al–Cu(–Fe) intermetallics including quasicrystalline and approximant phases using X-ray emission spectroscopy. These parameters are the density of states at the Fermi energy EF, the average binding energy due to Al 3p states and the second moment of their distribution. Hence, we demonstrate that the contribution to the compound stability arising from localization of Al 3p states via the Hume-Rothery mechanism is maximum for the icosahedral state. We also show that Al-transition metal sp-d hybridization is enhanced in the presence of icosahedral order. Finally, we confirm the already pointed out relationship between depth of the pseudo-gap at EF and apparent surface energy of the samples.

Complex metallic alloys as new materials for additive manufacturing

Abstract Additive manufacturing processes allow freeform fabrication of the physical representation of a three-dimensional computer-aided design (CAD) data model. This area has been expanding rapidly over the last 20 years. It includes several techniques such as selective laser sintering and stereolithography. The range of materials used today is quite restricted while there is a real demand for manufacturing lighter functional parts or parts with improved functional properties. In this article, we summarize recent work performed in this field, introducing new composite materials containing complex metallic alloys. These are mainly Al-based quasicrystalline alloys whose properties differ from those of conventional alloys. The use of these materials allows us to produce light-weight parts consisting of either metal–matrix composites or of polymer–matrix composites with improved properties. Functional parts using these alloys are now commercialized.

Self-Organized Molecular Films with Long-Range Quasiperiodic Order

Self-organized molecular films with long-range quasiperiodic order have been grown by using the complex potential energy landscape of quasicrystalline surfaces as templates. The long-range order arises from a specific subset of quasilattice sites acting as preferred adsorption sites for the molecules, thus enforcing a quasiperiodic structure in the film. These adsorption sites exhibit a local 5-fold symmetry resulting from the cut by the surface plane through the cluster units identified in the bulk solid. Symmetry matching between the C60 fullerene and the substrate leads to a preferred adsorption configuration of the molecules with a pentagonal face down, a feature unique to quasicrystalline surfaces, enabling efficient chemical bonding at the molecule-substrate interface. This finding offers opportunities to investigate the physical properties of model 2D quasiperiodic systems, as the molecules can be functionalized to yield architectures with tailor-made properties.

Atomic and electronic structure of quasiperiodic and crystalline Mg-61 at.% Al alloys

Mg-Al alloys with composition Mg-61 at.% Al display two types of structure: a stable crystalline state (the so-called β-Mg2Al3) and a metastable one showing both quasiperiodicity and inflation symmetry. The atomic as well as electronic structures of stable crystalline β-Mg2Al3 and a metastable Mg39Al61 phase that exhibits quasiperiodicity and inflation symmetry have been studied thanks to x-ray spectroscopy techniques. Extended x-ray absorption fine-structure experiments performed above the Al and Mg K absorption edges have probed the local atomic order around Mg as well as Al atoms. X-ray emission and absorption spectroscopies have investigated occupied and unoccupied electronic states around Mg and Al. The local order has been found to be the same around Al atoms in both alloys, whereas around Mg atoms differences are seen for higher shells than first neighbours. This suggests that the same clusters must be involved in both phases and that the quasiperiodicity is connected with modification of the Mg atom environment forming the linkage between large clusters. On the other hand, the electronic distributions show differences from β-Mg2Al3 to the quasiperiodic phase that are consistent with the local order studies. A pseudogap is observed in the Mg and Al electronic structures which is more marked in the quasiperiodic phase than the crystalline alloy. By analogy to quasicrystals, we suggest the enhancement observed is due to the occurrence of an inflation mechanism in the arrangement of the clusters in the quasiperiodic alloy with respect to the β-Mg2Al3 crystal.

Influence of leaching on surface composition, microstructure, and valence band of single grain icosahedral Al-Cu-Fe quasicrystal

The use of quasicrystals as precursors to catalysts for the steam reforming of methanol is potentially one of the most important applications of these new materials. To develop application as a technology requires a detailed understanding of the microscopic behavior of the catalyst. Here, we report the effect of leaching treatments on the surface microstructure, chemical composition, and valence band of the icosahedral (i-) Al-Cu-Fe quasicrystal in an attempt to prepare a model catalyst. The high symmetry fivefold surface of a single grain i-Al-Cu-Fe quasicrystal was leached with NaOH solution for varying times, and the resulting surface was characterized by x-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), scanning electron microscopy (SEM), and atomic force microscopy (AFM). The leaching treatments preferentially remove Al producing a capping layer consisting of Fe and Cu oxides. The subsurface layer contains elemental Fe and Cu in addition to the oxides. The quasicrystalline bulk structure beneath remains unchanged. The subsurface gradually becomes Fe3O4 rich with increasing leaching time. The surface after leaching exhibits micron sized dodecahedral cavities due to preferential leaching along the fivefold axis. Nanoparticles of the transition metals and their oxides are precipitated on the surface after leaching. The size of the nanoparticles is estimated by high resolution transmission microscopy to be 5-20 nm, which is in agreement with the AFM results. Selected area electron diffraction (SAED) confirms the crystalline nature of the nanoparticles. SAED further reveals the formation of an interface between the high atomic density lattice planes of nanoparticles and the quasicrystal. These results provide an important insight into the preparation of model catalysts of nanoparticles for steam reforming of methanol.

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).

The atomic structure of the threefold surface of the icosahedral Ag?In?Yb quasicrystal

We report a study of the atomic structure of the threefold icosahedral (i-)Ag-In-Yb quasicrystal surface using scanning tunneling microscopy (STM) and low energy electron diffraction (LEED). The LEED confirms that the surface exhibits quasicrystalline long-range order with the threefold symmetry expected from the bulk. The STM reveals large atomically flat terraces separated by steps of different heights. A comparison of atomically resolved STM images for the terraces and the step-height distribution with the bulk structure of isostructural i-Cd-Yb shows that the terraces are formed at bulk planes intersecting the centers of the rhombic triacontahedral clusters that make up the bulk structure of the system. However, the stability of particular terraces may be influenced by the density of atoms in the interstices (glue atoms that bind the clusters) in the terraces and also by the chemical environment in the underlying atomic plane. The surface exhibits screw dislocations, which is explained in terms of a continuous atomic density along the threefold axis.