Quasicrystalline Approximants Research Articles

STEM Structural Investigation of RE‐Au‐Si 1/1 Approximants

AbstractRE‐Au‐Si (RE=Ho, Tb) systems are 1/1 Tsai‐type quasicrystalline approximants with a cluster center decoration that can vary from a disordered tetrahedron to a rare‐earth atom. The local atomic structure of three different samples was observed by scanning transmission electron microscopy and interpreted in the light of high‐angle annular dark field simulated scanning transmission electron microscopy images. It is found that the combination of these two methods allows to identify differences in the chemical decoration of the cluster centers through quantitative analysis of line profiles.

Variation of Pressure-Induced Valence Transition with Approximation Degree in Yb-Based Quasicrystalline Approximants

Variation of Pressure-Induced Valence Transition with Approximation Degree in Yb-Based Quasicrystalline Approximants

Microstructural characterization of rapidly solidified Al-13.5 at.% Cr and Al-13.5 at.% V alloys for catalytic applications

Intermetallic compounds, due to their well-defined stoichiometry, arrangement of atoms and controlled crystal structure, are a promising alternative to expensive noble metal catalysts. In this paper, the catalytic properties of Al-13.5 at.% Cr and Al-13.5 at.% V alloys, corresponding to the quasicrystalline approximants Al45Cr7 and Al45V7, were investigated for the first time.The alloys in the form of fragmented brittle ribbons were produced by the melt-spinning. The microstructure of the ribbons, both in the as-spun state and after heat treatment (100 h at 600 °C), was characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. In the as-spun state, the ribbons showed a multiphase microstructure. In addition to the Al45Cr7 phase in the Al-Cr alloy and the Al3V phase in the Al-V alloy, they also contained α(Al) solid solution and icosahedral quasicrystalline phases. After heat treatment, the alloys became almost single phase, consisting mainly of stable monoclinic phases: Al45Cr7 or Al45V7. The catalytic performance of the phenylacetylene hydrogenation reaction was tested on as-spun and heat-treated alloys that had previously been pulverized and sieved to select a powder fraction of less than 32 µm. All the tested materials show high substrate conversion, above 80% after 1 h reaction, along with high activity rate. The homogenized powders demonstrated a slightly better properties in relation to as-spun materials. These results confirm the potential of intermetallic catalysts, including the tested alloys, in hydrogenation reactions and verify the possibility of using the metallurgical method to obtain catalytically active materials.Graphical abstract

Heavy fermion behaviour in Pr-based quasicrystalline approximants

Heavy fermion behaviour in Pr-based quasicrystalline approximants

Formation enthalpies of Al–Mn–Pd and the structure of the i-AlMnPd quasicrystal

This paper reports formation enthalpies of phases in the Al–Mn–Pd ternary alloy system as calculated from first principles using electronic density functional theory. We consider all crystal structures as reported in the assessed phase diagrams of the ternary and its binary alloy subsystems (Al–Mn, Al–Pd, and Mn–Pd), as well as additional reported or hypothetical structures. Icosahedral and decagonal quasicrystalline approximants are among the structures that we predict to be stable, or nearly so. Our results suggest the need for careful experimental reexamination of phase stability in each of the alloy systems, in tandem with further efforts to refine crystallographic and ab-initio structures.

Two-dimensional square and hexagonal oxide quasicrystal approximants in SrTiO3 films grown on Pt(111)/Al2O3(0001).

The formation of two-dimensional oxide dodecagonal quasicrystals as well as related complex approximant phases was recently reported in thin films derived from BaTiO3 or SrTiO3 perovskites deposited on (111)-oriented Pt single crystals. Here, we use an all-thin-film approach in which the single crystal is replaced by a 10 nm thick Pt(111) buffer layer grown by molecular beam epitaxy on an Al2O3(0001) substrate. An ultra-thin film of SrTiO3 was subsequently deposited by pulsed laser deposition. The film stacking and structure are fully characterized by diffraction and microscopy techniques. We report the discovery of two new complex phases obtained by reduction of this system through high temperature annealing under ultrahigh vacuum conditions. The formation of a new large square approximant with a lattice parameter equal to 44.4 Å is evidenced by low-energy electron diffraction and scanning tunneling microscopy (STM). Additionally, a new 2D hexagonal approximant phase with a lattice parameter of 28 Å has been observed depending on the preparation conditions. Both phases can be described by two different tilings constructed with the same basic square, triangle and rhombus tiles possessing a common edge length of about 6.7 Å. Using the tiling built from high resolution STM images, we propose an atomic model for each approximant which accounts for the experimental observations. Indeed, the STM images simulated using these models are found to be in excellent agreement with the experimental ones, the bright protrusions being attributed to the topmost Sr atoms. In addition our theoretical approach shows that the adhesion of the oxide layer is rather strong (-0.30 eV Å-2). This is attributed to charge transfer, from the most electropositive elements (Sr and Ti) to the most electronegative ones (Pt and O), and to hybridization with Pt-states. Density of states calculations indicate differences in the electronic structure of the two approximants, suggesting different chemical and physical properties. This all-thin-film approach may be useful to explore the formation of complex two-dimensional oxide phases in other metal-oxide combinations.

