Icosahedral Alloys Research Articles

ChemInform Abstract: Magnetic Properties of Al‐Based Icosahedral Alloys

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Electron transport in Al 65Cu 20Co 15− xFe x quasicrystals

We have studied the electrical resistivity and magnetoresistance for the system Al 65Cu 20Co 15− x Fe x (0≤ x≤15). The sample with x=0 could be indexed as the decagonal (D) phase, whereas the sample with x=15 forms the icosahedral (I) phase. The sample with intermediate x (2.5≤ x≤12.5), however, consists of both phases. The temperature coefficient of the resistivity (TCR) is positive when x≤17.5. The magnitude of the TCR for these samples is smaller than for icosahedral alloys. However, the other samples exhibit a negative TCR except below 30 K. The magnetoresistance of the samples with pure I or D phase, exhibits an increase of over 1% in a field of 45 kOe. The samples with intermediate x on the other hand, show relatively smaller increases. These results have been analyzed qualitatively in the framework of existing models.

Decahedral and icosahedral Cu-Au alloy particles grown under a controlled-potential region in acid solutions

Decahedral and icosahedral Cu-Au alloy particles have been formed by electrochemical deposition of Cu and Au in perchloric acid and sulfuric acid solutions at electrode potentials where formation of decahedron and icosahedron Au particles has not been previously observed. This observation is thought to occur due to a contraction of the Au overlayer, induced by the presence of an incompletely discharged ${\mathrm{Cu}}^{+}$ ion deposited on the Au overlayer of the alloy particles in the underpotential deposition region of Cu. This phenomenon is therefore analogous to the alkali-metal-induced reconstruction of Au surfaces. However, decahedral and icosahedral Cu-Au alloy particles were not observed in the Cu-Au alloy particles when the Cu content of the alloy exceeded 30%. It is known that the topmost layer of the reconstructed Au(111) surface is contracted by about 4% and that the lattice spacing of the Cu-Au alloy containing 30% Cu is shortened by about 3% with respect to the Au bulk lattice. Therefore, the lattice mismatch between the Au overlayer and the substrate of Cu-Au alloy particles containing more than 30% Cu is small enough so that no decahedral and icosahedral particles are formed.

Structural models of quasicrystals: Significance of positron annihilation studies

We discuss implications of our results of positron annihilation studies of various icosahedral quasicrystalline alloys on the structural models of this new phase. We have studied Al-Mn, Al-Mn-Si, Al-Cu-Li and Al-Cu-Fe and a two component positron annihilation lifetime spectrum seems characteristics of all these icosahedral quasicrystalline alloy phases. Analysis of the lifetime spectra yields vacancy sizes varying from monovacancies to hexavacancies with concentration of 1 to 8 ppm in various quasicrystalline alloys. These vacancy concentrations are about three orders of magnitude less than that observed in metallic glasses. We also notice that in the case of metastable quasicrystalline alloys such as Al-Mn and Al-Mn-Si, these vacancy clusters disappear during the crystallization process resulting in single lifetime spectra. On the other hand, for stable quasicrystalline alloys, two component life time spectra, which indicate the presence of vacancy clusters, continue to exist even after prolonged heat treatment. Our results seem incompatible with the space filling Penrose tiling or random tiling models and favour cluster-based models.

Magnetic properties of Al-based icosahedral alloys

One of the fundamental questions which has not been answered sufficiently well till today is whether a nonclassical symmetry such as an icosahedral one would induce any special and distinctive features in the physical properties of the systems. Most theoretical investigations indicate that there are unique features in the electronic structure on account of icosahedral symmetry which could for example, influence the magnetic properties significantly. Experimental studies, particularly, on Al-based icosahedral alloys, have shown a large variation in magnetic properties ranging from diamagnetic to Curie like and Pauli paramagnetism on one hand and ferromagnetic and spin glass like behaviour on the other. In the present article, we will concentrate on some aspects of moment formation and variation in magnetic properties in Al-Mn based icosahedral systems and their dependence on the method of preparation which presumably controls and introduces the variable disorder. We will also try to answer some pertinent questions related to magnetism in icosahedral systems such as extent and origin of moment formation existence of magnetic and nonmagnetic Mn sites and distribution of interatomic distances reponsible for variation in magnetic properties.

Electronic Structure of Icosahedral Alloys Studied by Ultrahigh Energy Resolution Photoemission Spectroscopy.

