Search

The intermetallic Al5Co2 is defined as a structurally complex material and is considered a low-order quasicrystalline approximant. A single crystal of Al5Co2(001) was obtained by the Czochralski method. The sample was characterized by X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), and X-ray photoelectron diffraction (PED). The surface composition was also analyzed by XPS, indicating only Al and Co compounds. In the current research, the crystal structure was qualitatively analyzed using the LEED patterns for different incident beam energies indicating a (1 × 1) termination, also in accordance with some literature works. The structure study was performed by applying the standard software MSCD and showed a (1 × 1) pattern. In addition, four different termination models for this termination were tested. The reliability factor indicated that the best termination belongs to the Al-rich surface layer. [ABSTRACT FROM AUTHOR]

We report on the dynamics of a magnetic-field-driven antiferromagnetic-to-paramagnetic quantum phase transition in monocrystalline Ce3Al via transverse-field muon spin rotation (TF-µSR) experiments down to low temperature of ∼ 80 mK. The quantum phase transition is of a spin-flip type and takes place on the Ce–Al magnetic chains as a result of competition between the indirect exchange and the Zeeman interaction of the Ce moments with the external field, applied along the chain direction (also the direction of the antiferromagnetic axis). The Ce moments are not static at T → 0, but fluctuate in their direction due to the Heisenberg uncertainty principle. Upon applying the magnetic field sweep, the fluctuations exhibit the largest amplitude at the quantum critical point, manifested in a maximum of the muon transverse relaxation rate at the critical field. The quantum nature of fluctuations observed in the TF-µSR experiments is reflected in the temperature independence of the average local magnetic field component along the external magnetic field at the muon stopping site(s) and the muon transverse relaxation rate within the investigated temperature range 1.5 K–80 mK. Quantum fluctuations are fast on the muon Larmor frequency scale, τ 0 < 10–10 s. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Following 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 Al9(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 I41/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. The discovery of a new Al–Au–Ir intermetallic compound is described; its crystal structure resolution is described and the Au/Ir mixed occupancy is discussed. [ABSTRACT FROM AUTHOR]

Recently, non-noble intermetallic compounds have shown promising catalytic performances in the partial hydrogenation of alkynes and alkenes. In this work, the properties of γ-Al4Cu9(1 1 0) toward the gas-phase hydrogenation of butadiene were 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(0 1 0), which was evaluated previously for the same reaction, it is more selective and more stable. The combination of catalytic tests with pre- and postreaction Auger electron spectroscopy measurements and comparative tests with Cu(1 1 0) shows that Cu governs the reaction on γ-Al4Cu9(1 1 0). However, the lower activity of the (more Cu-rich) 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. [ABSTRACT FROM AUTHOR]

A new ternary phase with a composition of Al3AuIr has been found in the Al-rich area of the Al-Au-Ir system. Differential thermal analysis indicates a melting point of 990 °C, and single-crystal X-ray diffraction measurements reveal that this ternary phase adopts a Ni2Al3 structure type (space group P...m1) with a = 4.2584(5) Å and c = 5.1991(7) Å. This compound is isostructural to the Al3Cu1.5Co0.5 phase also found in the Al-rich part of the Al-Cu-Co ternary diagram. Experimental evidence combined with ab initio calculations point toward an Al3AuIr phase stabilized by a Hume-Rothery mechanism. Quantum chemical calculations indicate two-center and multicenter interactions in the Al3AuIr phase. Layered distribution of two-center interactions separated by regions with four- and five-center bonds suggests a preferential cleavage of the material at puckered planes perpendicular to the [001] direction. [ABSTRACT FROM AUTHOR]

