Electronic Densities of States and Pseudo-gaps in Al-based Complex Intermetallics

The results of an investigation of the electronic structure of the valence band of various Al-based complex intermetallic alloys, carried out using the X-ray emission spectroscopy technique, are presented and discussed. It is shown that the electronic structure results from an interplay between covalency and repulsive interactions that contributes to the formation of a pseudo-gap at the Fermi level in the Al electronic distributions.</p

A new ternary compound with the BGa8Ir4 structure type in the Al-Au-Ir system

International audienceFollowing the recent determination of the Al3AuIr structure, a new ternary phase has been identified in the Al-Au-Ir phase diagram. It has a chemical composition Al 9 (Au;Ir)4 with an apparently low gold content. Its crystal structure has been determined with single-crystal X-ray diffraction. The new compound crystallizes in the tetragonal crystal system and has been successfully solved in space group I4 1/acd (Pearson symbol tI104) with lattice parameters a = 8.6339 (2) and c = 21.8874 (7) Å. Atomic environments are described as well as similarities with the BGa8Ir4 compound

Single crystal growth, crystal structure and surface characterisation of the binary phase Al45Cr7.

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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

Intermetallic compounds as potential alternatives to noble metals in heterogeneous catalysis: The partial hydrogenation of butadiene on alpha-Al4Cu9(110)

SSCI-VIDE+ECI2D+LPIInternational audienceNon-noble intermetallic compounds have recently shown promising catalytic performances for the partial hydrogenation of alkynes and alkenes. In this work, the properties of alpha-Al4Cu9(110) toward the gas-phase hydrogenation of butadiene have been investigated at total pressures of 2-20 mbar and temperatures of 110-180 °C. The model catalyst is active and 100% selective to butenes. Moreover, although less active than Al13Fe4(010), which had been previously evaluated for the same reaction, it is more selective and more stable. The combination of catalytic tests with pre- and postreactionAuger electron spectroscopy measurements, and comparative tests with Cu(110), shows that copper governs the reaction on alpha-Al4Cu9(110). However, the lower activity of the (Curicher) sputtered Al4Cu9 surface with respect to the annealed one, and the differences between Al4Cu9 and Cu surfaces in terms of butene isomer distribution, butene conversion kinetics and sensitivity to poisons, demonstrate the unique character of the intermetallic compound

Al 4 Ir: An Al–Ir Binary-Phase Superstructure of the Ni 2 Al 3 Type

International audienceA binary phase with Al4Ir composition has been discovered in the Al–Ir binary system. Single-crystal X-ray diffraction analysis reveals that it crystallizes in the trigonal space group P3c1 with the unit cell parameters a = 12.8802(2) Å and c = 9.8130(2) Å. This structure is derived from the Ni2Al3 structure type. The supercell is due to the ordering of the aluminum atoms, which replace the nickel atoms in the prototype structure. The crystal structure was directly imaged by atomic-scale scanning transmission electron microscopy, and the misalignment of the Al site responsible for the supercell has been clearly evidenced. Its metastable nature has been confirmed by differential thermal analysis measurements. The atomic and electronic structures of Al4Ir have also been investigated by density functional theory. The structural optimization leads to lattice parameters and atomic positions in good agreement with the experimental ones. The compound is metallic, with a minimum in the density of states located more than 1 eV above the Fermi energy. This suggests a metastable system, in agreement with the electron count found much above 18 electrons per Ir atom, deviating from the Hume-Rothery rule and with the presence of occupied antibonding states revealed by the crystal orbital Hamiltonian population analysis. The relative stability of the compound is ensured by the hybridization between sp-Al and d-Ir states within Ir-centered clusters, while covalent-like interactions in-between the clusters are indicated by the analysis of the electron localizability function

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.Reprinted with permission from ACS Nano 8 (2014): 3646–3653, doi:10.1021/nn500234j. Copyright 2014 American Chamical Society.</p

A new complex hexagonal phase in the Ce-Au-Sn system and its structure relationship with the 1/1 approximant

International audienceWe report the formation of a new structurally complex intermetallic phase in the Ce-Au-Sn system. This phase has been observed in an ingot with nominal composition Ce16.7Au72.2Sn11.1. In addition, we have identified by transmission electron microscopy operating in two-beam conditions the occurrence of planar defects, characterized as grains boundaries located between the cubic 1/1 CeAuSn approximant and the new phase. Tilt series of electron diffraction patterns of this new structure have been recorded along two main directions. These measurements have been used for the reconstruction of the reciprocal space. The resulting cell is hexagonal, with ah ≈ 1.53 nm and ch ≈ 1.52 nm. It is structurally related to the Gd13Cd58 structure type (P63/mmc), where some Cd atoms are icosahedrally coordinated. The orientation relationships between the two complex structures across planar boundaries have been deduced from the lattice fringes transmission electron microscopy images

Catalytic properties of Al13TM4 complex intermetallics: influence of the transition metal and the surface orientation on butadiene hydrogenation

Complex intermetallic compounds such as transition metal (TM) aluminides are promising alternatives to expensive Pd-based catalysts, in particular for the semi-hydrogenation of alkynes or alkadienes. Here, we compare the gas-phase butadiene hydrogenation performances of o-Al13Co4(100), m-Al13Fe4(010) and m-Al13Ru4(010) surfaces, whose bulk terminated structural models exhibit similar cluster-like arrangements. Moreover, the effect of the surface orientation is assessed through a comparison between o-Al13Co4(100) and o-Al13Co4(010). As a result, the following room-temperature activity order is determined: Al13Co4(100) < Al13Co4(010) < Al13Ru4(010) < Al13Fe4(010). Moreover, Al13Co4(010) is found to be the most active surface at 110°C, and even more selective to butene (100%) than previously investigated Al13Fe4(010). DFT calculations show that the activity and selectivity results can be rationalized through the determination of butadiene and butene adsorption energies; in contrast, hydrogen adsorption energies do not scale with the catalytic activities. Moreover, the calculation of projected densities of states provides an insight into the Al13TM4 surface electronic structure. Isolating the TM active centers within the Al matrix induces a narrowing of the TM d-band, which leads to the high catalytic performances of Al13TM4 compounds