Decagonal quasicrystal in the Al–Pd–Mn system

Abstract

A stable decagonal quasicrystal in Al₇₀Pd_30?xMn_x alloys (x = 10–20) was examined by electron diffraction and high-resolution electron microscopy. The decagonal quasicrystalline grains are formed with definite crystallographic relationships to adjacent icosahedral and Al₃Mn crystalline grains. The structure of the decagonal phase, which is formed as the main phase at near Al₇₀Pd₁₀Mn₂₀ composition, is a mixture of decagonal quasicrystalline regions with some linear phason strain and microcrystalline regions. The structures of both regions may be interpreted in terms of quasiperiodic and periodic tilings, constructed with two types of bond lengths, S (about 2 nm) and L (= τ · S, where τ is the Golden ratio), of the same atom cluster with decagonal symmetry.     

急冷Al80Mn20合金准晶T相的晶化动力学

本文研究了急冷Al₈₀Mn₂₀合金准晶T相的晶化过程。DSC实验证实在晶化过程中激活能保持不变,从而得到了晶化转变机理实验曲线,并确定了晶化激活能和频率因子的数值,它们分别为224kJ/mol和1.2×10¹²s^-1。分析结果表明准晶T相的晶化是由长程扩散所控制的,并在晶化过程中没有显著的新核形成。 关键词：     

Symmetry-dependent Mn-magnetism in Al<Subscript>69.8</Subscript>Pd<Subscript>12.1</Subscript>Mn<Subscript>18.1</Subscript>

We investigated the stability of magnetic moments in Al_69.8Pd_12.1Mn_18.1. This alloy exists in both, the icosahedral (i) and the decagonal (d) quasicrystalline form. The transition from the i- to the d-phase is achieved by a simple heat treatment. We present the results of measurements of the ²⁷Al NMR-response, the dc magnetic susceptibility, and the low-temperature specific heat of both phases. In the icosahedral compound, the majority of the Mn ions carries a magnetic moment. Their number is reduced by approximately a factor of two by transforming the alloy to its decagonal variety. For both compounds, we have indications for two different local environments of the Al nuclei. The first reflects a low density of states of conduction electrons and a weak coupling of the Al nuclei to the Mn-moments. The second type of environment implies a large d-electron density of states at the Fermi level and a strong coupling to the magnetic Mn moments. Spin-glass freezing transitions are observed at T^deca_f=12 K for the decagonal, and T^ico_f=19 K for the icosahedral phase.     

Short-range order of quasicrystals after ball milling

Thermodynamically stable icosahedral Al₆₅Cu₂₀Fe₁₅ alloy is studied using⁵⁷Fe Mössbauer 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 Mössbauer 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.     

Aging of supersaturated Al-Cu-Co decagonal phase

Aging of the supersaturated Al₆₈Cu₁₁Co₂₁ decagonal phase was studied by transmission and scanning electron microscopy. Changes in the twofold [12100] electron diffraction patterns are reported. The decagonal phase aged at 700–800°C exhibited a reduction of the intensities of the quasiperiodic odd-n reflections and the appearance of extra-reflections along quasiperiodic directions.     

Endothermic quasicrystalline-to-crystalline phase transition in Al6CuMg4

Electron microscopic and differential scanning calorimetric (DSC) studies have been performed to investigate the phase transitions in the rapidly solidified Al₆CuMg₄ alloy system. We present evidence, for the first time, that anendothermic quasicrystalline to crystalline transition occurs in this system around 340°C. This is an unexpected feature—for conventional wisdom would lead one to expect exothermic behaviour as was seen in Al₈₆Mn₁₄—and points to the fact that stability of certain quasicrystalline phases may be much larger than hitherto expected. Some comments on the recently reported large quasicrystal for the Al₆CuLi₃ system are also made in the light of our observation.     

Cluster structure of the surface in the amorphous,quasicrystalline, and liquid states

The electronic structure of the Zr₆₀Ni₂₀Ti₂₀, Al₆₅Cu₂₀Fe₁₅, and Al₈₆Mn₁₄ systems has been investigated by X-ray photoelectron spectroscopy and band calculations of the total and partial densities of electronic states have been carried out by the TB-LMTO-ASA method with the aim of studying the chemical bonds and the nearest environment of atoms in the liquid, amorphous, and quasicrystalline states. A direct proof of the inheritance of the chemical structure of the liquid state in the amorphous and quasicrystalline states has been obtained for the first time. The similarity of the electronic structure and the nearest environment of atoms indicates identity of clusters in the liquid, amorphous, and quasicrystalline states. The degree of hybridization of valence electrons of atoms in clusters increases in going from the quasicrystalline to the amorphous state due to the increase the number of transition metal atoms in clusters.     