The results of a systematic ultrahigh resolution ultraviolet photoemission study of the electronic structure of stable icosahedral alloys ${\mathrm{Al}}_{64}$${\mathrm{Cu}}_{24}$${\mathrm{Fe}}_{12}$, ${\mathrm{Al}}_{65}$${\mathrm{Cu}}_{20}$${\mathrm{Fe}}_{15}$, ${\mathrm{Al}}_{70}$${\mathrm{Pd}}_{20}$${\mathrm{Mn}}_{10}$, and ${\mathrm{Al}}_{70.5}$${\mathrm{Pd}}_{21}$${\mathrm{Re}}_{8.5}$ are presented. It is shown that all icosahedral alloys have a clearly developed Fermi edge and are thus metallic down to the temperature of measurement (14-45 K). A marked decrease of the spectral intensity towards the Fermi level in these alloys is demonstrated to be consistent with the existence of the theoretically predicted pseudogap. With an experimental resolution of 6 meV, no evidence of the theoretically predicted spikiness of the density of states could be observed.

Triacontahedral growth morphology in icosahedral ZnMg-Y alloy

We report here the first observation of triacontahedral growth morphology in a Zn-Mg-Y face-centred icosahedral alloy. Some considerations on the local order in this phase are given by a structural study of several crystalline phases in the binary Zn-Mg, Zn-Y and Mg-Y systems. Our experimental work shows that the icosahedral phase can coexist with both Mackay-type and Bergman-type (or Frank-Kasper-type) crystalline approximant phases of the Zn-Mg phase diagram. However, the occurrence of triacontahedral growth morphology allows us to classify it among the Bergman-type icosahedral alloys.

Elastic constants of Al-based icosahedral quasicrystals

The elastic constants of several Al-based icosahedral alloys were measured over a temperature range from 4K to 1073 K by the rectangular parallelepiped resonance method. The values of the bulk moduli of these quasicrystals are similar to those of conventional cubic crystals having similar melting temperatures, while the values of Young's and shear moduli are larger in comparison with conventional aluminium alloys. The latter feature is considered to be due to the strong directional bonding in the quasicrystals.

Modulated quasicrystal structures

Abstract A model for the formation of modulated structures frequently observed in icosahedral alloys of the Al[sbnd]Cu[sbnd]Fe type and representing an intermediate phase at the icosahedral phase-crystalline approximant transition is proposed. An assumption that this structural modulation may be due to a weak decagonality of the icosahedral phase is advanced. In our model, two different types of modulated structure can be distinguished. Such a decagonality originating both from phason strains and from a probable anisotropy of icosahedral shells has been considered.

A GENERAL VIEW OF METAL CLUSTERS AND ICOSAHEDRAL ALLOYS IN THE APERIODIC MULTIPLE TWINNING MODEL: CHEMICAL CONCENTRATION GRADIENTS, GROWTH RULE, INTERATOMIC DISTANCES AND NUCLEAR MAGNETIC RESONANCE

Recent experiments by electron diffraction and nuclear magnetic resonance (NMR) reported in the literature are consistent with the defective structure inherent to the aperiodic multiple twinning model proposed some years ago by the author for inhomogeneous metal clusters. A similar structural model is suggested for icosahedral (i) alloys, represented in this paper by i-Al—Mn, a binary system formally analogous to the metal-vacancy i-cluster. Determination of the mass density and interatomic distances for i-Al—Mn is easily accomplished from the strain distribution and the stoichiometric composition. A discussion of the nonzero nuclear quadrupole interaction of dilute quadrupolar nuclei in cubic crystals leads to a reexamination of Bloembergen and Rowland’s theory of NMR in cubic alloys and to a computation of the second-order quadrupole broadening of the 55 Mn NMR line in i-Al—Mn according to the model. Skin-depth effects are invoked in order to explain the observed variable intensity ratio of 55 Mn to 27 Al NMR lines in i-Al—Mn, as due to inhomogeneous quasicrystals differing in grain size.

Electronic structure of icosahedral alloys: The case of Al65Cu20Os15

Measurements with photoemission spectroscopy in the photon-energy range 35–130 eV have been used to determine the valence band of the stable icosahedral Al65Cu20Os15. Resonant photoemission near the Os 5p → 5d and 4f → 5d transitions has been employed to show that the feature in the valence band with the maximum intensity at 1.5 (1) eV below the Fermi level is predominantly of the Os 5d character. This has been additionally verified by conducting the photoemission measurements in the constant-initial-state mode and by using the effect of the Cooper minimum in the photoionization cross section of the Os 5d orbitals. The valence band feature with the maximum intensity at 3.7 (1) eV below the Fermi level has been shown as being due mainly to the states of the Cu 3d character. The Os 5d and Cu 3d empirical partial density of states have been determined from the photoemission spectra. The decrease of intensity towards the Fermi level has been interpreted as being indicative of the presence of a theoretically predicted pseudogap around the Fermi level. It has been indicated, however, that the Fermi cut-off also contributes to the observed intensity decrease. It has been demonstrated that the energy resolution of the spectroscopic measurements performed so far on quasicrystals was not high enough to unambiguously determine the presence of such a pseudogap. No unusual features in the valence band of icosahedral Al65Cu20Os15, which could be ascribed to its quasiperiodic nature, have been observed within the resolution of the experiment. High energy-resolution spectroscopic measurements were also shown to be essential to observe the theoretically predicted spikiness of the density of states in quasicrystals. A critical review of published spectroscopic data on the electronic structure of quasicrystals has also been presented.

