Covalent bonds in AlMnSi icosahedral quasicrystalline approximant

Electron density distributions were obtained using the maximum entropy method with synchrotron radiation powder data. In the metallic Al12Re, metallic bonding was observed for the icosahedral Al12 cluster with central Re atom. In the nonmetallic alpha-AlMnSi 1/1 approximant, covalent bonds were found in the electron density distribution of the Mackay icosahedral cluster without central atom. Rather than the Hume-Rothery mechanism, the covalency of Al (Si) icosahedron and that between Al (Si) and Mn atoms is considered to be the origin of the pseudogap and nonmetallic behavior of alpha-AlMnSi.     

Specific heat and electrical resistivity of an icosahedral-structure Zr<Subscript>70</Subscript>Pd<Subscript>30</Subscript> alloy and of its amorphous and crystalline analogs

Binary icosahedral and crystalline phases of the Zr₇₀Pd₃₀ alloy were obtained in crystallization from the amorphous state during heat treatment. The specific heat and electrical resistivity of the icosahedral, amorphous, and crystalline phases were measured and compared. An increase in the electronic density of states on the Fermi surface, lattice softening, and an increase in the electron-phonon coupling constant were observed to occur with decreasing structural order. Despite the high valence electron density in the icosahedral phase, where the electronic densities of states are twice those in the crystal, the electrical resistivity of the icosahedral phase is ～50 times as high. Superconductivity was observed for the first time in the icosahedral phase of a binary system of transition metal atoms, Zr₇₀Pd₃₀.     

Evaluation of structural vacancies for 1/1-Al–Re–Si approximant crystals by positron annihilation spectroscopy

The size of structural vacancies and structural vacancy density of 1/1-Al–Re–Si approximant crystals with different Re compositions were evaluated by positron annihilation lifetime and Doppler broadening measurements. Incident positrons were found to be trapped at the monovacancy-size open space surrounded by Al atoms. From a previous analysis using the maximum entropy method and Rietveld method, such an open space is shown to correspond to the centre of Al icosahedral clusters, which locates at the vertex and body centre. The structural vacancy density of non-metallic Al₇₃Re₁₇Si₁₀ was larger than that of metallic Al₇₃Re₁₅Si₁₂. The observed difference in the structural vacancy density reflects that in bonding nature and may explain that in the physical properties of the two samples.     

Simulation of the lattice dynamics of an Al-Cu-Fe icosahedral quasicrystal

The total and partial vibrational spectra of aluminum, copper, and iron atoms in an Al-Cu-Fe icosahedral quasicrystal are calculated by the recursive method. The calculations are based on the 1/1 crystal approximant. The interaction of atoms in the Al-Cu-Fe quasicrystal is described within the EAM model. The calculated spectra are in satisfactory agreement with the experimental data on neutron inelastic scattering.     

Structural transformation from the AlCuFe icosahedral phase to the 1/1 cubic (AlSi)CuFe approximant phase; three dimensional models of translation defects

Unusual results are obtained by High Resolution Electron Microscopy (HREM) studies of the 1/1 cubic approximant phase of the (AlSi)CuFe icosahedral phase. We study, in a 3D model of AlCuFe icosahedral phase, the possibilities of transformation of this structure into a periodic cubic structure. We propose a model of transformation coherent with the experimental results. Received 31 May 1999 and Received in final form 23 December 1999     

Formation of Tsai-type 1/1 approximants in In-Pd-RE (RE: rare earth metal) alloys

The formation of the 1/1 crystal approximant phase (1/1 phase) to the icosahedral phase (i phase) in In-Pd-RE (RE: rare earth metal) systems has been investigated. A new series of 1/1 phases were found in In₅₃Pd₃₃RE₁₄ (RE; Y, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, and Tm) alloys. For Y, Sm, Gd, Tb, Dy, and Ho, the 1/1 phases were found in annealed alloys, indicating that they are thermodynamically stable. The atomic structure of the 1/1 phases was directly observed by high-angle annular dark-field imaging performed via scanning transmission electron microscopy, revealing that the 1/1 phases consisted of a periodic arrangement of Tsai-type icosahedral clusters. Further, the atomic size effect on i phase formation, as well as formation conditions previously reported for other Tsai-type i and 1/1 phases were examined. It was found that the ratio of the atomic radius of base metals such as In and Pd affects i phase formation. Moreover, the appropriate range of the radius ratio for i phase formation was narrower than that for 1/1 phase formation.     

