Magnetic behavior,transport properties and nanocrystallization of melt-spun YxCe50 − xCu42Al8 (0 ≤ x ≤ 50) amorphous system

Amorphous Y_xCe_50 ? xCu₄₂Al₈ (0 ≤ x ≤ 50) ribbons prepared by melt-spinning on the Cu wheel were subjected to X-ray diffractometry (XRD), differential scanning calorimetry (DSC) and to the measurements of magnetization and resistivity in the temperature range from 1.7 to 300 K. Effective activation energies, characteristic crystallization temperatures and enthalpies of as-quenched alloys have been determined. Two stages of crystallization have been observed in most samples (except shallow effects in Ce₅₀Cu₄₂Al₈). The activation energies have been calculated from the Kissinger relation to be 247 ± 18 and 570 ± 56 kJ/mol for Y₂₅Ce₂₅Cu₄₂Al₈ and Y₅₀Cu₄₂Al₈, respectively. The absence of the endothermic effect for x = 50, usually associated with a glass transition, below the primary crystallization event, indicates the presence of dispersed polyamorphous packing with a wide range of local glass transitions. The magnetization versus temperature plot for Y₂₅Ce₂₅Cu₄₂Al₈ points to a magnetic ordering at temperatures considerably below 50 K. This observation has been confirmed by the temperature dependence of resistivity, which exhibits a singularity at the same temperature. Moreover, a negative temperature coefficient of resistivity, characteristic of disordered systems, was observed. The electrical resistivity in the Y₂₅Ce₂₅Cu₄₂Al₈ amorphous alloy is governed by the weak localization of electrons.     

Nano-heterogeneous structure of (1-x)KNbO3–xSiO2 glasses in the low glass-forming oxide content range 0.05 ≤ x ≤ 0.3

Glasses with a high content of niobium oxide are of significant interest for electro-optics and nonlinear optics. In the present paper we report the results of the investigation of the submicroscopic structure and nonlinear optical properties of (1-x)KNbO₃–xSiO₂ (KNS) glasses (x = 0.05–0.30) by XRD, SANS, electron microscopy and second harmonic generation (SHG) technique. Vitreous samples were fabricated by rapid melt cooling, via pressing the melt by steel plates, quenching between rotating metal rolls or splat cooling in air or nitrogen flow. Glasses with x < 0.15 are shown to possess a micro-inhomogeneous structure with regions enriched by SiO₂. On the contrary, as-quenched glasses with x > 0.15 are found by SANS to be homogeneous, but form nanostructures enriched by SiO₂ after heat-treatment. At temperatures below ~(T_g + 50 °C), SiO₂-enriched regions grow slightly, whereas their chemical composition shifts considerably closer to SiO₂. The data on the nano-inhomogeneous structure enables clarifying the complicated T_g(x) dependence of KNS glasses. SHG-active KNbO₃ phase precipitates at later stages of crystallization when the glass starts to lose its transparency, and crystallization of perovskite-like KNbO₃ is accompanied by the enhancement of SHG efficiency by several orders of magnitude.     

Epitaxial lateral overgrowth of non-polar GaN(1 1̄ 0 0) on Si(1 1 2) patterned substrates by MOCVD

m-Plane GaN was grown selectively by metal–organic chemical vapor deposition (MOCVD) on patterned Si(1 1 2) substrates, where grooves aligned parallel to the Si〈1 1 0〉 direction were formed by anisotropic wet etching to expose the vertical Si{1 1 1} facets for growth initiation. The effect of growth conditions (substrate temperature, chamber pressure, and ammonia and trimethylgallium flow rates) on the growth habits of GaN was studied with the aim of achieving coalesced m-plane GaN films. The epitaxial relationship was found to be GaN(1 1? 0 0) || Si(1 1 2), GaN[0 0 0 1] || Si[1 1 –1], GaN[1? 1? 2 0] || Si[1 1? 0]. Among all growth parameters, the ammonia flow rate was revealed to be the critical factor determining the growth habits of GaN. The distribution of extended defects, such as stacking faults and dislocations, in the selectively grown GaN were studied by transmission electron microscopy in combination with spatially resolved cathodoluminescence and scanning electron microscopy. Basal-plane stacking faults were found in the nitrogen-wing regions of the laterally overgrown GaN, while gallium-wings were almost free of extended defects, except for the regions near the GaN/Si{1 1 1} vertical sidewall interface, where high dislocation density was observed.     

