Al Pd Mn Quasicrystals Research Articles

Singular Continuous and Nonreciprocal Phonons in Quasicrystal AlPdMn.

In quasicrystals lacking translational symmetry but having highly ordered structures, understanding how phonons propagate in their aperiodic lattices remains an unsolved issue. We present an inelastic neutron scattering study on acoustic phonon modes of icosahedral quasicrystal AlPdMn, revealing hierarchical pseudo-gap structure in low-energy acoustic modes. Additionally, phonon intensities are asymmetric in energy and wave vectors with respect to the Bragg peak, indicating characteristic nonreciprocal phonon propagation in quasicrystals.

Liquid Exfoliation of Icosahedral Quasicrystals

AbstractThe realization of quasicrystals has attracted a considerable attention due to their unusual structures and properties. The concept of quasicrystals in the atomically thin materials is even more appealing due to the in‐plane covalent bonds and weak interlayer interactions. Here, it is demonstrated that 2D quasicrystals can be created/isolated from bulk phases because of long‐range interlayer ordered aperiodic arrangements. An ultrasonication‐assisted exfoliation of polygrained icosahedral Al–Pd–Mn quasicrystals at room temperature shows the formation of a large area of mono‐ and few layers in threefold quasicrystalline plane. The formation of these layers from random grain orientation consistently indicates that the threefold plane is most stable in comparison to the twofold and fivefold planes in icosahedral clusters. The above experimental observations are further supported with help of theoretical simulations. The mono‐ and few‐layered aperiodic planes render plentiful active sites for the catalysis of hydrogen evolution reaction. The threefold 2D quasicrystalline plane exhibits a hydrogen evolution reaction overpotential of ≈100 mV (160 times less than bulk counterpart) and long‐term durability. These systems constitute the first demonstration of quasicrystalline monolayer ordering in a free‐standing thin layer without requiring the support of periodic or aperiodic substrate.

Wetting and adhesion properties of quasicrystals and complex metallic alloys

This paper focuses at wetting on Al-based quasicrystals and complex metallic alloys (CMAs), which comprise a significant number of crystalline compounds of changing lattice complexity, according to composition. Such compounds are thermodynamically stable and may be prepared into various sample shapes that allow easy measurement of surface physical properties in air. Surface energy (γS) is one of the few fundamental properties of condensed matter: it defines the equilibrium shape of a crystal, it determines the interfacial behaviour of any piece of liquid or solid against another body, etc. The paper summarizes our attempts to determine the surface energy of a large variety of CMAs, including the stable, icosahedral AlCuFe and AlPdMn quasicrystals, all equipped with their native oxide layer when placed in ambient conditions. Experimental evidence is given that the surface energy correlates to the electronic density of states underneath the oxide layer as long as its thickness remains below 10 nm. Correlation to the thickness of the oxide on the one hand and on the other to specific features of the electronic density of states will be emphasized, in line with the varying complexity of the studied CMA compounds. Potential application to low-stick cookware will be addressed with a view at finding alternatives to fluorinated surface layers.

Finite size specimens with cracks of icosahedral Al—Pd—Mn quasicrystals

Icosahedral quasicrystals are the most important and thermodynamically stable in all about 200 kinds of quasicrystals currently observed. Beyond the scope of classical elasticity, apart from a phonon displacement field, there is a phason displacement field in the elasticity of the quasicrystal, which induces an important effect on the mechanical properties of the material and makes an analytical solution difficult to obtain. In this paper, a finite element algorithm for the static elasticity of icosahedral quasicrystals is developed by transforming the elastic boundary value problem of the icosahedral quasicrystals into an equivalent variational problem. Analytical and numerical solutions for an icosahedral Al—Pd—Mn quasicrystal cuboid subjected to a uniaxial tension with different phonon—phason coupling parameters are given to verify the validity of the numerical approach. A comparison between the analytical and numerical solutions of the specimen demonstrates the accuracy and efficiency of the present algorithm. Finally, in order to reveal the fracture behavior of the icosahedral Al—Pd—Mn quasicrystal, a cracked specimen with a finite size of matter is investigated, both with and without phonon—phason coupling. Meanwhile, the geometry factors are calculated, including the stress intensity factor and the crack opening displacement for the finite-size specimen. Computational results reveal the importance of phonon—phason coupling effect on the icosahedral Al—Pd—Mn quasicrystal. Furthermore, the finite element procedure can be used to solve more complicated boundary value problems.

