Narrow Results

Glasses from the xMnO · (100-x)[Bi₂O₃ · GeO₂] system, with 0≤ x ≤ 50 mol%, have been obtained and characterized by IR spectroscopy in order to obtain information about the influence of MnO on the local structure of the Bi₂O₃ · GeO₂ glass matrix. The presence and the dependence of the structural units BiO₃, BiO₆ and GeO₄ on the MnO content was analyzed.

X-ray diffraction, electrical and magnetic measurements performed on Pb₂Mg_1-xCo_xWO₆ solid solutions with 0≤x≤1 are reported. By cobalt substitution for magnesium the diffraction patterns indicate the transition from orthorhombic structure characteristics for Pb₂MgWO₆ to the cubic structure specific for Pb₂CoWO₆. The Curie temperature decreases with cobalt content from 38°C to 32°C. The magnetic data indicate that the cobalt ions are in a bivalent state and, for x>0.1, experience negative magnetic interactions.

Structural and dielectric measurements were performed on Pb(Mg_0.5Mo_0.5)O₃ (PMM). Polycrystalline samples of PMM were synthesized by solid-state reaction technique at normal pressure. The X-ray measurements indicate formation of single-phase perovskite structure with absence of Mg and Mo ions ordering. The temperature dependence of the relative dielectric constant suggests that PMM undergoes a phase transition (FE or AFE) at 63°C.

EPR and FT-IR spectroscopy have been used to investigate the B₂O₃–Bi₂O₃ glass matrix containing CuO in order to obtain more information about the local structure of these glasses. The EPR absorption spectra revealed the presence in the glass structure of the Cu²⁺ ions in axially distorted octahedral environments. No superexchange interaction of Cu²⁺ was detected. In the samples with x≥5 mol%, mixed valence states of copper ions were revealed. The FT-IR measurements indicate the presence in the glass structure of the distorted [BiO₆] polyhedra, tri- and tetra-borate units (BO₃, BO₄) and its dependence by the copper content.

We report systematic studies of Ba₂NdSbO₆ as substrates for the production of YBa₂Cu₃O_7-δ superconducting thin films. Chemical stability and crystallographic coupling between Ba₂NdSbO₆ and YBCO were examined by characterizing Ba₂NdSbO₆-YBa₂Cu₃O_7-δ (0 to 100 vol.%) polycrystalline composites. X-ray diffraction experiments showed that Ba₂NdSbO₆ belongs to the complex cubic perovskite family. Moreover, we determine that these materials are chemically stables, e.g. there is no chemical reaction at the interface, and the lattice parameters evidenced a matching ~ 2%. Morphological characterization of our samples was performed through scanning electron microscopy, which revealed the existence of separated grains of Ba₂NdSbO₆ and YBa₂Cu₃O_7-δ. Compositional analysis of samples was performed by energy dispersive X-ray experiments, which showed the inexistence of impurities or undesired chemical elements. DC susceptibility measurements permitted us to determine that the presence of Ba₂NdSbO₆ does not affect the critical temperature of the superconducting transition of YBa₂Cu₃O_7-δ. Our results evidenced that Ba₂NdSbO₆ is an excellent candidate as a substrate for the fabrication of YBa₂Cu₃O_7-δ superconducting thin films.

Glasses of the xFe₂O₃ · (100 - x)[2B₂O₃ · SrO] system (0 ≤ x ≤ 40 mol%) were prepared and studied by electron paramagnetic resonance (EPR) and magnetic susceptibility measurements. EPR investigations indicated the compositional domains favorable to the presence of isolated iron ions or to those magnetically clustered. The magnetic susceptibility data evidenced the presence of both Fe³⁺ and Fe²⁺ ions, with their relative content depending on the Fe₂O₃ concentration. Dipolar and superexchange interactions involving iron ions were revealed depending on the iron content of the sample.

The two equilibrium structures of the compound Li₃AlB₂O₆ have been investigated via the minimization of the total energy within Local Spin Density Approximation (LSDA) based on the Density Functional Theory (DFT) in our work. The calculated lattice parameters are all in good agreement with their corresponding experimental values. The relative stability of the two structures are determined. We find that the structure suggested by He et al. is more stable than that proposed by Abdullaev et al. at zero pressure conditions. The reasons for which the structure suggested by He et al. is more stable are also described. Then the electronic properties of the compound Li₃AlB₂O₆ including the density of states and energy band structure are successfully obtained and compared for the two structures. We find that the properties of insulator decreases from the structure suggested by He et al. to the structure proposed by Abdullaev et al.

