Study of the electronic structure of the Al-Cu-Fe quasicrystalline system

The photoelectron spectra, photon absorption spectra, and electron energy loss spectra have been measured in reflection geometry for the clean surface of quasicrystalline samples and ω-phase samples of identical chemical composition that belong to the Al-Cu-Fe system. The investigations performed made it possible to reveal some specific features of the electronic structure of quasicrystals; these features can be successfully used in construction of the model of the actual electronic structure of quasicrystalline materials. The resonant photoelectron spectra have been measured in the range of photon energies corresponding to the Fe2p and Cu2p absorption edges. A sharp increase (by a factor of 3–4) in the spectral intensity is found in the valence band near the Fe2p edge. Near the Cu2p edge, the Auger electron emission resonantly increased, while the emission from the valence band almost did not change.     

Study of melt-grown quasicrystalline samples in the Cd-Yb and Al-Cu-Fe systems

The regimes of preparation of bulk samples of the quasicrystalline phase in the Cd-Yb and Al-Cu-Fe systems have been investigated. A thermodynamically stable quasicrystalline phase Cd_5.7Yb and its approximant Cd₆Yb with a high degree of texture along the cooling axis have been obtained. X-ray diffraction analysis made it possible to reveal changes in the structure and phase composition of bulk samples with variation in the melting temperature and annealing for different methods of sample preparation. It is shown that the presence of a temperature gradient along the furnace axis significantly affects the volume of the quasicrystalline phase. It is established that, with a decrease in the volume of the quasicrystalline phase mixed with the crystalline phase, the character of the temperature dependence of resistivity changes from the semiconductor type to the metallic type and the microhardness decreases according to a linear law.     

Preparation of Al-Cu-Fe quasicrystalline powdered alloys and related materials by mechanical activation

A mechanochemical method is developed for preparing quasicrystalline powdered alloys in the Al-Cu-Fe system and related materials. Quasicrystals are designed and proposed for use in practice as filling materials for metal and polymer matrices. The structure and mechanical properties of the designed materials are described. The possible fields of application of the materials under consideration are discussed.     

Temperature dependence of the tribological properties of laser re-melted Al-Cu-Fe quasicrystalline plasma sprayed coatings

The aim of this study was to investigate the effect of temperature on tribological properties of plasma-sprayed Al-Cu-Fe quasicrystal (QC) coating after laser re-melting treatment. The laser treatment resulted in a more uniform, denser and harder microstructure than that of the as-sprayed coatings. Tribological experiments on the coatings were conducted under reciprocating motion at high frequency in the temperature range from 25 to 650 °C. Remarkable influence of temperature on the friction behavior of the coating was recorded and analyzed. Microstructural analysis indicated that the wear mechanisms of the re-melted QC coatings changed from abrasive wear at room temperature, to adhesive wear at 400 °C and severe adhesive wear at 650 °C owing to the material transfer of the counterpart ball. It was also observed that the ratio of the icosahedral (i)-phase to β-Al₅₀(Fe,Cu)₅₀ phase in the coating was higher after test at 400 °C than that at 650 °C. The variation of the ratio i/β of coating and of the property of the counterpart ball and coating with the temperature are the two main factors influencing the wear mechanisms and value of the friction coefficient.     

Precipitation of icosahedral quasicrystalline and crystalline approximant phases in Zr-Cu-(Co, Rh or Ir) metallic glasses

Zr₇₀Cu₃₀, Zr₇₀Cu₂₀Co₁₀, Zr₇₀Cu₂₀Rh₁₀ and Zr₇₀Cu₂₀Ir₁₀ glassy alloys were prepared by the single roller melt-spinning method and the crystallization process was studied by differential scanning calorimetry, X-ray diffraction and transmission electron microscopy, with an emphasis on the initial stage. The Zr₇₀Cu₃₀ metallic glass crystallizes through the direct precipitation of the stable crystalline phase from the matrix. The addition of Co, Rh and Ir to the Zr₇₀Cu₃₀ metallic glass induces the precipitation of metastable phases prior to the formation of the stable ones. The metastable phases are a Ti₂Ni-type compound for Zr₇₀Cu₂₀Co₁₀, a mixture of the Ti₂Ni-type compound and an icosahedral quasicrystalline phase (I-phase) for Zr₇₀Cu₂₀Rh₁₀, and the I-phase for Zr₇₀Cu₂₀Ir₁₀ metallic glasses. The different effects of Co, Rh and Ir addition are explained based on their difference in atomic radius.     