Induction of an atomically thin ferromagnetic semiconductor in 1T′ phase ReS2 by doping with transition metals

Two-dimensional 1T′ phase ReS₂, a transition metal dichalcogenide, has a unique structure and electronic properties that are independent of thickness. The pure phase is a nonmagnetic semiconductor. Using density functional theory calculations, we show that ReS₂ can be tuned to a magnetic semiconductor by doping with transition metal atoms. The magnetism mainly comes from the dopant transition metal and neighboring Re and S atoms as a result of competition between exchange splitting and crystal field splitting. ReS₂ doped with Co can be considered as a promising diluted magnetic semiconductor owing to its strong ferromagnetism with long-range ferromagnetic interaction, high Curie temperature (above room temperature) and good stability. These findings may stimulate experimental validation and facilitate the development of new atomically thin diluted magnetic semiconductors based on transition metal dichalcogenides.     

Structural properties of Pb2MnW(1-x)Re(x)O6 double perovskites

Pb2MnW(1-x)Re(x)O6 samples have been synthesized and their structure determined by powder x-ray diffraction. These samples undergo a first order structural phase transition between 413 and 445 K depending on the composition. Above this temperature, the samples are cubic. Below the transition temperature, solid solutions are found for x ≤ 0.2 and x ≥ 0.5. The W-rich samples adopt an orthorhombic cell whereas the Re-rich compounds are monoclinic. In the intermediate region, 0.2 < x < 0.5, both phases coexist. X-ray absorption spectra did not reveal significant changes in the local structure for Pb, Mn or Re atoms across the structural phase transition. All the atoms exhibit distorted environments in the whole series. In the case of Pb and W(Re) atoms, the local distortion remains in the high temperature phase. Samples with x ≤ 0.2 also show a sharp discontinuity in the dielectric permittivity at the phase transition temperature indicating the presence of a concomitant electrical ordering in the bulk grains. Such an anomaly in the dielectric constant is not observed for the x ≥ 0.5 samples, compatible with the lack of dipole ordering for this composition range. The different electrical behaviours also explain the differences in the entropy content for the two types of transition.     

A new behaviour of the interstitial element H or D in solution in rare-earth metal

The study of the resistivity of the Lu-(H or D) system makes clear certain characteristics of the α phase and indicates a change for the interstitial element in the transition range 170–400°C. On both sides of this domain an appreciable difference is observed in the properties of the solid solution. The α1 phase of the domain below 170°C has a higher resistivity and a maximum hydrogen concentration independent of the temperature. For the phase α of the domain above 400°C, the expansions both with the concentration growth and with the rise in temperature are isotropic contrary to the α1 phase.The resistivity and X-ray diffraction studies, taking into account the lattice symmetry, allow us to conclude that a change may occur in the type of interstices filled by the hydrogen atoms, tetrahedral for the α phase and octahedral with H atoms associating in pairs for the α1 phase. Among the lanthanide metals, we find that only lutetium and thulium, in uniting with H or D, have the low temperature α1 phase.     

Is the negative temperature coefficient of the resistivity of the quasicrystals due to chemical disorder?

We have found that the electronic transport of the binary icosahedral (i) Cd-Yb is extremely sensitive to a minute substitution of Mg for Cd atoms; the positive temperature coefficient of the resistivity (TCR) at low temperatures seen in the binary i Cd-Yb disappears by addition of only 0.1 at. % Mg and, moreover, the TCR stays negative well up to 60 at. % Mg. Such sensitiveness of the resistivity in the very dilute Mg concentration region, which is a consequence of the long coherence length (>28 A) of the conduction electrons in the quasiperiodic lattice, has led us to an unexpected conclusion: The negative TCR in the ternary i phase is due to partial chemical disorder; i.e., it is not a consequence of the quasiperiodicity.     