Decagonal quasicrystals

Following the discovery of two dimensional quasicrystals in rapidly solidified Al-Mn alloys by us and L. Bendersky in 1985, a number of fascinating studies has been conducted to unravel the atomic configuration of quasicrystals with decagonal symmetry. A comprehensive mapping of the reciprocal space of decagonal quasicrystals is now available. The interpretation of the diffraction patterns brings out the comparative advantages of various indexing schemes. In addition, the nature of the variable periodicity can be addressed as a form of polytypism. The relation between decagonal quasicrystals and their crystalline homologues will be explored with emphasis on Al₆₀Mn₁₁Ni₄ and ‘Al₃Mn’. It will also be shown that decagonal quasicrystals are closely related to icosahedral quasicrystals, icosahedral twins and vacancy ordered phases.     

Microstructural evolution in Al–Mn–Cu–(Be) alloys

This study investigates the effects of alloying elements on the microstructural evolution of Al-rich Al–Mn–Cu–(Be) alloys during solidification, and subsequent heating and annealing. The samples were characterised using scanning electron microscopy, energy dispersive X-ray spectroscopy, synchrotron X-ray diffraction, time-of-flight secondary-ion mass spectroscopy, and differential scanning calorimetry. In the ternary Al₉₄Mn₃Cu₃ (at%) alloy, the phases formed during slower cooling (≈1?K?s^?1) can be predicted by the known Al–Mn–Cu phase diagram. The addition of Be prevented the formation of Al₆Mn, decreased the fraction of τ₁-Al₂₉Mn₆Cu₄, and increased the fraction of Al₄Mn. During faster cooling (≈1000?K?s^?1), Al₄Mn predominantly formed in the ternary alloy, whereas, in the quaternary alloys, the icosahedral quasicrystalline phase dominated. Further heating and annealing of the alloys caused an increase in the volume fractions of τ₁ in all alloys and Be₄Al (Mn,Cu) in quaternary alloys, while fractions of all other intermetallic phases decreased. Solidification with a moderate cooling rate (≈1000?K?s^?1) caused considerable strengthening, which was reduced by annealing for up to 25% in the quaternary alloys, while hardness remained almost the same in the ternary alloy.     

Structural evolutions of the mechanically alloyed Al<Subscript>70</Subscript>Cu<Subscript>20</Subscript>Fe<Subscript>10</Subscript> powders

Elemental mixtures of Al, Cu, Fe powders with the nominal composition of Al₇₀Cu₂₀Fe₁₀ were mechanically alloyed in a planetary ball mill for 80 h. Subsequent annealing of the as-milled powders were performed at 600–800°C temperature range for 4 h. Structural characteristics of the mechanically alloyed Al₇₀Cu₂₀Fe₁₀ powders with the milling time and the heat treatment were investigated by X-ray diffraction (XRD), differential scanning calorimeter (DSC) and differential thermal analysis (DTA). Mechanical alloying of the Al₇₀Cu₂₀Fe₁₀ did not result in the formation of icosahedral quasicrystalline phase (i-phase) and a long time milling resulted in the formation of β-Al(Cu,Fe) solid solution phase (β-phase). The i-phase was observed only for short-time milled powders after heat treatment above 600°C. The β-phase was one of the major phases in the Al₇₀Cu₂₀Fe₁₀ alloy. The w-Al₇Cu₂Fe₁ phase (w-phase) was obtained only after heat treatment of the short-time milled and unmilled samples. The present investigation indicated that a suitable technique to obtain a large amount of quasicrystalline powders is to use a combination of short-time milling and subsequent annealing.     

The study of Al3(Mn,Pd) crystal and Al–Mn–Pd decagonal quasicrystal by spherical aberration-corrected scanning transmission microscopy and atomic-resolution energy dispersive X-ray spectroscopy