The effect of thermal treatment on the magnetic state and cluster-related disorder of icosahedral Al–Pd–Mn quasicrystals

The evolution of the magnetic state and the cluster-related disorder upon thermal treatment of the icosahedral i-Al–Pd–Mn quasicrystals were studied on single-crystalline samples subjected to different thermal annealing sequences involving short and long annealing times and slow or fast cooling modes. In this way, thermal equilibration of the samples was systematically changed by varying the amount of vacancies and thermal strains. The results show that different thermal treatments can reduce or increase reversibly the paramagnetic magnetization of the i-Al–Pd–Mn samples of nominally the same composition by a factor up to 26. Magnetic susceptibility and magnetization measurements have demonstrated comparable importance of the vacancies and thermal strains on the size of the mean effective magnetic moment per Mn atom. While the reduced vacancy concentration reduces the mean effective moment, thermal strains induced by rapid cooling increase it. Theoretical ab-initio calculations of the effect of vacancies and thermal strains on the magnetic properties of a periodic approximant model to the i-Al–Pd–Mn phase predict that vacancies at specific Al sites introduce magnetic moments at the Mn sites, which are otherwise nonmagnetic in the perfect structure. Removal of thermal strains by relaxation of the structure containing vacancies acts in the opposite way by decreasing the moments. X-ray diffraction experiments with full reciprocal space sampling revealed that the cluster-related disorder cannot be improved noticeably by annealing at either 800 °C or 600 °C even for annealing times as long as three months, suggesting that this disorder may be intrinsic to the i-Al–Pd–Mn phase, needed for the electronic or entropic stabilization of the structure.

ChemInform Abstract: Magnetism in Quasicrystals

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Oxygen-induced surface faceting of the threefold-symmetry surface of icosahedral AlPdMn quasicrystal

We have investigated the oxidation behavior of the threefold-symmetry surface of icosahedral AlPdMn quasicrystal by low-energy electron diffraction and Auger electron spectroscopy. Upon oxygen exposure at 740 K, the predominantly planar surface develops facets of an Al-oxide layer, each facet oriented along a high-symmetry direction of the icosahedral bulk structure. While the facets along the fivefold-symmetry directions are structurally well-ordered, the facets along the twofold- and threefold-symmetry directions are structureless. The results are compared with the oxide formation on the fivefold-symmetry surface of icosahedral AlPdMn quasicrystal.

Thermoelectric performance of Al–Pd–Mn quasicrystals: comparison with (1/1, 2/1-)AlPdMnSi approximants and improvement by Ga substitution for Al

Abstract We report the thermoelectric properties of polygrain icosahedral Al–Pd–Mn quasicrystal and compare with those of (1/1, 2/1-)AlPdMnSi approximant crystals. To improve the dimensionless figure of merit (ZT), we substitute Ga for Al atoms in Al–Pd–Mn quasicrystal in accordance with the guiding principle of “weakly bonded rigid heavy clusters” (WBRHCs). Although the thermoelectric properties do not obey the WBRHCs principle, the maximum ZT increased by a factor of 1.4 from 0.18 for Al71Pd20Mn9 to 0.26 for Al68Ga3Pd20Mn9 quasicrystal.

Origin of magnetic moments in i-Al–Pd–Mn quasicrystal

Abstract Using ab-initio density-functional methods in the generalized gradient approximation we have investigated the origin of magnetic moments on Mn atoms in icosahedral (i) Al–Pd–Mn quasicrystals. Two major mechanisms contribute to the formation of magnetic moments on Mn atoms located at special sites – loosely packed environments of the Mn atoms and the interaction of the Mn atoms with nearest-neighbor Pd atoms leading to a shift of the Mn d-band to lower binding energies and an increased partial density of states at the Fermi level. Mn–Pd nearest neighbors exist only at special sites located at intersections of the pseudo-Mackay and the Bergman clusters building the icosahedral structure. A change in the chemical decoration of these sites can lead to a complete disappearance of magnetism in i-Al–Pd–Mn. Total energy calculations show that the models with a chemical ordering disfavoring moment formation on Mn atoms are also lowest in energy.

