Study of electrical and thermoelecrical properties of sulfides Tm x Mn1–x S

Variable-valence Tm _x Mn_1–x S (0 ? x ? 0.15) compounds have been synthesized and their structural, electrical, and thermoelectrical properties have been studied in the temperature range of 80–1100 K. The regions of existence of solid solutions of sulfides Tm _x Mn_1–x S with the NaCl-type fcc lattice have been determined. It has been found that, as thulium ions are substituted for manganese cations, the electrical resistivity increases, and the lattice parameter increases more sharply than that corresponding to the Vegard’s law. The study of the temperature dependences of the thermopower coefficient has revealed that the current carrier sign is retained to 500 K for all the substitution concentrations, and the charge carrier type changes from the hole type to the electron type with variations in the temperature. The experimental data have been explained in terms of the exciton model.     

Structural and electronic properties of GaN x As1−x alloys

The structural and electronic properties of cubic GaN _x As_1−x with N-concentration varying between 0.0 and 1.0 with step of 0.25 were investigated using the full potential–linearized augmented plane wave (FP-LAPW) method. We have used the local density approximation (LDA) and the generalized gradient approximation (GGA) for the exchange and correlation potential. In addition the Engel-Vosko generalized gradient approximation (EVGGA) was used for the band-structure calculations. The structural properties of the binary and ternary alloys were investigated. The electronic band structure, total and partial density of states as well as the electron charge density were determined for both the binary and their related ternary alloys. The energy gap of the alloys decreases when we move from x=0.0 to 0.25; then it increases by a factor of about 1.8 when we move from 0.25 to 0.5, 0.75 and 1.0 using EVGGA. For both LDA and GGA moving from x=0.0 to 0.25 causes the band gap to close, showing the metallic nature of the GaN_0.25As_0.75 alloy. When the composition of N moves through x=0.25, 0.5, 0.75 and 1, the band gap increases.     

First-principles study of the structural and elastic properties of AuxV1–x and AuxNb1–x alloys

Ab initio total energy calculations, based on the Exact Muffin-Tin Orbitals (EMTO) method in combination with the coherent potential approximation (CPA), are used to calculate the total energy of Au_xV_1–x and Au_xNb_1–x random alloys along the Bain path that connects the body-centred cubic (bcc) and face-centred cubic (fcc) structures as a function of composition x (0 ≤ x ≤ 1). The equilibrium Wigner–Seitz radius and the elastic properties of both systems are determined as a function of composition. Our theoretical prediction in case of pure elements (x = 0 or x = 1) are in good agreement with the available experimental data. For the Au–V system, the equilibrium Wigner–Seitz radius increase as x increases, while for the Au–Nb system, the equilibrium Wigner–Seitz radius is almost constant. The bulk modulus B and C₄₄ for both alloys exhibit nearly parabolic trend. On the other hand, the tetragonal shear elastic constant C′ decreases as x increases and correlates reasonably well with the structural energy difference between fcc and bcc structures. Our results offer a consistent starting point for further theoretical and experimental studies of the elastic and micromechanical properties of Au–V and Au–Nb systems.     

Electronic and optical properties of ZnS x Se1−x alloys

A series of calculations from first principles have been carried out to study structural, electronic, and optical properties of ZnS_xSe_1−x alloys. Our results show that the lattice constant scales linearly with sulfur composition. The imaginary parts of the dielectric function are calculated, which are in good agreement with the experimental data. We have also interpreted the origin of the spectral peaks on the basis of band structure and density of states. Additionally, we find that no bowing effect in the absorption edge is observed, unlike other II-VI semiconductor alloys.      

Morphological and luminescent evolution of near-infrared-emitting CdTe x Se1?x nanocrystals