Refractive index and dispersion of glasses with different degrees of linking

The wavelength dependence of the refractive index of 20 chalcogenide glasses with different structures and different degrees of linking is analysed by the use of a 2-term Selmier dispersion formula. The infrared term, which characterizes the contribution of oscillation transitions to the refractive index, is to be neglected in the first approximation in contrast to the term describing the interband optical transitions. That means, that the refractive index dispersion can be described by a single-oscillator-model of the form n² ? 1 = A₁λ²/(λ²-λ²₁). For glass-forming compounds the resonance wavelength λ₁ detected by the dispersion calculations is in good agreement with the position of the strongest maximum of the ?₂-spectrum. In a first approximation the refractive index n and the materials dispersion dn/dλ are representable as linear functions of the parameters A₁, a measure of strength of the optical interband transitions and λ₁, the resonance wavelength of the “electron-oscillator”. The model used is equivalent to the single-effective-oscillator-model of Wemple and Di Domenico.     

Chiral phases in a system of biaxial molecules

Chiral phases in a system of biaxial molecules are investigated based on a theory of biaxial liquid crystals, which we have presented in past work. Four chiral phases are identified, corresponding to a uniaxial nematic phase, two biaxial nematic phases, and a discotic phase, respectively. We find that in the chiral uniaxial nematic phase, the pitch of the chiral structures does not depend on temperature. In contrast, in the two chiral biaxial nematic phases, as well as in the chiral discotic phase, the pitch is temperature-dependent.     

On the curious structural phases in Al-deficient (Al62Cu23Cr15) and Al-rich (Al68Cu17Cr15) quasicrystalline alloys

In the present work investigations on the effect of stoichiometric variations on the Occurrence and stabilization of quasicrystalline (qc) and related phases have been carried out. Based on the explorations of several Al-deficient and Al-rich versions of the ideal nominal composition i.e. Al₆₅Cu₂₀Cr₁₅, it has been found that the alloy compositions corresponding to Al₆₂Cu₂₃Cr₁₅ (Al-deficient) and Al₆₈Cu₁₇Cr₁₅ (Al-rich) exhibit several structural subtleties. The Al-deficient alloy, has been found to exhibit crystalline bcc and fcc phases with a = 8.90 Å and a = 17.98 Å respectively. In addition to these phases, a new crystalline bcc variant (a = 15.42 Å) originating from the bcc (a = 8.90 Å) phase has been found. Also a curious superstructure of the fcc (a = 17.98 Å) has been observed. The Al-rich alloy typified by Al₆₈Cu₁₇Cr₁₅, on the other hand, does not exhibit any structural variants, instead it shows nearly pure i-phase.     

Reflection spectra and optical constants of quasicrystalline Al-Pd-Re films in the IR region

Optical properties of quasicrystalline Al-Pd-Re films have been investigated in the mid and far IR regions. Films were deposited on sapphire substrates using layer-by-layer ion-plasma sputtering with subsequent vacuum annealing. Reflection spectra have been measured in a wide IR spectral range from 50 to 7500 cm^?1. The complex dielectric function of the films was determined from the experimental data by dispersion analysis of the reflection spectra. The frequency dependence of the real part of the optical conductivity was calculated.     

Ionic conductivity of the phases of five types formed in the BaF2-GdF3 system

The ionic conductivity has been measured in ceramic phases of five structure types found in the BaF₂-GdF₃ system: the fluorite type (CaF₂), its trigonal and tetragonal distortions, the tysonite type (LaF₃), and the orthorhombic ??-YF₃ modification. The phases have been obtained by solid-phase synthesis from BaF₂ and GdF₃ mixtures in hermetic nickel containers at 925, 964, and 1067°C for 108?C360 h in a fluorine atmosphere. Their conductivity ?? is compared in correlation with the composition and structure type. The highest conductivity values are found for the tysonite Gd_{1 ? y} Ba _y F_{3 ? y} phase (0.10 ?? y ?? 0.25): (1?2) × 10^?3 S/cm at 683 K. The ordered Ba_0.60Gd_0.40F_2.40 and Ba_0.57Gd_0.43F_2.43 phases with fluorite-derived structures and different degrees of order are characterized by the lowest conductivities: i(1.5?3.5 × 10^?5) S/cm.     

Investigation of the thermal diffusion during the formation of a quasicrystalline phase in thin Al-Pd-Re films

The layer mixing during the formation of the Al₇₀Pd₂₀Re₁₀ icosahedral quasicrystalline phase in thin (55 nm) Al-Pd-Re layered film systems subjected to vacuum annealing has been studied. It is shown that a combined layer of Pd and Al atoms (with the Al₃Pd₂ phase dominating) is formed in the first stage (at 350°C), while the rhenium layer remains invariable. In the second annealing stage (at 450°C), the β′-AlPd phase is formed and the Re layer is diffused. In the third stage (700°C), Pd and Re atoms are uniformly distributed throughout the film with the formation of a quasicrystalline phase.     