Magnetism in quasicrystals

The present understanding of magnetic properties of quasicrystals (QCs) is reviewed. In AlMn and AlPdMn QCs, only a few Mn atoms, with concentrations from a few per cent down to 10−4 carry a localized magnetic moment. These moments are coupled via RKKY interactions, leading to a canonical spin-glass ordering at low temperature. They also exhibit a Kondo effect and AlPdMn QCs turned out to be a model system for studying experimentally the competition between Kondo and RKKY interactions in the absence of long-range magnetic ordering. Besides, the presence of localized moments has very unusual consequences for the electronic transport in AlPdMn QCs. In RMgZn and RMgCd QCs (R is a magnetic rare earth), with R concentration around 9%, a spin-glass like freezing is observed. However the short-range antiferromagnetic correlations detected above the freezing temperature are unusual and are certainly influenced by the quasiperiodicity.

Theory of quasicrystal surfaces: Probing the chemical reactivity by atomic and molecular adsorption

The adsorption of oxygen and carbon atoms and of carbon monoxide molecules on a fivefold surface of icosahedral Al–Pd–Mn quasicrystals has been investigated using ab initio density-functional calculations. The quasicrystalline surface has been modeled by periodically repeated slabs cut from rational approximants to the quasicrystalline structure. Atomic and molecular adsorption have been studied for a large number of possible adsorption sites by performing three-dimensional static relaxations of the adsorbate/substrate complex. Four different scenarios for the dissociative adsorption of the CO molecule have been investigated via nudged-elastic band calculations of the transition states. Al and Mn-metal atoms present at the surface bind C and O atoms rather strongly, while Pd atoms are unstable adsorption sites: during relaxation, the adsorbate drifts to the nearest strong-binding site. The chemical reactivity with respect to a CO molecule varies very strongly across the surface. The adsorption close to Mn sites is promoted by rather strong covalent effects, but CO is only physisorbed at Al sites via weak polarization forces. On the basis of the observed local variations of the adsorption strength, we develop scenarios for dissociation and determine the potential energy barriers for this processes. We find that CO adsorbed close to a transition-metal atom can dissociate via an activated process, but the dissociation rate is expected to be rather low because of a high dissociation barrier and a “late” transition state. CO adsorbed close to Al atoms will desorb before dissociation. Surface vacancies present as a consequence of the irregular coordination of the Mackay cluster in the quasiperiodic structure will act as strongly attractive traps for diffusing molecules. Mn surface atoms are located in the center of truncated Mackay clusters. In scanning tunneling electron microscopy (STM) these truncated clusters are imaged as “white flowers”. Surface vacancies are responsible for the “dark stars” in the STM images. Since both are molecular adsorption sites, our study predicts that these characteristic features should be affected by CO adsorption.

CdTe and PbTe nanostructures on the oxidized pentagonal surface of an icosahedral AlPdMn quasicrystal

By means of congruent evaporation, we have deposited CdTe and PbTe onto the oxidized fivefold-symmetry surface of an icosahedral AlPdMn quasicrystal. This procedure results in the formation of nanocrystals in both cases. While the azimuthal orientations of the crystallites are random, the polar orientations are well defined. The crystalline CdTe and PbTe domains expose their (1 1 1) and (0 0 1) faces, respectively, which are aligned parallel to the pentagonal surface of the quasicrystal. The nanometric size of the domains is not a result of the lattice mismatch between the growing film and the substrate as usually observed in molecular-beam epitaxy, but of the limited size of the oxide domains of the substrate surface.

High-resolution inelastic X-ray scattering measurements of an Al72Pd20Mn8 alloy above the melting point

The dynamic structure factor S(Q,ω) of the melt of an icosahedral quasicrystal, Al72Pd20Mn8, was measured at 1223K near the melting point, Tm=1140K, for momentum transfers, Q, from 1.5nm−1 to 5.55nm−1 by means of inelastic X-ray scattering technique using synchrotron radiation at SPring-8. The composition of Al72Pd20Mn8 is a special one in AlPdMn ternary alloys, since icosahedral AlPdMn quasicrystal is formed directly from the melt. The acoustic mode was observed in the low Q region, and a sound velocity was estimated to be 4170±100m/s.

Stabilization of the pentagonal surface of the icosahedral AlPdMn quasicrystal by controlled Si absorption

The Debye-temperature of the pentagonal surface of the icosahedral AlPdMn quasicrystal (QC) is measured by means of low-energy electron diffraction after the absorption of different amounts of Si. We observe an increase of the surface Debye-temperature from 300 ± 7 K for the freshly prepared surface to 330 ± 7 K after the absorption of 60-Å Si. Because the quasicrystalline order persists at the surface in spite of the diffusion of Si into the substrate, we suggest that the diffusion is dominated by a vacancy-mediated process.