Square-wave voltammetry and impedance spectroscopy in tin doped melts with the compositions xNa2O · 15Al2O3 · (85 − x)SiO2 (x = 8.5, 11 and 16)

Glasses with the base compositions xNa₂O · 15Al₂O₃ · (85 ? x)SiO₂ (x = 8.5, 11 and 16) doped with 0.5 mol% SnO₂ were investigated by both square-wave voltammetry and impedance spectroscopy in the temperature range from 1300 to 1600 °C. Each recorded square-wave voltammogram exhibits a well pronounced peak attributed to the Sn²⁺/Sn⁴⁺-redox pair. Impedance spectra were measured in a frequency range from 0.1 to 10⁵ s^?1 as a function of the superimposed dc-potential and were simulated using an equivalent circuit taking into account the resistivity of the melt, the electrochemical double layer, a resistor attributed to a kinetically hindered electron transfer and a Warburg parameter which accounts for the diffusion process of Sn⁴⁺ and Sn²⁺ to and from the electrode. Additionally, two impedance elements, a resistor and a capacitance both attributed to adsorption processes were necessary to fit the impedance spectra.     

Synthesis and structure–property relations in xCuO–(1 − x)Bi2O3 (0.5 ⩽ x ⩽ 0.9) (C1–C5: x = 0.5, 0.6, 0.7, 0.8, 0.9) glasses

Synthesis of the xCuO–(1 ? x)Bi₂O₃ (0.5 ? x ? 0.9) (C1–C5: x = 0.5, 0.6, 0.7, 0.8, 0.9) glasses was done via nitrate–citrate gel route. Glassy phase is ascertained by XRD studies. Magnetic susceptibility results in the range 4.2–400 K show weak paramagnetic nature with exchange integrals ～0.024–0.13 eV in the glasses. The electron paramagnetic resonance (EPR) in the range 4.2–363 K shows g ≈ 2.0 and the trend of the g-matrix elements g_|| > g_⊥ > g_e for the glasses C1–C5 at 4.2 K are due to the Cu²⁺ (3d⁹) paramagnetic site in the glasses which is in a tetragonally elongated octahedron [O_1/2–CuO_4/2–O_1/2] having D_4h symmetry. IR spectroscopic results show the presence of octahedron [BiO_6/2]^3? and [CuO_6/2]^4? units and pyramidal [BiO_2/2O]^? unit in the glasses.     

Structure and properties of the 70Li2S · (30 − x)P2S5 · xP2O5 oxysulfide glasses and glass–ceramics

The 70Li₂S · (30 ? x)P₂S₅ · xP₂O₅ (mol%) oxysulfide glasses were prepared by the melt quenching method. The glasses were prepared in the composition range 0 ≦ x ≦ 10. The glass–ceramics were prepared by heating the glasses over crystallization temperatures. The PO_nS_3?n (n = 1–3) oxysulfide units were produced in the glasses and glass–ceramics by partial substituting P₂O₅ for P₂S₅. In particular, the P₂OS₆^4? unit would be produced by substituting a small amount of P₂O₅ for P₂S₅. The oxygen atoms were incorporated into the Li₇P₃S₁₁ crystal structure because the diffraction peaks of the oxysulfide glass–ceramic shifted to the higher angle side. The glass–ceramic with 3 mol% of P₂O₅ exhibited the highest conductivity of 3.0 × 10^?3 S cm^?1 and the lowest activation energy for conduction of 16 kJ mol^?1. The P₂OS₆^4? dimer units in the oxygen-incorporated Li₇P₃S₁₁ crystal would improve conductive behavior of the Li₂S–P₂S₅ glass–ceramics.     

Growth of Pb on Si(3 3 5)–Au surface

The growth, crystallographic structure and electronic properties of ultrathin Pb films grown on a vicinal silicon surface are investigated with reflection high energy electron diffraction (RHEED) and specific resistivity measurement techniques. A Si(3 3 5) surface with a perfect distribution of monoatomic steps separated with (1 1 1) terraces induced by a submonolayer amount of Au is used as a substrate. In the early stage, Pb growth is anisotropic. Apparently, the presence of steps forces the growth of short crystalline Pb chains along the steps. The layer is amorphous in the perpendicular direction. With the increasing thickness, a phase transition takes place between 3 and 4 monolayers (ML) that makes crystalline order also across the terraces. A further increase in thickness causes the layer surface to repeat the substrate morphology. It consists of regularly distributed monoatomic steps and narrow (1 1 1) terraces.     

