A Quantum Chemical Study on Hexanuclear Cluster Halides of Rare Earth Elements and Their Interstitial Compounds

The electronic structures of rare earth cluster halides R(R_6X_(12)) and their interstitial compounds R_7X_(12)Zwere studied by the DV-X_(?) method (R=Sc,Y,Pr,Gd or Er;X=Cl,Br or I;Z=B,C,N,Fe,Co or Ru).Theresults show that because f electrons in empty rare earth cluster are screened,their orbitals are more difficult tooverlap each other,a deficiency of skeleton orbitals in cluster causing the system to be unstable.They are easilycondensed into chain compound R_2X_3 or R_5X_8.If a light atom of main group is embedded into octahedralcluster,bonding orbitals formed from interstitial atom and rare earth cluster strengthen clusterskeleton bond in the system to reach structural stability.If embedded atom belongs to transition metal,bondingorbitals composed of that of interstitial atom and rare earth cluster take the place of original cluster skeletonorbitals to form heteronuclear metal cluster (or double-coordination compound).     

Ozonation of Drinking Water: A Design Methodology

The formation of potentially carcinogenic organic halides has been shown to result from drinking water disinfection with chlorine. xidative treatment of organic halide precursors with ozone prior to chlorination has surfaced as an attractive technique for reducing the formation of these compounds. In addition to reduction of precursor levels, preozonation has been reported to effect other beneficial results in water treatment. This paper presents design methodologies to optimize the implementation of the ozonation process for water treatment applications. Pre-design considerations common to all ozonation design processes are discussed. Subsequently, design procedures for the ozone generation and contacting systems are reviewed.     

The effect of pH and halides on the corrosion process of stainless steel bipolar plates for proton exchange membrane fuel cells

Stainless steel is attractive as material for bipolar plates in proton exchange membrane fuel cells, due to its high electrical conductivity, high mechanical strength and relatively low material and processing cost. Potentiostatic and potentiodynamic tests were performed in H₂SO₄ solutions on AISI 316L stainless steel bipolar plates with etched flow fields. The effect of pH and presence of small amounts of fluoride and chloride on the corrosion rate and interfacial contact resistance of the stainless steel bipolar plate were investigated. The tests performed in electrolytes with various pH values revealed that the oxide layer was thinner and more prone to corrosion at pH values significantly lower than the pH one expects the bipolar plate to experience in an operating proton exchange membrane fuel cells. The use of solutions with very low pH in such measurements is thus probably not the best way of accelerating the corrosion rate of stainless steel bipolar plates. By use of strongly acidic solutions the composition and thickness of the oxide layer on the stainless steel is probably altered in a way that might never have happened in an operating proton exchange membrane fuel cell. Additions of fluoride and chloride in the amounts expected in an operating fuel cell (2 ppm F⁻ and 10 ppm Cl⁻) did not cause significant changes for neither the polarization- nor the contact resistance measurements. However, by increasing the amount of Cl⁻ to 100 ppm, pitting was initiated on the stainless steel surface.     

Basic properties of the F-type centers in halides, oxides and perovskites

We present a short survey of the optical properties of primary radiation-induced point defects in alkali halides, simple oxides and some ABO₃ perovskites. We discuss in details the optical properties of single electron F and F⁺ centers in rock-salt (f.c.c.) alkali halides and oxides and show that the Mollwo-Ivey law well-known for the F-type centers in alkali halides may be extended for other rock-salt structure insulators. We also discuss the major differences in point defect production mechanisms in halides and oxides. We show that the Rabin-Klick diagram may be generalized for a whole family of alkali halides. The F-type center migration and aggregation into metal colloids in alkali halides and oxides is also discussed.     

Hartree–Fock study of near-edge gap states in CaF2 with Na, Cl or Sr impurities

The energy of formation and electronic structure of Na⁺, Cl⁻ and Sr²⁺ impurity centers in CaF₂ have been computed using ab initio Hartree–Fock theory and the supercell approach. The work extends and complements recent results [Solid State Commun. 118 (2001) 569] for Mg²⁺ as a substitutional impurity in CaF₂. For Na⁺ substituting for Ca²⁺ [S(Na)], charge compensation by an F⁻ vacancy [V(F)] or by a second, interstitial Na⁺ [I(Na)] are both considered. In all cases, geometry optimization is done by relaxing the positions of nearest- and next-nearest-neighbors to minimize the total energy. After correction for electron correlation, the energies of formation increase in the order Mg²⁺2+−+ results are in agreement with previous Mott–Littleton formation energies. Ion charges, charge density maps and Mulliken bond populations are obtained to show the nature of bonding in the vicinity of the defect. Na⁺ leads to states just above the CaF₂ valence band maximum (VBM), and Na⁺ (and also Mg²⁺) produce states just below the conduction band minimum. The results are in qualitative agreement with available optical data for Na⁺ and Mg²⁺ impurity effects on CaF₂ near-edge absorption but show that gap states are important in addition to perturbed excitons. A Cl⁻ impurity yields a narrow band of states above the VBM which may significantly affect the deep-ultraviolet transmission of CaF₂. Sr²⁺ does not appear to produce states in the CaF₂ gap.     