For the first time, to the best of our knowledge, a realistic approach for deriving an estimate by superior values of the surface energy of complex metallic alloys based on aluminium is proposed. It turns out that the surface energy of quasicrystals lies in the range 0.5–0.8 J/m2, in contrast to pure aluminium (1.15 J/m2) orλ-Al13Fe4(2.0 J/m2) andω-Al7Cu2Fe (2.2 J/m2) intermetallics. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Steel is the most commonly manufactured material in the world. Its performances can be improved by hot-dip coating with the low weight aluminum metal. The structure of the Al∥Fe interface, which is known to contain a buffer layer made of complex intermetallic compounds such as Al 5 Fe 2 and Al 13 Fe 4 , is crucial for the properties. On the basis of surface X-ray diffraction, combined with theoretical calculations, we derive in this work a consistent model at the atomic scale for the complex Al 13 Fe 4 (010)∥Al 5 Fe 2 (001) interface. The epitaxial relationships are found to be [130] Al 5 Fe 2 ∥[010] Al 13 Fe 4 and [1 1̅0] Al 5 Fe 2 ∥[100] Al 13 Fe 4 . Interfacial and constrained energies, as well as works of adhesion, calculated for several structural models based on density functional theory, identify the lattice mismatch and the interfacial chemical composition as main factors for the stability of the interface. Molecular dynamics simulations suggest a mechanism of Al diffusion to explain the formation of the complex Al 13 Fe 4 and Al 5 Fe 2 phases at the Al∥Fe interface.

We have determined the crystal structure and the magnetic state of the CeFe 9 Si 4 intermetallic compound. Our revised structural model (fully ordered tetragonal unit cell, I 4/ mcm ) agrees with the previous literature report, except for some minor quantitative differences. Magnetically, the CeFe 9 Si 4 undergoes a ferromagnetic transition at the temperature T C ≈ 94 K. Ferromagnetism in the combined Ce-Fe spin system is a result of interplay between the localized magnetism of the Ce sublattice and the Fe band (itinerant) magnetism. Ferromagnetic ordering obeys the rather general rule that the exchange spin coupling between atoms possessing more than half-full d shells with atoms possessing less than half-full d shells is antiferromagnetic (where the Ce atoms are considered as light d elements). Since in rare-earth metals from the light half of the lanthanide series, the magnetic moment is directed opposite to the spin, this results in ferromagnetism. The magnetoresistance and the magnetic specific heat show an additional temperature-dependent feature (a shoulder) deep inside the ferromagnetic phase that is considered to originate from the influence of the magnetization on the electronic band structure via the magnetoelastic coupling, which alters the Fe band magnetism below T C . The ferromagnetic phase of CeFe 9 Si 4 is magnetically soft.

The intermetallic Al 5 Co 2 is defined as a structurally complex material and is considered a low-order quasicrystalline approximant. A single crystal of Al 5 Co 2 (001) was obtained by the Czochralski method. The sample was characterized by X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), and X-ray photoelectron diffraction (PED). The surface composition was also analyzed by XPS, indicating only Al and Co compounds. In the current research, the crystal structure was qualitatively analyzed using the LEED patterns for different incident beam energies indicating a (1 × 1) termination, also in accordance with some literature works. The structure study was performed by applying the standard software MSCD and showed a (1 × 1) pattern. In addition, four different termination models for this termination were tested. The reliability factor indicated that the best termination belongs to the Al-rich surface layer.

A binary phase with Al 4 Ir 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 P 3 c 1 with the unit cell parameters a = 12.8802(2) Å and c = 9.8130(2) Å. This structure is derived from the Ni 2 Al 3 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 Al 4 Ir 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.