We report ab initio molecular dynamics simulations of carbon clusters in free space and inside C₆₀ using SIESTA. We have studied the stability and geometries of small carbon clusters consisting of 2–12 carbon atoms inside the C₆₀ molecule and in free space by optimizing the atomic geometries. We have found that the C–C bond length is in agreement with the 1.40 and 1.45 values reported earlier. We find that the clusters inside the C₆₀ are more stable than clusters in free space. Binding energy per carbon atom initially increases with number of carbon atoms in the cluster and then decreases after maximizing for the 9-atom cluster. For more than 9 carbon atoms in the cluster inside C₆₀, C atoms of the cluster start forming bond with the C₆₀ cage and the C₆₀ structure gets distorted. We have done calculations for charge transfer and chemical reactivity. The calculations of ionization potential and electron affinity show that clusters in free space are less reactive compared with C_n@C₆₀. Charge transfer calculations show that the bond formation of C atoms with the C₆₀ cage is accompanied with a transfer of charge from carbon cluster to C₆₀.

Hydrogenated amorphous SiC films (a-Si_1-xC_x:H) were prepared by DC magnetron sputtering technique on p type Si(100) and corning 9075 substrates at low temperature, by using 32 sprigs of silicon carbide (6H-SiC). The deposited film a-Si_1-xC_x:H was realized under a mixture of argon and hydrogen gases. The (a-Si_1-xC_x:H) films have been investigated by scanning electronic microscopy equipped with EDS system (SEM-EDS), X-rays diffraction (XRD), secondary ions mass spectrometry (SIMS), Fourier transform infrared spectroscopy (FTIR), UV-visible-IR spectrophotometry, and photoluminescence (PL). XRD results showed that the deposited film was amorphous with a structure of a-Si_0.81C_0.19:H corresponding to 19 at.% carbon. The photoluminescence response of the samples was observed in the visible range at room temperature with two peaks centered at 463 nm (2.68 eV) and 542 nm (2.29 eV). The structural properties and the origin of the luminescence were discussed.

Structural and optical properties of F-center (two electrons trapped by an oxygen vacancy) defect in hematite have been studied using a quantum-chemical model. Calculated absorption energies, 0.9 eV and 3.6 eV, are discussed in terms of the available experimental data. An explanation for the origin of experimentally observed electron depletion in hematite is proposed.

The LaCoO₃ single crystal, 4 mm in diameter and 30 mm in length, has been grown by optical floating zone method. The magnetic measurements show that the coercivity and the remanence are 5 Oe and 1.5 × 10^-3 emu/g at 5 K, respectively. The ZFC and FC magnetic susceptibility curves overlap and the 1/χ(T) curve shows a linear variation over the interval 5 K < T < 35 K in 1000 Oe. As the temperature increases from 35 K to 90 K, the ZFC curve in 1000 Oe firstly reduces nonlinearly, and then appears as a wave crest. The χ under ZFC is higher than that under FC in the temperature range of 55~90 K. In 50 000 Oe, a slope change of 1/χ(T) at about 12 K is observed.

Structure and optical characteristics of nickel (Ni) single crystal (100) surface and thin films have been evaluated by the combined use of reflection high-energy electron diffraction (RHEED) and spectroscopic ellipsometry (SE). Nickel films were fabricated by thermal evaporation at 100°C while Ni single crystal was grown by vertical Bridgman method. RHEED analysis indicates that the grown Ni films were polycrystalline. Spectral dependencies of refractive index n(λ) and extinction coefficient k(λ) determined in the spectral range λ ~ 250–1030 nm indicate the typical behavior of metals. The refractive index of Ni films is found to be slightly higher compared to that of Ni(100) crystal.

Synthesis of the Y₃Ba₅Cu₈O₁₈ superconducting material by the standard solid state reaction is reported. DC resistivity measurements reveal a bulk T_c = 97.86 K, which was determined by the criterion of the maximum in the numerical temperature derivative of electrical resistivity. Structure characterization was performed by means of the X-ray diffraction (XRD) technique. A Rietveld refinement of XRD patterns shows that the material crystallizes in an orthorhombic structure, space group Pmm2 with cell parameters a = 3.9211(3) Å, b = 3.8514(1) Å and c = 31.0170(0) Å. In order to study the pairing mechanism close to T_c, conductivity fluctuation analysis was performed by the method of logarithmic temperature derivative of the conductivity excess. Close and above T_c, the conductivity fluctuation analysis revealed the occurrence of two fluctuation regimes characterized by the critical exponents λ_3D = 0.48 and λ_2D = 0.97. These regions were interpreted as corresponding to 3D and 2D Gaussian regimes, respectively. By the utilization of the Ginzburg–Landau theory, we experimentally determined the Ginzburg number. Our results are in agreement with that reported on YBa₂Cu₃O_7-δ, but an enhancement of the Gaussian fluctuation regimes was experimentally detected. The correlations of the critical exponents with the dimensionality of the fluctuation system for each Gaussian regime were performed by using the Aslamazov–Larkin theory. Closer to T_c, a genuinely critical regime was identified. Scaling of our results permits to establish that the dynamical of fluctuation system has the universality class described by the 3D-XY model. Below T_c, it is observed that for several values of transport current density, this material evidences a paracoherent regime followed by a coherent transition, which occurs in the regime of the zero resistance state.