Quasicrystals and related approximant phases in Mg-Zn-Y

As-cast microstructure of Mg-rich Mg(68)Zn(28)Y(4) has been investigated by a detailed transmission electron microscopy study. The as-cast Mg(68)Zn(28)Y(4) alloy consisted of three different types of phases: 10-20 m size primary solidification phase, dendritic phase grown from the primary phase and a eutectic structure formed at the later stage of solidification. The primary solidification phase has an icosahedral structure with a large degree of phason strain. 1/1 rhombohedral approximant phase with lattice parameters a=27.2 A and =63.43 degrees is first observed in Mg-Zn-Y system. The rhombohedral structure can be obtained by introducing phason strain in the six-dimensional face centered hyper-cubic lattice. The decagonal phase nucleates with orientation relationship with the icosahedral phase, and Mg(4)Zn(7) nucleates with orientation relationship with the decagonal phase, indicating a close structural similarity between the three phases. Gradual depletion of Y during solidification plays an important role in heterogeneous nucleation of decagonal and Mg(4)Zn(7) phases from icosahedral and decagonal phases, respectively.     

An electronmicroscopic study of hexagonal phases related to quasicrystals in Zn-Mg-RE alloys

A number of hexagonal phases with inflated/deflated a lattice parameters but equal c parameters are found to exist with compositions close to that of the icosahedral phase. Surprisingly, no cubic or orthorhombic approximants are found. The closeness of the hexagonal phases to the icosahedral phase is shown through the electron diffraction features. Through stereographic analysis of the hexagonal phases it is shown that they consist of three icosahedral units rotated by 120 degrees about a common twofold axis, forming the sixfold axis. It is suggested that this phase falls into a class of crystals closely related to both the icosahedral and decagonal quasicrystals, but are not approximants to them. These phases can instead be considered to be approximant to the hexagonal quasicrystal.     

Electronic properties of quasicrystals an experimental review

The electronic properties of a large number of icosahedral-crystal systems have been studied experimentally. These systems include alloys of the simple metals and those that contain transition metals. Some of the icosahedral phases (i-phases) are thermally stable while others are metastable; and their degree of structural order varies within each of the stability classes. The importance of sample quality to the exposition of intrinsic properties is emphasized, particularly for systems with high resistivities. As a result, experiments on single-phase samples from the diversity of systems studied have shed light on the physics of quasicrystalline alloys. Comparison made with the approximant and amorphous phases have provided important insights to the understanding of quasicrystallinity, randomness, and atomic-potential effects on electronic properties. Reduction in the electronic density of states (DOS) at the Fermi level relative to the free-electronic value is observed in the stable i-phase systems studied to date. Examples are the GaMgZn, AlCuLi, AlCuFe, and AlCuRu systems. Although the simple-metal i-alloys such as AlCu (Mg, Li) and (Al, Ga)MgZn within their metastable phase fields are found to have nearly free-electron DOS, the anomalies in their transport behaviour and compositional dependence of electronic properties have revealed the important role of the Fermi-surface-Jones-zone boundaries (FS-JZB) interaction in these measurements. Also, central to the FS-JZB interaction criterion of phase stability is the existence of a pseudogap in the DOS which is most evident in the stable i-phases. Unusual transport behaviour has been reported for the structurally ordered i-phases that contain transition metals. These materials exhibit semi-metallic transport behaviour characterized by a high resistivity (～4000–30000 μΩcm), a large resistivity ratio ρ(4·2 K)/ρ(300 K) ranging from 1·5 to 4 as well as a low carrier concentration of 10^?2 to 10^?3 electron per atom. Adding to the anomalous electron transport are the strong temperature dependences of the Hall coefficient and thermopower leading to a change of sign. Further understanding of the ordered i-phases has been advanced through studies of amorphous phases and crystalline counterparts of i-phases known as rational approximants. Based on these studies, the key findings are reported: (i) even for simple-metal systems in the weak scattering regime, differences in the electronic properties between the structural phases begin to emerge as the FS-JZB interaction increases in strength, (ii) semi-metallic conductivity and other anomalous transport properties are also observed in the approximant-crystals that contain transition metals (e.g. AlMnSi, AlFeCu), (iii) on the other hand, disordered i-phases and amorphous phases containing transition metals are found to possess metallic-glass-like properties. Experimental results reported indicate that the electronic properties of an ordered icosahedral quasicrystal are similar to those of a crystal in the presence of a strong FS-JZB interaction. Quite surprisingly, the elastic scattering time of electrons in the sp-d band ordered phases is found to be much longer than that in the nearly-free-electron i-phases. The propensity towards electron localization is ascribed to the realization of a potentially strong FS-JZB interaction in an ordered structure, particularly that with the icosahedral point group symmetry. The data have also provided evidence for the existence of rapidly varying electronic structures within the pseudogap of a semi-metallic i-crystal and its crystal analogue, despite the short electronic mean free path. The electron transport at low temperature can be described by the weak localization and electron-electron interaction effects. Contact with theories on quasiperiodic systems is also made. Finally, results from the stable decagonal crystals are discussed.     