Two sets of Co₃O₄-KIT-6 composite catalysts were prepared. To prepare the first set of catalysts, mesoporous silica (KIT-6) samples with different pore sizes were obtained by changing the hydrothermal temperature during synthesis, and the same amount of Co₃O₄ (10?%) was loaded by impregnation. In the second set, KIT-6 prepared by hydrothermal synthesis at 100?°C was used to load different amounts of Co₃O₄ (up to 50?%). These materials were characterized by low-angle and wide-angle X-ray diffraction, transmission electron microscopy (TEM), and N₂ adsorption?Cdesorption. The TEM results showed that Co₃O₄ species do not disperse on the inner pores of KIT-6 homogeneously, nor do they form separate Co₃O₄ particles detached from KIT-6. Rather, Co₃O₄ crystals grow into the KIT-6 matrix, forming Co₃O₄-KIT-6 composite grains in some parts of the KIT-6 matrix, whereas there are empty mesoporous silica channels in other parts. The influences of textural properties of KIT-6 support and the loading of Co₃O₄ on the catalytic activity in N₂O decomposition were studied. It was found that the textural properties (e.g., pore size, surface area) of KIT-6 have little effect on the catalytic activity, whereas the amount of Co₃O₄ directly correlates with the catalytic activity.     

Magnetic and electric properties of Yb x Mn1 − x S alloys

Results from investigating the structural, magnetic, and electrical properties of Yb _x Mn_{1 ? x} S alloys (0 ≤ x ≤ 0.2) synthesized on the basis of manganese monosulfide are presented. Substituting manganese for ytterbium increases the concentration of charge carriers and lowers the activation energy. The observed anomalies in the temperature dependence of resistivity are explained by an impurity semiconductor model with donor 4f levels.     

Formation of Ti1–x Si x and Ti1–x Si x N films by magnetron co-sputtering

The paper reports on surface morphology, structure and microhardness of TiSi–N films formed by cosputtering from two target-facing unbalanced magnetrons, equipped with pure Ti and Si targets, on an unheated substrate rotating in front of both targets. The ratio Si/Ti in the TiSi–N film was achieved by modifying the magnitude of currents in the individual magnetrons and by the addition of nitrogen to the film. The rotation of the substrate has a strong effect on the film deposition rate and its morphology. The deposition rate is 3 times lower than that of the film deposited on a stationary substrate. The surface roughness of a polycrystalline Ti film deposited on the rotating substrate is considerably higher than that on a stationary substrate. On the contrary, the surface of an amorphous Si film is smooth and there is no difference between the roughness of Si films sputtered on stationary and on rotating substrates. The hardness of the film increases with increasing Si content and with the addition of nitrogen to the TiSi film. The Ti(26 at.%)Si(8.5 at.%)N(65 at.%)-film sputtered on an unheated rotating steel substrate, held at a floating potential, exhibited the best result with a hardness of 29 GPa.     

Specific features of the electronic structure and spectral properties of NdNi5 − x Cu x compounds

The spectral properties of the intermetallic compounds NdNi_{5 ? x} Cu _x (x = 0, 1, 2) have been studied using optical ellipsometry in the wavelength range 0.22–16 μm. It has been established that substitution of copper atoms for nickel leads to noticeable changes in the optical absorption spectra, plasma frequencies, and relaxation frequencies of conduction electrons. Spin-polarized calculations of the electronic structure of these compounds have been performed in the local spin density approximation allowing for strong electron correlations (LSDA + U method) in the 4f shell of the rare-earth ion. The calculated electron densities of states have been used to interpret the experimental dispersion curves of optical conductivity in the interband light absorption region.     

Optical and structural properties of Zn1−x Mg x O ceramic materials

This paper reports structural, optical and cathodoluminescence characterizations of sintered Zn_1?x Mg _x O composite materials. The effects of MgO composition on these film properties have been analyzed. X-ray diffraction (XRD) confirms that all composites are polycrystalline with prominent hexagonal wurtzite structure along two preferred orientations (002) and (101) for the crystallite growth. Above doping content x = 10 %, the formation of the hexagonal ZnMgO alloy phase and the segregation of the cubic MgO phase start. From reflectance and absorption measurements, we determined the band gap energy which tends to increase from 3.287 to 3.827 eV as the doping content increases. This widening of the optical band gap is explained by the Burstein–Moss effect which causes a significant increase of electron concentration (2.89 × 10¹⁸?5.19 × 10²⁰ cm^?3). The luminescent properties of the Zn_1?x Mg _x O pellets are studied by cathodoluminescence (CL) at room and liquid nitrogen temperatures under different electron beam excitations. At room temperature, the CL spectra of the Zn_1?x Mg _x O composites exhibit a dominant broad yellow-green light band at 2.38 eV and two ultraviolet emission peaks at 3.24 and 3.45 eV corresponding to the luminescence of the hexagonal ZnO and ZnMgO structures, respectively. For the doped ZnO samples, it reveals also new red peaks at 1.72 and 1.77 eV assigned to impurities’ emissions. However, the CL spectra recorded at 77 K show the presence of excitonic emission peaks related to recombination of free exciton (X _A), neutral donor-bound excitons (D⁰X) and their phonon replicas. The CL intensity and energy position of the green, red and ultraviolet emission peaks are found to depend strongly on the MgO doping content. The CL intensity of the UV and red emissions is more enhanced than the green light when the MgO content increases. CL imaging analysis shows that the repartition of the emitting centers in Zn_1?x Mg _x O composites is intimately connected to the film composition and surface morphology.     