An Al₃Mn-type Al₃(Mn, Pd) crystal and an Al–Mn–Pd decagonal quasicrystal (DQC) in an Al₇₀Mn₂₀Pd₁₀ alloy are studied using a spherical aberration (Cs)-corrected scanning transmission electron microscope (STEM) with high-angle annular dark-field (HAADF) and annular bright-field (ABF) techniques, together with atomic-resolution energy dispersive X-ray spectroscopy (EDS). Mn and Pd atomic positions in the Al₃(Mn, Pd) structure projected along the b-axis (pseudo-tenfold rotational axis) are represented by separate bright dots in observed HAADF-STEM images. Besides, Al as well as Mn and Pd atomic positions are represented as dark dots in ABF-STEM images. Most Mn and Pd atomic positions in the Al₃(Mn, Pd) structure can be observed on atomic-resolution EDS maps. On the basis of the good correlation between the STEM images and the EDS maps, and also considering the structure of the Al₃(Mn, Pd) crystal, which was determined by X-ray diffraction using a single crystal, observed HAADF and ABF-STEM images of the Al–Mn–Pd DQC have been interpreted. Pd and Mn atomic positions in the Al–Mn–Pd DQC can be detected on the observed EDS maps. It can be seen that Pd is enriched around the centre of the columnar clusters, having a decagonal section with 2 nm in diameter. It can therefore be concluded that Pd plays an important role in the stabilization of the decagonal clusters, which form the Al–Mn–Pd DQC structure.     

Atomic-scale mapping of twins and relevant defective structures in Al20Cu2Mn3 decagonal approximant

Twins or multiple twins occur frequently in the orthorhombic Al₂₀Cu₂Mn₃ approximant of decagonal quasi crystal (DQC). Due to the specific structural units, the twins in the Al₂₀Cu₂Mn₃ approximant usually exhibit the glide-reflection characteristics. Using aberration-corrected transmission electron microscope at the atomic scale, we have observed not only glide-reflection twins but also simple mirror-reflection twins in the Al₂₀Cu₂Mn₃ approximant. The two twinning modes are found to coexist in the present sample. These twins exhibit variant configurations at the twin boundaries where the tessellations of local subunits are imaged at an atomic scale. At the twin boundaries, diversified tiles such as star-like (S), bowtie-shaped and boat-shaped (B) are observed. The diversified tiles stacking with various manners allow the coexistence of the DQC and the approximant. Furthermore, the variants of B and S tiles are also found.     

Low temperature heat capacity and magnetic study of the Al80Mn20 icosahedral alloy

Low temperature heat capacity and magnetization measurements are reported for the Al₈₀Mn₂₀ alloy in the quasicrystalline icosahedral phase. The heat capacity, which was measured for temperatures ranging from 0.5 to 5.0 K and magnetic fields up to 11.7 kOe, shows a broad magnetic contribution around 1.0 K. The linear electronic contribution does not indicate an anomalously high density-of-states at the Fermi energy as predicted theoretically for quasicrystalline systems. The d.c. magnetization, which was measured from 2.0 to 300 K and with magnetic fields up to 50 kOe, indicates an effective number of one 11 μ_B localized magnetic moment for approximately every 100 Mn atoms. The low field susceptibility follows the Curie-Weiss law for temperatures ≧ 10 K, while a spin-glass-like ordering is observed at low temperatures.     

Local order of icosahedral quasicrystals studied by Mössbauer spectroscopy

Thermodynamically stable Al-Cu-Fe and Fe-doped ferromagnetic Al-Cu-Ge-Mn icosahedral quasicrystals are studied by⁵⁷Fe transmission Mössbauer spectroscopy and X-ray diffraction experiments. Al₆₅Cu₂₀Fe₁₅ quasicrystalline alloy was subjected to a mechanical grinding (MG) for up to 800 hours in a ball mill. Presence of the amorphous phase which co-exists with the quasicrystalline one is revealed in the early stage of MG. Mössbauer measurements were performed on icosahedral Al₄₀Cu_10–x Ge₂₅Mn₂₅Fe _x quasicrystal (x0.1; 3) in a temperature range from 10 K to 548 K. It was found that a magnetic transition occurs at about 30 K which is far belowT _c reported in the literature. It is concluded that AlGeMn ferromagnet which is present in the samples does not affect the magnetic transition observed and the transition is an intrinsic property of the Al-Cu-Ge-Mn host alloy.Samples of icosahedral quasicrystals were kindly provided by Profs. A. Inoue, T. Masumoto and P. H. Shingu. Ball milling was performed in Kyoto University by a courtesy of Prof. P. H. Shingu. This work was supported by the project for priority areas on properties of quasicrystals (No. 01630003) from the Japanese Ministry of Education, Science and Culture.     

Spin Glass Behavior in a Decagonal Quasicrystal Al-Mn-Fe-Ge System

The magnetic behavior of a decagonal quasicrystal Al₄₀Mn₂₅Fe₁₅Ge₂₀ system has been investigated by Mössbauer spectroscopy, magnetization and AC susceptibility measurements. Magnetization measured at low fields as a function of temperature shows a cusp-like peak around 41K. AC susceptibility also shows a sharp cusp, and the cusp shifts to a higher temperature as the frequency increases. These results strongly suggested that Al₄₀Mn₂₅Fe₁₅Ge₂₀ is a spin glass system with T _g =41K. From the temperature dependence of the Mössbauer spectrum, we observed the appearance of a hyperfine field below around 50K. The temperature is a little higher than the cusp temperature, which is rather usual in spin glass systems.     