Solidification microstructure of as-cast Mg–Zn–Y alloys

Composites consisting of icosahedral (i) phase and ductile α-Mg can be fabricated by controlling the alloy composition. With increasing Mg content, the primary solidification phase changed from the i phase to the α-Mg phase and single eutectic structure can be obtained at a composition of Mg₇₂Zn_23.5Y_3.5. The i phase showed a variation in structural order from the well-ordered icosahedral phase to the 1/1 rhombohedra1 approximant with lattice constants a=27.2 Å and α = 63.43°. The structural change in the i phase can be explained by microdomain formation due to compositional change during solidification. Annealing treatment improves the structural order of the i phase by homogenization. By controlling the alloy composition, a single eutectic structure consisting of a hard i phase and a ductile α-Mg phase could be obtained.     

Phase separation in the itinerant metamagnetic transition of Sr4Ru3O10

We have investigated the electronic transport properties of the triple-layered ruthenate Sr4Ru3O10. We observed surprising anomalous features near its itinerant metamagnetic transition, including ultrasharp magnetoresistivity steps, a nonmetallic temperature dependence in resistivity for upward field sweeps, and a resistivity drop in temperature dependence for downward field sweeps. These features suggest that the metamagnetic transition of Sr4Ru3O10 occurs via an electronic phase separation process with magnetic domain formation.     

Graphene-graphite phase transition at the surface of a carbonized metal

A phase transition leading to the transformation of a graphene layer into a multilayer graphite film at the surface of a carbonized metal has been experimentally studied on the atomic level under ultrahigh-vacuum conditions. It has been shown that this process is governed by dynamic equilibrium between edge atoms of graphene islands and a chemisorbed carbon phase, two-dimensional carbon “gas,” and is observed in the temperature range of 1000–1800 K. The features of the phase transition at the surfaces Ni(111), Rh(111), and Re(10-10) are similar, although the specific kinetic characteristics of the process depend on the properties of the substrate. It has been shown that change in the emissivity of the substrate after the formation of a multilayer graphite film increases the rate of the phase transition and leads to a temperature hysteresis.     

稳态Cu-Zr二十面体团簇电子结构的密度泛函研究

采用第一原理对以Cu为心的低能稳态Cu_nZ_(r13-n)(n=6,7,8,9)二十面体团簇的电子结构进行计算,结果表明:同一化学组分下,以Cu为心的Cu-Zr二十面体团簇中出现的同类原子聚集现象可以增强团簇的稳定性,降低费米能级(EF)上的电子数N(EF),这为低能稳态团簇拥有较小的N(EF)提供了深层次的理论解释.进一步的差分电子密度与Mulliken布居分析得知,Cu-Zr二十面体中共价键与离子键共存,成键态与反键态共存,且团簇在形成时壳层Zr与中心Cu原子是电子的提供者,壳层Cu是电子的获得者.该电荷转移方向是金属玻璃中以Cu为心的Cu-Zr二十面体团簇普遍遵循的规律,不随团簇的化学序参数及化学组分的变化而变化.计算的红外振动谱为实验上准确表征不同二十面体原子团提供了一种新的思路.     

Changes in the structure of nanoclusters of nickel and copper under the action of temperature

Molecular dynamics with tight-binding potential was used to study the processes of thermal impact on Ni nanoclusters with radii of up to 0.8 nm. The performed simulation demonstrated that under the action of heating, there is a structural transition from the initial face-centered cubic phase to a icosahedral modification. It was observed that upon an increase of cluster size, the transition temperature shifted to the melting point of the cluster. The obtained relationships were tested on the copper nanoparticles of similar size. It was concluded that the size of a metal cluster of 200–250 atoms is probably the limit below which it is impossible to maintain the initial face-centered cubic modification under actual conditions of exploitation.     

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.