Atomic and electronic structure of icosahedral Al-Pd-Mn alloys and approximant phases.

The electronic structures of the stable face-centered-icosahedral alloy ${\mathrm{Al}}_{70}$${\mathrm{Pd}}_{20}$${\mathrm{Mn}}_{10}$ and of a hierarchy of rational approximants to the icosahedral phase have been calculated using ab initio pseudopotential, linear-muffin-tin-orbital (LMTO), and tight-binding (TB)-LMTO techniques. The description of the aotmic structure is based on a projection from six-dimensional space, with acceptance volumes chosen such as to reproduce the observed diffraction data. For the lowest-order approximants (1/1 and 2/1 with 128 and 544 atoms in the periodically repeated cell), the electronic eigenvalues and eigenfunctions have been calculated self-consistently using LMTO and ab initio pseudopotential techniques. For the 1/1 approximant, we have also performed a relaxation of the idealized structure using the Hellmann-Feynman forces. For the higher-order approximants (we go up to the 8/5 approximants with 41 068 atoms), the electronic densities of states and the spectral functions have been calculated from the TB-LMTO Hamiltonian via a real-space recursion technique. The electronic density of states (DOS) of the higher-order approximants is characterized by a structure-induced minimum at the Fermi level, indicating the possibility of a Hume-Rothery-type electronic mechanism for the stabilization of the icosahedral phase. However, in the lowest-order approximants the DOS minimum is either flattened or shifted away from the Fermi energy. This is in contrast to the simple icosahedral alloys such as Al-Cu-Li, where the DOS minimum exists in the quasiperiodic phase and in the crystalline approximants. Hence it appears that for the face-centered icosahedral alloys, the structure-induced DOS minimum may be not only a generic, but also a specific property of the quasicrystalline phase. In addition to the spectral properties, we have also studied the character of the electronic states via a calculation of the participation ratio. We find that the states in the vicinity of the Fermi level tend to be more localized than in the rest of the valence band and that the localization is related to certain aspects of the quasicrystalline structure. This is important for understanding the anomalous transport properties of these alloys.

Electronic structure of decagonal Al65Co15Cu20 and Al70Co15Ni15.

Photoemission spectroscopy measurements in the photon-energy range 35--120 eV have been used to make determinations of the valence bands of high-quality decagonal alloys ${\mathrm{Al}}_{65}$${\mathrm{Co}}_{15}$${\mathrm{Cu}}_{20}$ and ${\mathrm{Al}}_{70}$${\mathrm{Co}}_{15}$${\mathrm{Ni}}_{15}$. Resonance photoemission near the Co 3p\ensuremath{\rightarrow}3d transition has been employed to show that the feature in the valence bands of both alloys with a maximum intensity at 0.7(2) eV below the Fermi level is predominantly of the Co 3p character. The feature at 3.7(1) eV in the valence band of ${\mathrm{Al}}_{65}$${\mathrm{Co}}_{15}$${\mathrm{Cu}}_{20}$ has been identified as being mainly of the Cu 3d character, whereas the feature due to the Ni 3d states overlaps with the Co 3d-like feature in the valence band of ${\mathrm{Al}}_{70}$${\mathrm{Co}}_{15}$${\mathrm{Ni}}_{15}$. Within the energy resolution, our results reveal that, contrary to the prediction of recent band-structure calculations and to the widespread qualitative interpretation of various electronic transport data, there is no pseudogap in the density of states at the Fermi level in both decagonal alloys. This shows that decagonal alloys differ significantly in their electronic structure from icosahedral alloys and that a Hume-Rothery mechanism cannot be invoked to explain their stability and electronic transport properties. The need of high-energy-resolution photoemission experiments, which were shown to be essential to observe the theoretically predicted fine structure in the density of states, has been emphasized. The possible influence of chemical and topological disorder on the electronic structure of high-quality stable quasicrystals was indicated. A review of published experimental data on the electronic structure of decagonal alloys has also been presented.

Chemical trends of icosahedral order in AlCuTM quasicrystals

Icosahedral alloys with the composition Al 62Cu 25.5TM 12.5 (TM ≡ V, Cr, Mn, Fe, Co, Ni) were prepared by melt spinning. Prevailing icosahedral phases were found for TM ≡ Cr, Mn and Fe. Phonon and random phason disorder were evaluated by an analysis of X-ray diffraction linewidths. A marked chemical trend was noted for phonon-type disorder, which increases in the sequence Fe < Mn < Cr. The parameter of the hypercubic lattice for the Mn-based icosahedral phase allows information to be derived on the metallic valence of Mn and its local coordination via the effective atomic radius.