Calorimetric study and Kissinger analysis of melt-spun DyMn6−xGe6−xFexAlx (1 ⩽ x ⩽ 2.5) alloys

Thermal properties of rapidly quenched alloys from the DyMn_6?xGe_6?xFe_xAl_x (1 ? x ? 2.5) series produced by melt-spinning have been investigated by differential scanning calorimetry (DSC). The DSC curves show two exothermic effects connected with crystallization processes. Crystallization temperatures and enthalpies ΔH have been estimated. The systematic changes in these parameters allow concluding that the crystallization exothermic events are independent. Effective activation energies E have been determined using the Kissinger analysis and relatively high values up to 480 ± 20 kJ/mol for DyMn₄Ge₄Fe₂Al₂ have been found indicating high thermal stability of the amorphous state in this alloy series.     

Electronic properties calculation of Ge1 − x − ySixSny ternary alloy and nanostructure

The band structure of Ge_{1 ? x ? y}Si_xSn_y ternary alloys, which are easier to grow than binary Ge_1-xSn_x alloys, and clearly offer a wider tunability of their direct band-gap and other properties, was calculated and investigated by using the empirical pseudo-potential plane wave method with modified Falicov pseudo-potential formfunction. The virtual crystal approximation (VCA) and 2 × 2 × 2 super-cell (mixed atoms) method were adopted to model the alloy. In order to calculate all of these properties, the empirical pseudo-potential code was developed. The lattice constant of the alloy varies between 0.543 to 0.649 nm. The regions in the parameter space that corresponds to a direct or indirect band gap semiconductor are identified. The Ge_{1 ? x ? y}Si_xSn_y ternary alloy shows the direct band gap for appropriate composition of Si, Ge and Sn. The direct energy gap is in the range 0–1.4 eV (from the VCA calculation), and 0–0.8 eV (from the super-cell calculation), depending on the alloy composition. Therefore, this alloy is a promising material for optoelectronic applications in both visible and infrared range, such as interband lasers or, solar cells. Furthermore, strain-free heterostructures based on such alloys are designed and, using the effective-mass Hamiltonian model, the electronic structure of GeSiSn quantum wells with arbitrary composition is investigated, in order to understand their properties and the potential of their use in devices.     

The electrical switching and thermal behavior of bulk Ge15Te85 − xSnx and Ge17Te83 − xSnx glasses

Bulk Ge₁₅Te_85 ? xSn_x and Ge₁₇Te_83 ? xSn_x glasses, are found to exhibit memory type electrical switching. The switching voltages (V_t) and thermal stability of Ge₁₅Te_85 ? xSn_x and Ge₁₇Te_83 ? xSn_x glasses are found to decrease with Sn content. The composition dependence of V_t has been understood on the basis of the decrease in the OFF state resistance and thermal stability of these glasses with tin addition. X-ray diffraction studies reveal that no elemental Sn or Sn compounds with Te or Ge are present in thermally crystallized Ge–Te–Sn samples. This indicates that Sn atoms do not interact with the host matrix and form a phase separated network of its own, which remains in the parent glass matrix as an inclusion. Consequently, there is no enhancement of network connectivity and rigidity. The thickness dependence of switching voltages of Ge₁₅Te_85 ? xSn_x and Ge₁₇Te_83 ? xSn_x glasses is found to be linear, in agreement with the memory switching behavior shown by these glasses.     

Crystallization kinetics of a-Se75Te25 − xGax (x = 0, 5, 10 and 15 at wt %) glassy system

Bulk glasses of a-Se₇₅Te_25 ? xGa_x (x = 0, 5, 10 and 15 at wt %) have been prepared by melt quenching technique. These samples were structurally characterized by using X-ray diffraction. Kinetic of crystallization in these glasses was studied under non-isothermal conditions using differential thermal analysis (DTA). DTA is performed at different heating rates of 5, 10, 15, 20 and 30 °C/min. The values of glass transition (T_g) and crystallization peak temperature (T_p) are found to be composition and heating-rate dependent. The obtained results have been analyzed in terms of activation energy of glass transition (E_g) using Kissinger's and Mahadevan et al. relations. Values of E_g obtained by the two relations are in agreement with each other. The results indicate that the crystallization process is a three-dimensional growth.     

Microstructural analysis of Ga2S3–2MCl (M = K, Rb, Cs) glasses using Raman scattering

Raman scattering spectra of Ga₂S₃–2MCl (M = K, Rb, Cs) glasses have been conducted at room temperature. Based on the analysis of the local co-ordination surroundings of Cs⁺ ions, the similarities and differences of Raman spectra for the glass Ga₂S₃–2CsCl and the bridged molecular GaCl₃ were explained successfully. Through considering the effect of M⁺ ions on mixed anion units [GaS_4?xCl_x] and bridged units [Ga₂S_6?xCl_x] and the corresponding microstructural model, the Raman spectral evolution of the Ga₂S₃–2MCl (M = K, Rb, Cs) glasses was reasonably elucidated.     

