Structure of boron-oxygen groups in crystalline,molten, and glassy alkali-metal and alkaline-earth metaborates

The structure of boron-oxygen groups in the Li₂O · B₂O₃, Na₂O · B₂O₃, Cs₂O · B₂O₃, and BaO · B₂O₃ metaborates in different states of aggregation has been studied by conventional and high-temperature Raman spectroscopy. The structural changes that occur in these groups in going from the crystalline state to the liquid and glassy states depend on the nature of the cation. In the liquid state, the metaborates contain boron-oxygen groups in the form of [BO₂]^? monomeric triatomic anions.     

Influence of repeller potential on ion-molecule reactions in methane and carbon tetrafluoride mixtures

Investigations of ion-molecule reactions were performed for methane-carbon tetrafluoride mixtures of different compositions. Concentration of methane in these mixtures ranged from 10% to 90% (at 10% increment) and total mixture pressure was fixed at 13.3 Pa. Measurements were made using a quadrupole mass spectrometer with a high-pressure ion source. Primary ions CH₄⁺, F⁺, CF⁺ and CF₃⁺ were produced by electrons with energy of 300 eV. Secondary ions CH₅⁺, C₂H₃⁺, C₂H₄⁺, C₂H₅⁺, C₂H₇⁺ and CF₃⁺ were observed as the result of ion-molecule reactions.The influence of repeller potential on ion-molecule reactions efficiency was examined.     

Mechanical properties of and cracks and wrinkles in vacuum-deposited MgF2, carbon and baron coatings

The mechanical properties of coatings of MgF₂, carbon and boron were investigated. The internal stress in the MgF₂ coatings on glass substrates, measured by the cantilever method, was tensile and was about 4×10⁹dyncm^?2, while those of carbon and boron were compressive and were about 7×10⁹dyncm^?2 and 2×10⁹dyncm^?2 respectively. Young's moduli of MgF₂ and carbon were determined by the vibrating reed method to be 15×10¹¹dyncm^?2 and 2×10¹¹dyncm^?2 respectively. Cracks were observed in sufficiently thick MgF₂ coatings on glass substrates and the adhesion energy was estimated to be 270 erg cm^?2 from the measured values of the internal stress, Young's modulus and crack initiation thickness. Wrinkles were generated under certain conditions in carbon and boron coatings. It was found that exposure of the coatings to the atmosphere was essential to the initiation of the wrinkles.     

Effect of soil compositions on the electrochemical corrosion behavior of carbon steel in simulated soil solution

In this study, effect of cations, Ca²⁺, Mg²⁺, K⁺, and anions, SO₄^2–, HCO₃^–, NO₃^– on electrochemical corrosion behavior of carbon steel in simulated soil solution was investigated through potentiodynamic polarization curves and electrochemical impedance spectroscopy. The results indicate that the Ca²⁺and Mg²⁺ can decrease the corrosion current density of carbon steel in simulated soil solution, and K⁺, SO₄^2–, HCO₃^–, and NO₃^– can increase the corrosion density. All the above ions in the simulated soil solution can decrease its resistivity, but they have different effect on the charge transfer resistivity. This finding can be useful in evaluating the corrosivity of certain soil through chemical analysis, and provide data for construction engineers.     

New ternary oxides of Re,Os, Ir and Pt with cubic crystal structures

A body-centered cubic structure related to the cubic KSbO₃ strcture has been found for phases with compositions close to Ba_0.5IrO₃, Sr_0.5IrO₃, Ba_0.5OsO₃, Sr_0.5OsO₃, and Sr_0.5ReO₃. A primitive cubic structure also related to the cubic KSbO₃ structure has been found for phases with compositions close to NaOsO₃, BiPtO_3.5, and BiOsO_3.5. The ternary oxides Cd₂Ir₂O₇, Ca₂Ir₂O₇, and Ca_1.7Os₂O₇ have been prepared with the cubic pyrochlore structure. All iridium phases contain Ir⁵⁺ and are the first oxide examples of iridium in an oxidation state higher than four.     

Thermally stable luminescence and energy transfer in Ce, Mn doped Sr2Mg(BO3)2 phosphor

Sr₂Mg(BO₃)₂:Ce³⁺,Li⁺ and Sr₂Mg(BO₃)₂:Ce³⁺,Li⁺,Mn²⁺ phosphors have been synthesized by conventional solid state reaction technology at 900 °C for 12 h in reducing atmosphere. The phase purity, photoluminescence (PL) properties, thermal stability, energy transfer and luminescent decay curves have been investigated. Sr₂Mg(BO₃)₂:Ce³⁺,Li⁺,Mn²⁺ phosphors show blue and deep-red¹ emission bands. The deep-red emission band is attributed to the energy transfer from Ce³⁺ to Mn²⁺. The fluorescence lifetimes of Ce³⁺ in co-doped sample are shorter than that in single doped one, which confirms that the energy transfer takes place. The phosphors have weak thermal quenching. The luminescence properties of Sr₂Mg(BO₃)₂:Ce³⁺,Li⁺,Mn²⁺ make the phosphor a new bicolor emitting material.     