The vibrational density of states of three-dimensional quasicrystals

We have computed the vibrational densities of states (DOS) both of several ideal approximants of icosahedral quasicrystals, up to 13/8, and of an actual Al70Pd21Mn9 icosahedral alloy, in the framework of the harmonic approximation. The dynamic matrix has been constructed for a spring potential in the cluster approximation. In order to compute the DOS we have used the continued-fraction expansion of the Green's function and the coefficients of this expansion have been obtained with the recursive technique. The DOS of the 13/8 approximant is in agreement with the one already obtained in the literature, by using the commensurate approximation and the direct diagonalization. This strongly suggests that, provided the size of the cluster is large enough, the two methods are equivalent. On the other hand, recursive technique algorithms are much more convenient from the point of view of computing time. The DOS computed for the actual case of AlPdMn shows a satisfactory qualitative agreement with the neutron-weighted DOS, measured for the same compound. This is an interesting result, since it indicates that, in spite of the simple assumption adopted for the potential, the model describes quite well the main features of the vibrational spectrum of this quasicrystal. Further improvements in this direction can then be made.

Thermal, structural and magnetic properties of icosahedral Al86Cr8Fe6 alloy

Abstract Icosahedral alloy Al86Cr8Fe6 and its crystallization products have been studied with X-ray diffraction, differential scanning calorimetry, 57Fe Mossbauer spectroscopy, and magnetic susceptibility methods. The presence of a multiplicity of Fe sites is demonstrated, and this is interpreted as evidence of intrinsic disorder characteristic of icosahedral alloys. The analysis of the Mossbauer spectra of icosahedral alloys in terms of two doublets is shown to be methodologically incorrect, and consequently the basis of the so-called two-site model is unfounded. The crystallization products of the studied icosahedral alloy are Al13Fe4 and A113Cr2-xFe x- A small magnetic moment is present both in the icosahedral sample and in its crystallization products. The origin of the magnetic moment in icosahedral alloys is discussed in view of recent theoretical and experimental advances in the understanding of the electronic structure of these alloys.

Are decagonal quasicrystals the Hume-Rothery phases?

Photoemission spectroscopy measurements in the photon energy range 40–100 eV have been used to make the first determinations of the valence bands of the high quality decagonal alloys Al 65Co 15Cu 20 and Al 70Co 15Ni 15. Within the energy resolution our results reveal that, contrary to the prediction of recent band-structure calculations and to the widespread qualitative interpretation of various electronic transport data, there is no pseudogap in the density of states at the Fermi level. This suggests that decagonal alloys differ significantly in their electronic structure from icosahedral alloys and that a Hume-Rothery mechanism does not play an important role in accounting for their stability and electronic transport properties.

メカニカルアロイングによる非晶質Ａｌ７５Ｃｕ１５Ｖ１０合金の作製とその構造変化

Al75Cu15V10 powder mixture was mechanically alloyed in a conventional planetary ball mill under an Ar atmosphere. The alloying process and the structure change induced by additional heating were investigated using X-ray diffractometry and differential scanning calorimetry. An amorphous alloy was obtained directly in the powder milled for 15 to 30h. The amorphous alloy obtained after 20h of mechanical alloying showed that by subsequent heating the amorphous to Mackay type icosahedral phase transition occurred in the region between 720 and 740K. The interatomic distances and coordination numbers of nearest neighbors in the amorphous and the Mackay type icosahedral alloys were determined from the principal peak of the radial distribution functions. It was found that atomic structures of these alloys were very similar to those of the same composition prepared by liquid quenching.

Mössbauer studies of Al75Cu15−xFexV10 icosahedral alloys

57Fe Mossbauer effect measurements have been performed at room temperature on the alloy series Al75Cu15−xFexV10 withx=0.15 (amorphous and icosahedral phases), 3 and 6 (icosahedral phases). There is more disorder in the icosahedral phase of thex=0.15 alloy than in its amorphous phase. The bimodal character of the distribution of quadrupole splittings becomes less pronounced with increasingx and disappears forx≥6. This is interpreted in terms of two classes of transition metal sites in Al-transition metal-based icosahedral alloys.

Short-range order of quasicrystals after ball milling

Thermodynamically stable icosahedral Al65Cu20Fe15 alloy is studied using57Fe Mossbauer experiments. Its quasicrystalline structure is subjected to a low energy process of mechanical grinding up to 800 hours. The influence of ball milling on the electric field gradient magnitudes is discussed using an analysis of the Mossbauer spectra to different fitting models. The presence of an amorphous phase which co-exists with the quasicrystalline one is revealed in the early stage of mechanical grinding.