Classical and Quantum Magnetic Ground States on an Icosahedral Cluster

Recent discovery of various magnetism in Tsai-type quasicrystalline approximants, in whose alloys rare-earth ions located on icosahedral apices are coupled with each other via the Ruderman--Kittel--Kasuya--Yosida interaction, opens an avenue to find novel magnetism originating from the icosahedral symmetry. Here we investigate classical and quantum magnetic states on an icosahedral cluster within the Heisenberg interactions of all bonds. Simulated annealing and numerical diagonalization are performed to obtain the classical and quantum ground states. We obtain qualitative correspondence of classical and quantum phase diagrams. Our study gives a good starting point to understand the various magnetism in not only quasicrystalline approximants but also quasicrystals.

Composition Effect of Kondo Behavior in Au–Al–Ce Quasicrystalline Approximants

Composition Effect of Kondo Behavior in Au–Al–Ce Quasicrystalline Approximants

Two-dimensional oxide quasicrystal approximants with tunable electronic and magnetic properties.

Recently, the discovery of the quasiperiodic order in ultra-thin perovskite films reinvigorated the field of 2-dimensional oxides on metals, and raised the question of the reasons behind the emergence of the quasiperiodic order in these systems. The effect of size-mismatch between the two separate systems has been widely reported as a key factor governing the formation of new oxide structures on metals. Herein, we show that electronic effects can play an important role as well. To this end, the structural, thermodynamic, electronic and magnetic properties of freestanding two-dimensional oxide quasicrystalline approximants and their characteristics when deposited over metallic substrates are systematically investigated to unveil the structure-property relationships within the series. Our thermodynamic approach suggests that the formation of these aperiodic systems is likely for a wide range of compositions. In addition, the magnetic properties and work functions of the thin films can be controlled by tuning their chemical composition. This work provides well-founded general insights into the driving forces behind the emergence of the quasiperiodic order in ternary oxides grown on elemental metals and offers guidelines for the discovery of new oxide quasicrystalline ultra-thin films with interesting physical properties.

Band engineering in Al-TM (TM=Rh, Ir) quasicrystalline approximants via alloying and enhancement of thermoelectric properties

The binary Al–Ir 1/0 quasicrystalline approximant is predicted to be a narrow band gap semiconductor with a large Seebeck coefficient; however, it has not yet been realized practically because the presence of Al vacancies causes excess hole doping, which results in a small Seebeck coefficient. Here, we synthesized and measured the true density, lattice constant, and thermoelectric properties of Al-(Rh,Ir) quasicrystalline approximants with a series of Al73.3(RhxIr1-x)26.7 (x = 0, 0.25, 0.5, 0.75, 1.0) compositions. The number of atoms per unit cell calculated using the true density, nominal composition, and lattice parameter increased with increasing Rh content x. The compositional dependence of the Seebeck coefficient and dimensionless figure of merit zT showed a maximum value at x = 0.25, 850 K. The results indicated that Rh doping could effectively suppress Al vacancies, and the width of the band gap decreased with increasing x. In other word, a trade-off was observed between the amount of Al vacancies and the width of the band gap between the Al–Ir and Al–Rh 1/0 quasicrystalline approximants.

Nonwetting Behavior of Al-Co Quasicrystalline Approximants Owing to Their Unique Electronic Structures.