Comparative study of the sol-gel processes starting with different substituted Si-alkoxides

In the present work a comparative study of the hydrolysis-polycondensation processes of different Si-substituted alkoxides, leading to hybrid materials with covalent -Si-C- bonds, was carried out. The following alkoxides were used: tetraethoxysilane (TEOS), methyltriethoxysilane (MTEOS) and vinyltriethoxysilane (VTEOS). Using gas chromatography coupled with mass spectrometry (CG-MS), nuclear magnetic resonance (²⁹Si-NMR) and infrared spectrometry (IR), information about the sol-gel process in the mentioned systems were obtained. The differences in the reactivity of the studied alkoxides are connected with the steric effect of the organic substituents. The reactivity of the alkoxides in the early stages of the hydrolysis-polycondensation process increased in the order TEOS < VTEOS < MTEOS.     

Structural and transport properties of the AgI–AgBr system in its superionic and molten phases by computer simulation

Molecular dynamics simulations were performed to investigate the structural properties of AgI–AgBr system in its superionic and molten phases. Parrinelo, Rahman and Vashishta (RVP) type potentials are used for the simulation. The obtained pair distribution functions are in good agreement with the neutron diffraction experimental data. The transport coefficients in these two phases are also calculated from the equilibrium and non-equilibrium molecular dynamics simulation method. In molten phase, the ratios of partial conductivities on constant concentration are found to be constant at different temperatures.     

Effects of Nb on the formation of icosahedral quasicrystalline phase in Ti-rich Ti-Zr-Ni-Cu-Be glassy forming alloys

As-cast (Ti₄₀Zr₂₅Ni₈Cu₉Be₁₈)_100−xNb_x (0 ? x ? 5) (Ф3) glassy forming alloys were investigated in order to clarify the role of Nb on the formation of icosahedral quasicrystalline phase (I-phase) in Ti-rich Ti-Zr-Ni-Cu-Be glassy system. It is found that an I-phase is formed in Ti-Zr-Ni-Cu-Be glassy alloy by addition of Nb element; however, the nucleation rate of I-phase increases, whereas the grain growth rate decreases with increasing Nb content. Moreover, with increasing Nb content, the thermal stability against crystallization increases, while the temperature range of stability of the I-phase decreases.     

Induced SA phases and reentrant nematic phases in binary systems of homologous terminal-nonpolar compounds

The isobaric phase diagrams of binary systems are presented in which one component is represented by members of the homologous series of swallow tailed compounds. The occurrence of induced S_A phases and reentrant nematic phases was studied in dependence on the length of the alkyl chain of the swallow tailed compound. In some selected binary systems X-ray investigations and calorimetric measurements were performed. An attempt has been made to interprete the results by a simple packing model.     

Polymorphic transformation in TlSe and the electrical properties of phases

Alloys in the TlSe-Se system on the side of the TlSe compound in the phase diagram have been investigated using differential thermal, X-ray powder diffraction, and microstructural analyses. The phase diagram of the system has been constructed, and the temperature dependences of the electrical conductivity of the phases obtained have been examined. An analysis of the thermograms of alloys in the (TlSe)_0.96-Se_0.04 system has revealed a structural phase transition at a temperature of 470 ± 1 K. Investigations into the temperature dependences of the electrical conductivity in the range 120–450 K have demonstrated that the temperature dependence of the electrical conductivity for the (TlSe)_0.96-Se_0.04 alloy exhibits metallic behavior.     

The influence of silicon on the crystallographic phases of the niobium-gallium system studied by x-ray diffraction methods

The influence of silicon on the crystallographic phases of the binary niobium-gallium system is studied by X-ray diffraction methods. The samples were prepared by rapid quenching. Silicon substituted gallium in all phases of the binary system. It exists a complete pseudobinary solid solution Nb₅Ga₃-Nb₅Si₃. The maximum degree of substitution in the A 15 phase Nb₃Ga_1–xSi_x is x = 0.5. The presence of two A 15 phases with different cell parameters indicates the existence of a miscibility gap. Contrary to the expections the supercondusting transition temperatures decreases with an increasing degree of substitution.     

Some peculiarities of the formation of thallium-containing phases with perovskite-like structures

The authors systematize and analyse their own and described in literature experimental data on the composition and structure of high-temperature superconductors with thallium-containing phases. Some peculiarities of the isomorphic replacement and formation of thallium phases having single- and double-layered-type structure were found. Principles of crystallochemical formation of some phases, and chemical and thermal treatment allowing for their formation are described.