Synthesis of the Y₂Ba₅Cu₈O₁₇ superconducting material by the standard solid state reaction is reported. DC resistivity measurements reveal a bulk T_c = 104.95 K which was determined by the criterion of the maximum in the numerical temperature derivative of electrical resistivity. Structure characterization was performed by means of the X-ray diffraction (XRD) technique. A Rietveld refinement of XRD patterns shows that the material crystallizes in an orthorhombic structure, space group Pmm2 with cell parameters a = 3.8712(0) Å, b = 3.8481(4) Å and c = 27.1601(4) Å. In order to study the pairing mechanism close to T_c, conductivity fluctuation analysis was performed by the method of logarithmic temperature derivative of the conductivity excess. Close and above T_c, the conductivity fluctuation analysis reveal the occurrence of two fluctuation regimes characterized by the critical exponents λ_3D = 0.52 and λ_1D = 1.51, corresponding to 3D and 1D Gaussian regimes, respectively. The correlations of the critical exponents with the dimensionality of the fluctuation system for each Gaussian regime were performed by using the Aslamazov–Larkin theory.

Crystal structure, surface composition, infrared emission properties and surface electrical properties of tourmaline from Guangxi of China, when subjected to heat treatment in air atmosphere had been studied by some methods, including X-ray fluorescence spectrum (XRF), X-ray diffraction (XRD) meter, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), energy dispersion spectroscopy (EDS), scanning electron microscope (SEM) and Zeta potential analyzer, etc. Experimental results show that the unit cell of tourmaline would shrink during heat treatment because Fe${}^{2+}$ were oxidized. Moreover, the Fe${}^{3+}$/Fe${}_{\mathrm{\text{total}}}$ inside tourmaline can be raised after treatment. Infrared normal total emissivity of tourmaline reaches 0.87, and infrared radiation energy density is $4.56\times 10^2{\mathrm{\text{W/m}}}^2$. It can maintain excellent infrared emission properties at high temperature. Simultaneously, tourmaline presents negative Zeta potential in the aqueous solution, and its Zeta potential reaches −18.04 mV. Zeta potential of tourmaline was increased to −24.83 mV after heat treatment at 400${}^{\circ }$C, and decrease to −11.78 mV after heat treatment at 600${}^{\circ }$C. These findings may provide reference data for tourmaline’s application in the field of functional materials.

Molecular dynamics (MD) simulations and visualizations were explored to investigate the changes in structure of liquid aluminosilicates. The models were constructed for four compositions with varying Al₂O₃/SiO₂ ratio. The local structure and network topology was analyzed through the pair of radial distribution functions, bond angle, bond length and coordination number distributions. The results showed that the structure of aluminosilicates mainly consists of the basic structural units TO${}_y$ (T is Al or Si; y = 3, 4, 5). Two adjacent units TO${}_y$ are linked to each other through common oxygen atoms and form continuous random network of basic structural units TO${}_y$. The bond statistics (corner-, edge- and face- sharing) between two adjacent TO${}_y$ units are investigated in detail. The self-diffusion coefficients for three atomic types are affected by the degree of polymerization (DOP) of network characterized by the proportions of nonbridging oxygen (NBO) and Q${}^n$ species in the system. It was found that Q⁴ and Q³ tetrahedral species (tetrahedron with four and three bridging oxygens, respectively) decreases, while Q⁰ (with four nonbridging oxygen) increase with increasing Al₂O₃/SiO₂ molar ratio, suggesting that a less polymerized network was formed. The structural and dynamical heterogeneities, micro-phase separation and liquid–liquid phase transition are also discussed in this work.

This paper presents the results of a study on the production of nanofiber anisotropic nanoporous materials based on silk fibroin and cotton cellulose by electrospinning using a rotating screen-receiver of nanofibers in the form of a thin material. The difference in the anisotropic properties of the nanofiber material is shown by the method of birefringence, sorption of water vapor and filtration of a liquid-phase mixture. The possibility of using nanofiber nanoporous fibroin and cotton cellulose materials as a nanofilter of gaseous and liquid-phase mixtures has been shown.

Cleaved specimens of layered compounds Bi₂Sr₂(CaCuO₂)_nCuO₆, n = 0, 1 and 2 phases have been studied by convergent beam electron diffraction. The identity and the symmetry of the phases have been determined from the higher order Laue zone (HOLZ) rings and the symmetry of their [001] patterns. In the case of Bi₂Sr₂CuO₆, the [001] axial periodicity corresponds to the component of the wave vector of the monoclinic modulation along the c axis. Widely different axial periodicities have been obtained. This is interpreted as the manifestation of the variation of the monoclinic modulation. The presence of basal stacking fault lowers the symmetry of the [001] pattern.