Structure and magnetic properties of the Co x Pt100−x nanowire arrays

The development of photocathodes materials has become an important task for X-ray free electron laser and new generation of particle accelerator. The choice of the optimum cathode type and its further improvement is a fundamental issue for the progress in radio-frequency photoinjectors. Metallic photocathodes offer several advantages over the semiconductor ones, e.g. long lifetime and prompt response time on the photoemission. This paper reviews the requirements and the current status of metallic photocathodes prepared by pulsed laser ablation deposition technique. Magnesium, yttrium and lead are proposed as good alternative to copper photocathode which is generally used in radio-frequency photoinjectors. Parametric studies of the irradiation conditions are demanded to optimize the metallic thin film deposition. The main achievements on the morphology and structure characterization as well as photoemission testing of metallic photocathodes are presented and discussed.     

Magnetic and dynamic properties of Sm x Mn1 − x S solid solutions

The real and imaginary parts of the magnetic permeability at frequencies of 0.1, 1.0, and 10.0 kHz, as well as the electron paramagnetic resonance (EPR) line width and g-factor, have been measured in Sm _x Mn_{1 ? x} S (0.10 < x < 0.25) solid solutions in the temperature range 5–300 K. The logarithmic dependence of the maximum in the imaginary part of the magnetic permeability on the frequency and the power-law dependence of Imμ on the temperature have been determined. The mechanism of relaxation of the magnetic moment in the magnetically ordered and paramagnetic phases has been established. The experimental results have been explained in terms of the Heisenberg model with competing exchange interactions and the formation of the antiaspiromagnetic state at low temperatures.     

Experimental and theoretical investigations of the Knight shift of Pd and Ag in the alloy system Ag x Pd1−x

The Knight shift of Pd in Ag _x Pd_1–x has been determined for concentrationsx0.2. In full accordance with the expectations based on the behaviour of the magnetic susceptibility, it was found that the Knight shift of Pd is rapidly reduced in magnitude by adding Ag to Pd. To allow for a detailed interpretation of this finding, we have performed Korringa-Kohn-Rostoker coherent potential approximation (KKR-CPA) band structure calculations for Ag _x Pd_1–x . These calculations clearly demonstrate that the decrease in spin susceptibility with increasingx is accompanied with a decrease in core polarization. In contrast to Pd, the negative Knight shift of Ag on the Pd-rich side of the system is caused by the valence band contribution, as it is demonstrated by our calculations. This is caused by an intersite effect in analogy to the transferred hyperfine field found for non-magnetic elements dissolved in a magnetic host.     

Electrical,magnetic, and thermal properties of UCu5− x Pt x

We report measurements of the structural and electronic properties of UCu_{5??? x} Pt _x , a Pt-based analogue of the nFL system UCu_{5??? x} Pd _x , in the concentration range 0≤?x?≤?5. Forx?≤?2.5 and x?≥?4.5, the UCu_{5??? x} Pt _x system crystallizes in the AuBe₅ structure, but is mixed phase in the range 2.5?<?x?<?4.5. We observe a rapid suppression of long range antiferromagnetic order for small concentrations of Pt. The electrical resistivity, DC magnetic susceptibility, and specific heat of UCu_{5??? x} Pt _x in the concentration range 0.75?≤?x?≤?1 have temperature dependencies at low temperature (T?≤?10?K) consistent with the non-Fermi liquid behaviour found in UCu_{5??? x} Pd _x . Above x?=?1, there is a transition from non-Fermi liquid behaviour to Fermi liquid behaviour with no observation of any spin glass behaviour.     