Synthesis,characterization, theoretical investigation,and properties of monoclinic-phase InWO<Subscript>4</Subscript> hollow nanospheres

As a newly discovered member of the tungstate family, InWO₄ hollow nanospheres with a monoclinic wolframite structure were synthesized successfully. The crystal phase of InWO₄ was investigated via a combination of CASTEP geometric optimization and experimental simulation. InWO₄ has a space group of P2/c with two InWO₄ formula units per unit cell. The optimized cell dimensions are a = 5.16 Å, b = 5.97 Å, and c = 5.23 Å, with α = 90°, β = 92.11°, γ = 90°, giving a unit cell volume of 161.10 ų, which is consistent with the experimental measurements. More importantly, InWO₄ was a promising host material for different Ln³⁺ (Ln = Eu and Yb/Er) ions. For InWO₄:Yb³⁺/Er³⁺ excited at 980 nm, transitions from the ⁴G_11/2 (384 nm), ²H_9/2 (411 nm), and ⁴F_7/2 (487 nm) levels to the ground state (⁴I_15/2) of Er³⁺ were observed. In addition to the aforementioned properties, the InWO₄ hollow nanospheres can be used to improve the performance of dye-sensitized solar cells, which is chiefly attributed to theirlight scattering.

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Preparation and characterization of transition metal-modified lacunary Keggin 11-tungstophosphates supported on carbon

Transition metal-modified lacunary Keggin heteropolycompounds [PW₁₁O₃₉M]⁵⁻, with M = Ni²⁺, Co²⁺, Cu²⁺, or Zn²⁺, were synthesized and characterized. These heteropolycompounds were used to prepare the carbon-supported catalysts. Both the heteropolycompounds and the carbon-supported catalysts showed FT-IR spectra with the characteristic bands of these compounds and provided an indirect measure of the interaction strength between M and the oxygen of the central tetrahedron group of the Keggin structure. The anion decomposition of the modified compounds takes place at equal or lower temperature than that of the bulk and supported parent lacunary [PW₁₁O₃₉]⁷⁻ anion. The species present on the support surface may be highly dispersed as a non-crystalline form, as a result of the interaction with the support surface groups. The activity of the synthesized catalysts in isopropanol dehydration, reflecting their acidity, increases as the reduction temperature of the decomposition products of the heteropolycompounds decreases. It may be assumed that the generation of acid sites is a result of the interaction of hydrogen, donated by isopropanol during its decomposition, with the M cation, leading to M reduction and protons.     

Coordination of Ti4+ and Ge4+ ions in Na2O-TiO2-GeO2 glasses—an approach based on crystallization behaviour,X-ray absorption and IR spectroscopy

The coordination state of Ti⁴⁺ and Ge⁴⁺ ions in Na₂O-TiO₂-GeO₂ glasses has been investigated. Upon crystallization of the glasses, the thermodynamically metastable hexagonal GeO₂ crystals containing 4-fold coordinated Ti⁴⁺ ions together with stable Na₄Ge₉O₂₀ crystals containing 6-fold coordinated Ti⁴⁺ ions were precipitated. The distribution of Ti⁴⁺ ions between these two crystalline phases is a reflection of the coordination state of Ti⁴⁺ ions in the starting glass. The majority of Ti⁴⁺ ions are in 6-fold coordination at a small TiO₂/Na₂O ratio and they are contained in Na₂Ge₉O₂₀, while the majority of Ti⁴⁺ ions are in 4-fold coordination at a large TiO₂/Na₂O ratio and they are contained in GeO₂ crystals. Concentrations of 4- and 6-fold coordinated Ti⁴⁺ ions have been confirmed to be similar to that in Na₂O-TiO₂-SiO₂ glasses as measured by the X-ray absorption spectrometry of Ti⁴⁺ ions. The coordination state of Ge⁴⁺ ions in the glasses have been examined by infrared (IR) spectrometry. It is found that in Na₂O-TiO₂-GeO₂ glasses Ge⁴⁺ ions have 6-fold coordination preferentially, while Ti⁴⁺ ions have 4-fold coordination except at small contents of TiO₂, indicating that the addition of TiO₂ to Na₂O-GeO₂ glasses leads to the replacement of 4-fold coordinated Ge⁴⁺ ions by 4-fold coordinated Ti⁴⁺ ions.     