Good wetting is generally observed for liquid metals on metallic substrates, while poor wetting usually occurs for metals on insulating oxides. In this work, we report unexpected large contact angles for lead on two metallic approximants to decagonal quasicrystals, namely, Al5Co2 and Al13Co4. Intrinsic surface wettability is predicted from first principles, using a thermodynamic model based on the Young equation, and validated by the good agreement with experimental measurements performed under ultra-high vacuum by scanning electron microscopy. The atomistic details of the atomic and electronic structures at the Pb-substrate interface, and the comparison with Pb(111)/Al(111), underline the influence of the specific electronic structures of quasicrystalline approximants on wetting. Our work suggests a possible correlation of the contact angles with the density of states at the Fermi energy and paves the way for a better fundamental understanding of wettability on intermetallic substrates, which has potential consequences in several applications such as supported catalysts, protective coatings, or crystal growth.

High ductility aluminum alloy made from powder by friction extrusion

Friction extrusion has been used to consolidate and extrude aluminum alloy powder into bulk nanostructured rods. Extrudates exhibited 450 MPa ultimate tensile strength, 380 MPa yield strength, and 15.7% elongation at ambient temperature. Twice the elongation was achieved compared to conventional direct extrusion of the same material, with similar ultimate and yield strengths, and is attributed to extensive reduction of the matrix grain size and refinement and redistribution of nanoscale second phases. Quasicrystalline approximants within the starting powder were not observed in the extrudate, indicating shear processing was effective in solutionizing some fraction of the alloying elements and re-precipitating them as second phase nanoscale intermetallics.

Clusters in the Al13TM4 quasicrystalline approximants: role of the surface structures and properties

Clusters in the Al₁₃TM₄ quasicrystalline approximants: role of the surface structures and properties

Direct observation of heterogeneous valence state in Yb-based quasicrystalline approximants

We study the electronic structure of Tsai-type cluster-based quasicrystalline approximants, Au$_{64}$Ge$_{22}$Yb$_{14}$ (AGY-I), Au$_{63.5}$Ge$_{20.5}$Yb$_{16}$ (AGY-II), and Zn$_{85.4}$Yb$_{14.6}$ (Zn-Yb), by means of photoemission spectroscopy. In the valence band hard x-ray photoemission spectra of AGY-II and Zn-Yb, we separately observe a fully occupied Yb 4$f$ state and a valence fluctuation derived Kondo resonance peak, reflecting two inequivalent Yb sites, a single Yb atom in the cluster center and its surrounding Yb icosahedron, respectively. The fully occupied 4$f$ signal is absent in AGY-I containing no Yb atom in the cluster center. The results provide direct evidence for a heterogeneous valence state in AGY-II and Zn-Yb.

Bronze-mean hexagonal quasicrystal.

The most striking feature of conventional quasicrystals is their non-traditional symmetry characterized by icosahedral, dodecagonal, decagonal or octagonal axes. The symmetry and the aperiodicity of these materials stem from an irrational ratio of two or more length scales controlling their structure, the best-known examples being the Penrose and the Ammann-Beenker tiling as two-dimensional models related to the golden and the silver mean, respectively. Surprisingly, no other metallic-mean tilings have been discovered so far. Here we propose a self-similar bronze-mean hexagonal pattern, which may be viewed as a projection of a higher-dimensional periodic lattice with a Koch-like snowflake projection window. We use numerical simulations to demonstrate that a disordered variant of this quasicrystal can be materialized in soft polymeric colloidal particles with a core-shell architecture. Moreover, by varying the geometry of the pattern we generate a continuous sequence of structures, which provide an alternative interpretation of quasicrystalline approximants observed in several metal-silicon alloys.

Structural, mechanical and thermal characterization of an Al-Co-Fe-Cr alloy for wear and thermal barrier coating applications

A structural, mechanical and thermal characterization of an Al-Co-Fe-Cr coating alloy was performed. An atomized powder with atomic composition of Al71Co13Fe8Cr8 was thermally sprayed by high velocity oxygen fuel (HVOF) on a steel substrate. X-ray diffraction, scanning and transmission electron microscopy and differential scanning calorimetry were used to characterize the atomized powder and the sprayed coating. Vickers microhardness and pin-on- plate wear test were carried out for mechanical and tribological characterization and an infrared camera was used to evaluate the insulation capacity of the coating material. The results show that both the atomized powder and the coating material were composed predominantly by a quaternary extension of the hexagonal Al5Co2 phase and by the monoclinic Al13Co4. Both phases are quasicrystalline approximants of a decagonal Al-Co quasicrystal. The coating samples presented high values of micro-hardness, close to 500HV and substantially low friction coefficient values, around 0.05. The coatings were good thermal insulators, decreasing by 30% the surface temperature of a sample exposed to a hot plate in comparison with the carbon steel substrate.