Local iron symmetry in quasicrystals

Iron sites in quasicrystalline Al(TFe)-alloys have been investigated with Mössbauer spectroscopy. The quadrupole splitting of these materials is about 0.4 mm/s, showing that the iron atom is not in an icosahedral \((m\bar 3\bar 5)\) site. A measurement in an external magnetic field ofH _ext≈5 T yields the asymmetry parameter η=(V _xx ?V _yy )/V _zz . The value of η enables one to decide between the applicability of some mathematical structure models of quasicrystals. The results are: η=0.8 for decagonal Al₇(Mn_0.7Fe_0.3)₂ η=0.9 for icosahedral Al₆Cr_0.7Fe_0.3 using an one site approximation. This means that the iron atoms are in sites with a symmetry lower than axially symmetry, which suits very well for a Penrose lattice.     

Thermodynamic and kinetic properties of an icosahedral quasicrystalline phase in the Al-Pd-Tc system

The properties of a quasicrystalline phase in the Al-Pd-Tc system are studied for the first time. X-ray investigations demonstrate that the quasicrystalline phase in the Al₇₀Pd₂₁Tc₉ alloy has a face-centered icosahedral quasi-lattice with parameter a=6.514 ?. Annealing experiments have revealed that this icosahedral phase is thermodynamically stable. The heat capacity of an Al₇₀Pd₂₁Tc₉ sample is measured in the temperature range 3–30 K. The electrical resistivity and magnetic susceptibility are determined in the temperature range 2–300 K. The electrical resistivity is found to be high (600 μΩ cm at room temperature), which is typical of quasicrystals. The temperature coefficient of electrical resistivity is small and positive at temperatures above 50 K and negative at temperatures below 50 K. The magnetic susceptibility has a weakly paramagnetic character. The coefficient of linear contribution to heat capacity (γ=0.24 mJ/(g-atom K²)) and the Debye characteristic temperature (Θ=410 K) are determined. The origin of the specific features in the vibrational spectrum of the quasicrystals is discussed. __________ Translated from Fizika Tverdogo Tela, Vol. 42, No. 12, 2000, pp. 2113–2119. Original Russian Text Copyright ? 2000 by Mikheeva, Panova, Teplov, Khlopkin, Chernoplekov, Shikov.     

Are quasicrystals hume-rothery alloys?

In 1926 Hume Rothery discovered that for some simple alloys the electron to atom ratio e/a is a stability determining factor. We applied this energy band effect or Hume-Rothery rule to the quasicrystalline series Al₈₀Mn_20-xFe_x. The isomer shift of the Mössbauer spectra shows a maximum at x=9, where e/a=1.76. This strongly indicates a relatively low s-electron density, in agreement with the energy band effect.     

Luminescence of Mn ions in ordered and disordered LiAl5O8

The phosphorescence and excitation spectra of Mn ions in the ordered and disordered phases of LiAl₅O₈ have been measured. In both phases Mn²⁺ ions substitute for Al³⁺ ions in two different tetrahedral sites of the LiAl₅O₈ lattice. In both sites in the ordered phase, sharp zero-phonon transitions have been observed in the low temperature phosphorescence and excitation spectra - these transitions were considerably broadened in the disordered phase due to crystal field inhomogeneity in that phase. The deviation from neutrality caused by the Mn²⁺ ions in the ordered phase is largely compensated by Mn⁴⁺ ions occupying octahedral Al³⁺ sites. On disordering, a large proportion of Mn⁴⁺ is reduced to Mn²⁺, while the remainder takes up a site with a higher proportion of Li⁺ ions as next nearest neighbours. This leads to an increase in the ionicity of the Mn⁴⁺ site in the disordered phase and hence to a larger value of the Racah parameter B.     

Cluster-glass behavior in Al70Pd20Mn10

We carried out precise magnetization measurements in an Al₇₀Pd₂₀Mn₁₀ quasicrystalline system using a high-quality single crystal. A strange phenomena was observed at room temperature, which was a difference between zero field cooled and field cooled magnetization below 290 K. The thermoremanent magnetization also disappeared at 290 K. These experimental results suggest the forming of ferrimagnetic Mn ions clusters in Al₇₀Pd₂₀Mn₁₀ below 290 K. In addition, high field magnetization in the temperature range between 4.2 and 30 K up to 12 T was also measured to investigate the anisotropic molecular field between clusters.