In a search for unconventional heavy-Fermion compounds with the localized 4f moments distributed quasiperiodically instead of a conventional distribution on a regular, translationally periodic lattice, we have successfully synthesized a stable Ce 3 Au 13 Ge 4 Tsai-type 1/1 quasicrystalline approximant of the off-stoichiometric composition Ce 3+ x Au 13+ y Ge 4+ z ( x = 0.17, y = 0.49, z = 1.08) and determined its structural model. The structure is body-centered-cubic (bcc), with space group Im 3̅, unit cell parameter a = 14.874(3) Å, and Pearson symbol cI 174, and can be described as a bcc packing of partially interpenetrating multishell rhombic triacontahedral clusters. The cerium sublattice, corresponding to the magnetic sublattice, consists of a bcc packing of Ce icosahedra with an additional Ce atom in a partially occupied site (occupation 0.7) at the center of each icosahedron. The measurements of its magnetic properties and the specific heat have demonstrated that it is a regular intermetallic compound with no resemblance to heavy-Fermion systems. The partially occupied Ce2 site in the center of each Ce1 icosahedron, the mixed-occupied Au/Ge ligand sites between the Ce2 and Ce1 atoms, and the random compositional fluctuations due to nonstoichiometry of the investigated Ce 3+ x Au 13+ y Ge 4+ z alloy introduce randomness into the Ce magnetic sublattice, which causes a distribution of the indirect-exchange antiferromagnetic interactions between the spins. Together with the geometric frustration of the triangularly distributed Ce moments, this leads to a spin-glass phase below the spin freezing temperature T f ≈ 0.28 K.

A few low-order approximants to decagonal quasicrystals have been shown to provide excellent activity and selectivity for the hydrogenation of alkenes and alkynes. It is the case for the Al 13 Co 4 compound, for which the catalytic properties of the pseudo-2-fold orientation have been revealed to be among the best. A combination of surface science studies, including surface X-ray diffraction, and calculations based on density functional theory is used here to derive an atomistic model for the pseudo-2-fold o -Al 13 Co 4 surface, whose faceted and columnar structure is found very similar to the one of the 2-fold surface of the d -Al-Ni-Co quasicrystal. Facets substantially stabilize the system, with energies in the range 1.19-1.31 J/m 2 , i.e., much smaller than the ones of the pseudo-10-fold (1.49-1.68 J/m 2 ) and pseudo-2-fold (1.66 J/m 2 ) surfaces. Faceting is also a main factor at the origin of the Al 13 Co 4 catalytic performances, as illustrated by the comparison of the pseudo-10-fold, pseudo-2-fold and facet potential energy maps for hydrogen adsorption. This work gives insights toward the design of complex intermetallic catalysts through surface nanostructuration for optimized catalytic performances.

A new ternary phase with a composition Al 1+ x V 2 Sn 2- x ( x = 0.19) has been found during investigation of the Al-V-Sn ternary system. Single-crystal X-ray diffraction measurements reveal that this ternary phase crystallizes with an orthorhombic structure with a = 5.5931(1) Å, b = 18.8017(5) Å, and c = 6.7005(2) Å (space group Cmce ). This compound is thus isostructural to the GaV 2 Sn 2 structure type, showing a layered structure composed of vanadium cluster bands formed with pentagonal faces intercalated by Sn atom layers. High-resolution transmission electron microscopy measurements confirm the orthorhombic structure. Regarding lattice perfection, no dislocation could be identified within the probed Al 1.19 V 2 Sn 1.81 single-crystal lamella. Ab initio calculations reveal a reduction of the density of states at the Fermi level, which could be attributed to both a Hume-Rothery effect combined with strong spd hybridization.

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- Al 13 Co 4 (100), m -Al 13 Fe 4 (010) and m -Al 13 Ru 4 (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- Al 13 Co 4 (100) and o- Al 13 Co 4 (010). As a result, the following room-temperature activity order is determined: Al 13 Co 4 (100) < Al 13 Co 4 (010) < Al 13 Ru 4 (010) < Al 13 Fe 4 (010). Moreover, Al 13 Co 4 (010) is found to be the most active surface at 110°C, and even more selective to butene (100%) than previously investigated Al 13 Fe 4 (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 Al 13 TM 4 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 Al 13 TM 4 compounds.

Following the recent determination of the Al 3 AuIr 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 BGa 8 Ir 4 compound.

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.