In situand real-time probing of quasicrystal solidification dynamics by synchrotron imaging

Quasicrystal growth remains an unsolved problem in condensed matter. The dynamics of the process is studied by means of synchrotron live imaging all along the solidification of icosahedral AlPdMn quasicrystals. The lateral motion of ledges driving faceted growth at the solid-melt interface is conclusively shown. When the solidification rate is increased, nucleation and free growth of new faceted grains occur in the melt due to the significant interface recoil induced by slow attachment kinetics. The detailed analysis of the evolution of these grains reveals the crucial role of aluminum rejection, both in the poisoning of their growth and driving fluid flow.

Dynamics of phason diffusion in icosahedral Al–Pd–Mn quasicrystals

We present a direct experimental investigation of the dynamics of phason defects in an icosahedral quasicrystal. The activation parameters of the diffusion of phasons were determined as Δ H = 4.28 eV and t d 0 - 1 = 4 × 10 21 s - 1 .

Voids and pits on sputter-annealed fivefold terraces of icosahedral Al–Pd–Mn quasicrystals

Our scanning tunneling microscope studies on clean surfaces of fivefold icosahedral Al70.2Pd20.7Mn9.1 reveal that, after annealing at 900 K, two types of flat depressions are visible on terraces: voids (small, shallow, flat-bottomed depressions bordered by 2.5 Å steps) and pits (deeper, and often larger, depressions). They are different not only in their dimensions but in their behavior: voids preferentially nucleate on some of the terraces and, after annealing at 925 K, their coalescence and growth leads a new type of termination to be exposed. Pits, however, do not destroy the terraces on which they exist, under the experimental conditions. At the bottom of 4.1 Å-deep pits, 2.5 Å-deep voids nucleate, so that the total depth of the pit is 6.6 Å. We propose that 2.5 Å void-rich terraces are metastable terminations, and that these metastable terminations are also exposed at the bottom of the 4.1 Å-deep pits.

Kondo-like magnetic scattering in electrical transport of Al70−xBxPd20Mn10 (x = 0, 0.5, 1, 2 and 4) quasicrystals

The electrical conductivity and magnetization behaviour of single-phase Al70−xBxPd20Mn10 (x = 0, 0.5, 1, 2 and 4) quasicrystals have been measured. We observe that boron substitution for aluminium causes large changes in the σ vs. T behaviours. σ vs. (ln T)−1 plots shows linear behaviour. Scaling was achieved by plotting {(σ(T) − σmin)/(σ4 − σmin)} vs. [ln(T/Tmin)]−1. These samples show large positive magnetoresistance with H2 behaviour. In the Al–B–Pd–Mn and Al–Pd–Mn quasicrystals, the observed σ(T) behaviour has been explained using the weak localization theory, including Kondo-type spin-flip scattering and not due to spin-orbit scattering.

In situ observation of pore evolution during melting and solidification of Al–Pd–Mn quasicrystals by synchrotron X-ray radiography

It is now generally admitted that pores are intriguing special features of quasicrystals. Therefore, we have performed an ‘in situ’ and real time observation of the pore evolution during directional solidification and melting cycles of an icosahedral Al–Pd–Mn bi-grained sample, by synchrotron X-ray radiography. Rather surprisingly, no pore was observed to grow during the solidification stages. Nucleation and growth of pores were first seen during melting. These pores were subsequently shrinking either just before being absorbed or during resumption of directional solidification. It is concluded that the vacancy origin of pores is certainly valid whereas the vacancy supersaturation, needed thereby to explain their growth, would be more probably related to the peculiar structure of quasicrystal than to the destruction of the thermal equilibrium.

Core and valence level photoemission and photoabsorption study of icosahedral Al–Pd–Mnquasicrystals

The electronic structure of quasicrystalline Al–Pd–Mn is investigated by means of valenceand core level photoelectron spectroscopy. Variations of the photoionization cross section inthe constituents’ valence electronic levels as a function of photon energy are used toidentify contributions from the different atomic species, in particular near the Pd 4d Cooperminimum. Resonant photoemission at the Mn 2p absorption edge shows the contribution ofthe Mn 3d states to the density of states in a region near the Fermi level. Theasymmetry of Pd 3d and Mn 2p core level photoemission lines, and its differencefor emission from metallic and quasicrystalline phases, are utilized to infer thecontributions of the different constituents to the density of states at the Fermi level.