DC conductivity of new single and mixed alkali oxyfluorovanadate glasses

New glasses have been prepared according to these formulas (70-x)V₂O₅-30BaF₂-xAF, where AF=LiF or NaF and (60-x)V₂O₅-30BaF₂-10LiF-xAF, where AF=NaF and x=10, 15, 20, 25 and 30 mol%. Density of the glasses was measured and molar volume calculated and they correlated with the AF content. The dc conductivity has been measured in the temperature range from 302 to 453 K. The dc conductivity increases with temperature and V₂O₅ content, while it decreases with the alkali fluoride content. Conductivity has been correlated with the calculated polaron distance, R, and glass transition temperature, T_g. The activation energy, W, increases with the increase in the alkali fluoride, while it decreases with the V₂O₅ content. Some parameters like polaron distance, R, polaron radius, r_p, ion concentrations, n(V), n(Li) or n(Na), hopping energy, W_H, density of localized states at Fermi level, N(E_F), polaron coupling constant, γ_p, polaron band width, J, hopping mobility, μ, and carrier density for electronic conduction, N_c, were calculated to explain the conduction mechanism and behavior of the present glasses.     

Effect of Mie resonance on photocatalytic hydrogen evolution over dye-sensitized hollow C-TiO_2 nanoshells under visible light irradiation

Light utilization is one of the key factors for the improvement of photocatalytic perfo rmance.He rein,we design C-TiO_2 hollow nanoshells with strong Mie resonance for enhanced photocatalytic hydrogen evolution in a dye-sensitized system under visible light irradiation(λ≥420 nm).By tuning the inner diameters of hollow nanoshells,the Mie resonance in hollow nanoshells is adjusted for better excitation of dye molecules,which thus greatly enhances the light utilization in visible light region.This work shows the potential of Mie resonance in nanoshells can be an alternative strategy to increase the light utilization for photocatalysis.     

Photocatalytic and optical properties of titanium dioxide thin films prepared by metalorganic chemical vapor deposition

Titanium dioxide thin films have been deposited by metalorganic chemical vapor deposition (MOCVD) over sodalime glass substrates at substrate temperatures ranging from 250 °C to 450 °C. The effect of deposition temperature on the structure and microstructure of the obtained films has been studied by x-rays diffraction (XRD) and scanning electron microscopy (SEM), respectively. Diffraction patterns show the existence of a pure anatase phase beside a texture change with the increase of deposition temperature. Micrographs show grain fragmentation with the increase in deposition temperature. UV–Vis. spectra have been recorded by spectrophotometery. The optical energy gap has been calculated for the deposited films from the spectrophotometrical data. Photocatalytic experiments have been carried out. The photocatalytic activity has been found to decrease with the increase in deposition temperature.     

Quasi-periodic stability of normally resonant tori

We study quasi-periodic tori under a normal-internal resonance, possibly with multiple eigenvalues. Two non-degeneracy conditions play a role. The first of these generalizes invertibility of the Floquet matrix and prevents drift of the lower dimensional torus. The second condition involves a Kolmogorov-like variation of the internal frequencies and simultaneously versality of the Floquet matrix unfolding. We focus on the reversible setting, but our results carry over to the Hamiltonian and dissipative contexts.     

EPR,ENDOR and optical spectroscopy of Yb3+ ion in KZnF3 single crystals

The paramagnetic center of tetragonal symmetry formed by the Yb³⁺ ion in the KZnF₃ crystal has been studied using methods of EPR, ENDOR and optical spectroscopy. The location of the impurity ion and the structural model of the complex differing from the model of the Yb³⁺ center in KMgF₃ have been established. The empirical scheme of the energy levels of the Yb³⁺ ion has been found. The parameters of its interaction with the crystal electrostatic field and the hyperfine interaction with ligands of the nearest environment have been determined. The parameters of the crystal field were used for the analysis of the distortions of the crystal lattice in the vicinity of Yb³⁺. The parameters of the transferred hyperfine interaction have been calculated for the distances between Yb³⁺ and F⁻ ions of the nearest environment obtained taking into account the found distortions. They are in good agreement with the experimental values.     

The band gap and transmission characteristics investigation of local resonant quaternary phononic crystals with periodic coating

In this article, the investigation of the Lamb wave propagation in two-dimensional phononic crystals (PCs) composed of an array of periodic coating on a thin plate is presented. Compared with the traditional PCs usually consist of cylindrical scatters with uniform coatings in their exterior structure, the newly exterior coating structures with periodic alternant arrangement of two different materials are proposed. The band structures are calculated using finite element method. We discover that a complete band gap can be exhibited at low frequency. Furthermore, for a finite PCs plate, the computed transmission and resonance spectra shown an evident resonance nature which can be directly related to formation of the low-frequency gaps. The effects of different material parameters and arrangement mode of coating on the acoustic energy transmission and attenuation are also studied. Finally, the experimental transmission spectrum of the periodic coating PCs are also presented and compared with the numerical results. This study will provide useful support to the design of tuning band gaps and isolators in the low-frequency range.     

Assessment of DFT functionals with fluorine–fluorine coupling constants†

Density functional theory (DFT) calculations of nuclear magnetic resonance (NMR) spin–spin coupling constants (SSCCs) provide an important contribution for understanding experimentally observed values. It is known that calculated SSCCs using DFT methods correlate well with those experimentally measured. Unlike most of SSCCs, in fluorine compounds, fluorine–fluorine SSCC J_FF shows that the Fermi contact (FC) term is not dominant, particularly for J_FF in polyfluorinated organic molecules. In order to devise a DFT approach that would correctly reproduce the variation of SSCCs within a series of fluorine compounds, we test several DFT-based approaches, using different exchange and correlation functionals. Isotropic contributions to NMR fluorine–fluorine coupling constants (FC, spin-dipolar, SD, paramagnetic spin-orbit, PSO, and diamagnetic spin-orbit, DSO) have been calculated. Results show that DFT methods give appropriate values for ⁿJ_FF (n = 4 to 7), while for geminal and vicinal J_FF present large deviations from experimental values. For the latter SSCCs (²J_FF and ³J_FF), the four contributions (FC, SD, PSO and DSO) are analysed as a function of the local and nonlocal exchange in 1,1- and 1,2-difluoroethylene. Although FC term is not dominant for these SSCCs, the variation of this contribution with exchange is remarkable. On the other hand, SD and PSO contributions can be suitably computed without and with exact exchange, respectively.