Structural analysis of xCsCl(1 − x)Ga2S3 glasses

Sulphide glasses doped with rare-earth ions have been demonstrated to be suitable for photonic applications such as optical amplifiers, up-converters and fiber lasers. The substitution of metal halides into the glass network has been shown to result glasses with desirable properties in terms of quantum efficiency and fiber manufacture [J.R. Hector, J. Wang, D. Brady, M. Kluth, D.W. Hewak, W.S. Brocklesby, D.N. Payne, Journal of Non-Crystalline Solids 239 (1998) 176]. To assist in the understanding of this improvement a structural analysis of glasses with a composition xCsCl(1 ? x)Ga₂S₃ has been undertaken in order to examine the nature of the gallium environment. Information collected by high energy X-ray diffraction and neutron diffraction have been analyzed to permit the identification of the structural units as Ga centered tetrahedra. The interconnection between the tetrahedra was found to be predominantly corner sharing.     

The preparation and structural studies in the (80 − x)TeO2–20ZnO–(x)Er2O3 glass system

Er³⁺-doped tellurite glasses of the (80 ? x)TeO₂–20ZnO–(x)Er₂O₃ system (0.5 mol% ? x ? 2.5 mol%) have successfully been made by melt-quenching technique and their structure has been investigated by means of DTA and Raman spectroscopy. The DTA results show the thermal parameters; such as the glass transition temperature (T_g) and crystallization temperature (T_c) were determined. It is found that this system provides a stable and wide glass formation range in which the glass stability around 99–140 °C may be obtained. The Raman spectroscopy used the structural studies in the glass system. Two Raman shift peaks were observed around 640–670 cm^?1 and 720–740 cm^?1, which correspond to the stretching vibration mode of TeO₄ tbp and TeO₃ tp, respectively. It is found that the spectral shift in Raman spectra is depending on the Er₂O₃ content. This evolution is an indication of the changes in the basic unit of the glass structure.     

Role of interfacial crystallites in the crystallization of α-Fe2O3/α-Al2O3(0 0 0 1) thin films

In situ crystallization of α-Fe₂O₃/α-Al₂O₃(0 0 0 1) thin films was studied in real-time synchrotron X-ray scattering experiments. We find the coexistence of α-Fe₂O₃ (hexagonal) and Fe₃O₄ (cubic) interfacial crystallites (∼50-Å-thick), well aligned [0.02° full-width at half-maximum (FWHM)] to the α-Al₂O₃[0 0 0 1] direction, in the sputter-grown amorphous films. As the annealing temperature increases up to 750°C, the cubic stacking of the Fe₃O₄ crystallites gradually changes to the hexagonal α-Fe₂O₃ stacking, together with the growth of the well-aligned (WA) (0.02° FWHM) grains from the α-Fe₂O₃ crystallites. In the meanwhile, heterogeneous nucleation starts to occur on the substrate at ∼600°C, resulting in the formation of misaligned (1.39° FWHM) α-Fe₂O₃ grains. Our study reveals that the interfacial crystallites act as a template for the growth of the WA α-Fe₂O₃ grains.     

Enhanced 2.0 μm emission in oxyfluoride glass-ceramics containing nanocrystals MF2 (MF3): Ho3+,Tm3+ (M = Ca,Ba, and La)

Transparent glass-ceramics containing MF₂(MF₃):Ho³⁺,Tm³⁺ (M = Ca, Ba, and La) nanocrystals have been prepared by melt quenching and subsequent thermal treatment. X-ray diffraction and transmission electron microscopy analysis confirmed the precipitation of MF₂ (MF₃) nanocrystals among the glass matrix. Energy-dispersive X-ray spectroscopy results evidenced the incorporation of Tm³⁺ and Ho³⁺ into the MF₂ nanocrystals. Intense 2.0 μm emission originating from the Ho³⁺: ⁵I₇ → ⁵I₈ transition was achieved upon excitation with 808 nm laser diode. A large ratio of the forward Tm³⁺ → Ho³⁺ energy transfer constant to that of the backward process indicated high efficient energy transfer from Tm³⁺ (³F₄) to Ho³⁺ (⁵I₇), and benefited from the reduced ionic distances of Tm³⁺–Tm³⁺ and Tm³⁺–Ho³⁺ pairs and low phonon energy environment with the incorporation of rare earth ions into the precipitated MF₂ nanocrystals. The results indicate that oxyfluoride glass-ceramic is a promising candidate for 2.0 μm laser.