Magnetic properties and structure of Fe2 − x Mg x CrO4 chromites

The structural and magnetic properties of Fe_{2 ? x} Mg _x CrO₄ chromite synthesized by the ceramic method to receive analogs of natural minerals and to solve the rock magnetism problems are investigated. The dependences of cubic-lattice parameters and magnetic characteristics on composition, with slight deviation from linearity, are obtained. It is established in the course of experiments related to the partial thermal remanent magnetization in weak fields that some compositions are characterized by self-reversal under sample heating to 600°C in air. It is suggested that the self-reversal is caused by a nonuniform distribution of cations in chromite structure and phase transformation under oxidation.     

Electronic structure of ordered ferro- and antiferromagnetic phases of Cd1−x Mn x Te semimagnetic semiconductor

Self-consistent, spin-polarized SDF band structure calculations for the ordered ferromagnetic and antiferromagnetic phases of Cd_0.5Mn_0.5Te and MnTe are reported. The electronic structure, the spin-ordering-dependent hybridization mechanism and the nature of optical excitations are discussed. Also, striking differences between cubic and hexagonal phases of MnTe are discussed and explained.     

Magnetic and optical properties of Zn1– x Fe x O ( x = 0.05 and 0.10) diluted magnetic semiconducting nanoparticles

We have investigated the magnetic and optical properties of chemically low temperature-synthesized Zn_{1– x} Fe _x O (x = 0.05 and 0.10) diluted magnetic semiconducting nanoparticles (～7 nm). Observed magnetic behaviour of x = 0.05 samples showed that the net magnetic interaction was antiferromagnetic-like, a feature established by Curie–Weiss fit, concave Arrott–Belov–Kouvel (ABK) plots with the absence of spontaneous magnetization even at 5 K and stretched exponential-type time-dependent magnetization behaviour. Optimization of the Fe(x) dopant concentration in Zn_{1– x} Fe _x O gave the most favourable room-temperature ferromagnetism for x = 0.10, as supported by finite coercive field (～94.4 Oe) and remanent magnetization (0.011 µ_B/Fe ion) from strong hysteretic magnetization vs. magnetic-field curves at room temperature. The Curie temperature of the x = 0.10 sample was estimated at ～388 K. The existence of a room-temperature ferromagnetic phase was further established by the convex nature of the ABK plots with finite spontaneous magnetization. The observed magnetic behaviour for different x values is best explained by a magnetic polaron model.     

Mechanism and kinetics of solid-state amorphization by mechanical alloying of Al65Cu 35−x Nb x

In the present study, we have synthesized a number of Al-based Al₆₅Cu_35???x Nb_x ternary alloys by mechanical alloying and undertaken a detailed characterization of their microstructural evolution by X-ray diffraction, high-resolution transmission electron microscopy and positron annihilation spectroscopy. To predict the phase-equilibrium in a given ternary powder blend subjected to mechanical alloying, we have modified the Miedema model to incorporate the influence of interfacial energy contribution in solid-state amorphization and analytically calculate the enthalpy and Gibbs energy of ternary amorphous and nanocrystalline solid solutions. The predicted trend compares well with the experimental data. Finally, an attempt has been made to determine the mechanism of solid-state amorphization in Al₆₅Cu_35?xNb_x alloys utilizing both experimental results and model-based thermodynamic calculations.     

Characterization and optimization of Ce0.6Zr0.4−x Mn x O2 (x ≤ 0.4)

In situ synthesis method is used to synthesize g-C₃N₄-P25 composite photocatalysts with different mass rations. The experiment result shows that P25 particles with diameter at range of 20–30 nm were embedded homogenously in the sheets of g-C₃N₄. Coupling g-C₃N₄ with P25 can not only improve the visible light absorption, but also improve the visible light photocatalytic activity of P25. The g-C₃N₄-P25 nanocomposite has the higher photocatalytic activity than g-C₃N₄ under visible light. The optimal g-C₃N₄ content with the highest photocatalytic activity is determined to be 84 %, which is almost 3.3 times higher than that of individual g-C₃N₄ under the visible light. The enhanced visible light photocatalytic activity could be ascribed to the formation of g-C₃N₄ and TiO₂ heteojunction, which results in an efficient separation and transfer of photo-induced charge carriers. The electron spin resonance results show that the ^·O₂ ^? radicals are main active species for g-C₃N₄ and the g-C₃N₄-P25 nanocomposites.