Effect of sintanol DS-10 and halides on tin(II) reduction kinetics

Voltammetry and the EIS technique were applied to study effect of polyether sintanol DS-10 on tin(II) reduction kinetics in strong acidic sulphate solutions containing different halides. Diffusion-controlled Sn(II) reduction was found to be significantly retarded by strong inhibitive adsorption of sintanol. Surface coverage obtained from both impedance and kinetic data may be quantitatively described by means of Frumkin isotherm accounting for interaction between adsorbed particles. Partial suppression of the surface activity of sintanol was observed in the presence of halides. This effect was found to increase in the sequence: Cl⁻ < Br⁻ < I⁻⋅     

Synthesis and optical properties of elliptic Pb(OH)Br microdiskettes

The novel elliptic Pb(OH)Br microdiskettes were controllably synthesized by a simple sonochemical process. The structure characterizations of the microdiskettes were investigated in detail by means of XRD, SEM, and HRTEM. The results indicate that the orthorhombic phase of Pb(OH)Br with good crystallinity can be obtained. Meanwhile, controlled experiments show that the concentration of cetyltrimethylammonium bromide (CTAB) play an important role in the formation of the microdiskettes. Moreover, the ultraviolet-visible (UV-vis) absorbance spectra and photoluminescence (PL) microdiskettes show their ultraviolet absorption and green emitting behavior, indicating that the elliptic Pb(OH)Br microdiskettes have great potential to be applied in luminescent and optoelectronic devices.     

Influence of halides on the dissolution and passivation behavior of AZ91D magnesium alloy in aqueous solutions

The electrochemical behavior of AZ91D in various aqueous sodium halide solutions was investigated using open-circuit potential (E_oc), potentiodynamic polarization and ac impedance (EIS) techniques. Generally, the results reveal that during immersion a protective layer of a salt film is formed on the alloy surface whose passivation performance depends on the halide nature, its concentration and temperature. E_oc shifts positively with time until attaining a steady (E_st) value, which becomes less noble with increasing concentration or temperature of the test solution. At any given conditions, self-passivation was found to be favored in the order F⁻ > I⁻ > Br⁻ > Cl⁻. This sequence is consistent with that for surface film resistance (R_T) and its relative thickness (1/C_T). Nevertheless, in F⁻ medium each of the above parameters increases with [F⁻] up to a critical value of 0.3 M then decreases. Increasing concentration above 0.3 M induces large change in the microstructure of the outermost layer of the fluorinated extremely protective film and depassivation behavior predominates. In Br⁻ and I⁻ solutions, as well as the lower Cl⁻ concentrations (≤0.01 M), AZ91D exhibits pseudo-passive state over the polarization range from the corrosion potential (E_corr) to the knee point (E_pt) in the anodic scan, at which passivity breakdown occurs with rapid increase in the anodic current and hydrogen gas reaction. At Cl⁻ concentrations >0.01 M the negative difference effect (NDE) occurs under cathodic polarization where E_pt lies negative to E_corr. Addition of F⁻ to the Cl⁻ solution can induce large changes in the behavior of AZ91D. Equal concentration addition (1:1) produces the highest propensity of the surface to form passivating layer that can afford better protection.     

Crystal growth and characterization of CuI single crystals by solvent evaporation technique

Cuprous iodide (CuI) crystals are grown by slow evaporation technique in three different solvents. Large CuI single crystals with dimensions of 7.5 mm × 5 mm × 3 mm are obtained in pure acetonitrile solvent at 40 °C. The as-grown crystals are analyzed by X-ray diffraction, energy-dispersive X-ray analysis, differential scanning calorimetry, current-voltage characteristic and photoluminescence spectrum. The results show that the CuI crystal has the zinc-blende structure with no secondary phase. The elemental Cu/I ratio is 1.09:1. The melting point of the crystal is 875 K and two phase transitions occur from room temperature to its melting point. The electrical conductivity of CuI platelet crystal is in the range of 1.11-2.38 Ω⁻¹ cm⁻¹. Under ultraviolet excitation, the CuI crystals exhibit three emission bands with peak positions at 426, 529 and 671 nm. The nature of the luminescence is discussed.     

Catalytic effect of halide additives ball milled with magnesium hydride

The influence of various halide additives milled with magnesium hydride (MgH₂) on its decomposition temperature was studied. The optimum amount of halide additive and milling conditions were evaluated.