Unified cluster-based description of valence bands in AlIr, RuAl2, RuGa3 and Al–TM quasicrystalline approximants

Formation of the valence bands in AlIr, RuAl2 and RuGa3 was analysed on the basis of Wannier functions. It was sufficient to consider nine (s-, p- and d-like) Wannier functions per cluster centred on each transition metal for describing the valence bands. As in the case of Al-Cu-Ir quasicrystalline approximant, which we reported previously [Kitahara K, Takagiwa Y and Kimura K 2015 J. Phys. Soc. Jpn. 84 014703-1-8], covalent bonds between clusters form where the distance between neighbouring clusters is about 0.3 nm, making conduction states as antibonding functions. sp3d2-, sp3- and py-like Wannier functions are used for the covalent bonds in AlIr, RuAl2 and RuGa3, respectively.

Contribution to thermodynamic description of Al-Pd system

The homogeneity range around the Al3Pd stoichiometry, representing the family of quasicrystalline approximants denoted as ϵn and/or (Al3Pd), was involved in the calculated Al-Pd phase diagram. The calculations were performed by means of the CALPHAD method using the Thermo-Calc software. A novel thermodynamic description of the ϵn/(Al3Pd) phase was proposed based on the (Al%,Pd)3(Al,Pd%)1 two-sublattice model. Existing thermodynamic parameters of stoichiometric phases Al4Pd and Al21Pd8 were slightly adjusted. The transition between phases ϵn/(Al3Pd) and γ-Al21Pd8 was predicted thermodynamically at 619°C.

Assessment of phase constitution on the Al-rich region of rapidly solidified Al-Co-Fe-Cr alloys

The formation of quasicrystalline approximants in rapidly solidified Al-Co-Fe-Cr alloys was investigated. Alloys of atomic composition Al71Co13Fe8Cr8, Al77Co11Fe6Cr6 and Al76Co19Fe4Cr1 were produced using melt spinning and arc melting methods and their microstructural characterization was carried out by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Up to the present there is no consensus in the literature regarding the formation of quasicrystalline phase or quasicrystalline approximants in the Al71Co13Fe8Cr8 alloy. This work presents, for the first time, a detailed structural characterization of selected alloys in the Al-Co-Fe-Cr system close to the atomic composition Al71Co13Fe8Cr8. The results indicated the samples to be composed, mostly, by two intermetallic phases, which are quaternary extensions of Al5Co2 and Al13Co4 and are quasicrystalline approximants. Although the Al5Co2 phase has already been reported in the Al71Co13Fe8Cr8 alloy, the presence of the monoclinic Al13Co4 is now identified for the first time in the as cast state. In the binary Al-Co system a quasicrystalline phase is known to form in a rapidly solidified alloy with composition close to the monoclinic and orthorhombic Al13Co4 phases. This binary quasicrystalline phase presents an average valence electron per atom (e/a) between 1.7 and 1.9; thus, in addition to the Al71Co13Fe8Cr8 alloy, the compositions Al77Co11Fe6Cr6 and Al76Co19Fe4Cr1 were chosen to be within the region of formation of the quaternary extension of the Al13Co4 phase and also within the (e/a) of 1.7 to 1.9. However, no quasicrystalline phase is present in any of the studied alloys. The Al-Co-Fe-Cr system, around the compositions studied, is composed of quaternary extensions of Al-Co intermetallic phases, which present solubility of Fe and Cr at Co atomic sites.

Conical Dispersion and Effective Zero Refractive Index in Photonic Quasicrystals

It is recognized that for a certain class of periodic photonic crystals, conical dispersion can be related to a zero-refractive index. It is not obvious whether such a notion can be extended to a noncrystalline system. We show that certain photonic quasicrystalline approximants have conical dispersions at the zone center with a triply degenerate state at the Dirac frequency, which is the necessary condition to qualify as a zero-refractive-index medium. The states in the conical dispersions are extended and have a nearly constant phase. Experimental characterizations of finite-sized samples show evidence that the photonic quasicrystals do behave as a near zero-refractive-index material around the Dirac frequency.