Mass Spectrometric Study of Photoionization. I. Apparatus and Initial Observations on Acetylene,Acetylene-d2, Benzene,and Benzene-d6

A windowless vacuum ultraviolet monochromator and mass spectrometer are combined for the study of photoionization processes in the energy range 2000 to 600 A (6 to 21 eV). Details of the apparatus and techniques of operation are given and results are reported for an initial study of acetylene, acetylene-d₂, benzene, and benzene-d₆. Ionization energies of 11.406 and 11.416 eV are obtained for the 1π_u electron of C₂H₂ and C₂D₂, respectively. Vibrational levels of the ground state of the ion are observed with quantum intervals of 1855 cm⁻¹(C₂H₂) and 1775 cm⁻¹(C₂D₂). Ionization energies for the e_1g(π) electron of C₆H₆ and C₆D₆ are determined to be 9.24₂ and 9.24₅ eV, respectively. Quantum intervals for vibrational levels of the ground state ions are apparently equal for the two isotopic molecules and estimated to be 800 cm⁻¹. A second onset of ionization is observed at 11.5₃ eV for C₆H₆ and at 11.5₉ eV for C₆D₆. Results agree well with spectroscopic data.     

Oxidation of carbon monoxide over Cu- and Ag-NaY catalysts with aqueous hydrogen peroxide

A novel oxidation reaction of CO with aqueous H₂O₂ over Cu-NaY (2-15 wt%) and Ag-NaY (5-15 wt%) catalysts has been achieved at low temperatures (55-70 °C) using a flow mode system. The employed catalysts were prepared by the incipient wetness impregnation of NaY zeolite (Si/Al = 5.6, surface area = 910 m²/g) with an aqueous solution of known concentrations of copper acetate and silver nitrate. Solids were subjected to thermal treatment at 300-450 °C prior to catalytic measurements unless subjected to subsequent reduction with hydrogen at 350 °C. The physicochemical characterization of the catalysts was probed using X-ray diffraction (XRD), FT-IR and combined thermal analyses TGA-DrTGA. The XRD data indicated that, the Ag particles have an ordered location in the sodalite cavity and the center of a single six-ring. The FT-IR data also proved the presence of a new peak at 1385 cm⁻¹ that is assigned to Ag-coordinated with the framework.A slow induced oxidation of CO (induction period, t_ind) took place at the initial stage of the CO oxidation reaction after which the reaction obeyed first-order kinetics. The utilized metal ions are proposed to be reduced to lower oxidation states such as Cu⁺ and Ag⁰ during the first period of reaction, t_ind, where the reaction proceeded favorably on such sites. Such argument was evidenced by carrying out the oxidation reaction over H₂-reduced Cu10-NaY and Ag10-NaY catalysts. The reduction caused a decrease in the t_ind, giving an evidence that the lower oxidation states Cu⁺ and Ag⁰ are the active sites in the studied oxidation reaction. The enhancement in catalytic activity was interpreted in terms of the facile adsorption of CO on the low oxidation state species.     

Preparation and characterization of some single,mixed and doped crystals and instrumentation aspects

The preparation and characterization (salient ones) of KAl (SO₄)₂. 12H₂O, KCr (SO₄)₂·12H₂O, mixed crystals of both with 10 to 90% of each component, mixed crystals of CsCl with CuCl₂, doped crystals of KBr with K₃FeCN₆, mixed crystals (NH₄)₂SO₄ with CuSO₄ or NiSO₄, NaCl with growth improver Pb⁺², Mn⁺², metallic crystals of Zn, Bi, ionic crystals of alkali halides with Pb⁺², or Cd⁺², etc. are presented. Instrumentation aspects of a rotary crystallizer, a homogeniser, an ingot release mechanism and a zone refiner are shown.     

Electrooptical characteristics of nanoscale and bulk long persistent phosphor SrAl2O4 : Eu,Dy

Nanoscale and bulk SrAl₂O₄ : Eu²⁺, Dy³⁺ were prepared, respectively, by auto-combustion of citrate gelatin (gel) and high temperature solid state method. The crystalline structure was examined by X-ray diffraction. The morphological and size aspects of nanoscale sample were carried out with transmission electron microscopy. Spectral shift of nanoscale phosphor was confirmed by measuring excitation and emission spectra. The current-voltage characteristic and optical power were measured with a Keithley source meter 2410. Ionic conductivity plays the leading role when the sample is not irradiated by light and electron conductivity should be in the charge of the current gain when the sample is irradiated by light. The result showed that light irradiation simultaneously increased the luminescence intensity and the current. The current increasing range of nanoscale phosphor is smaller than that of bulk phosphor. It was further concluded that the excited state energy levels of emission centre overlap partly with the conduction band of host crystal. Otherwise, surface effect was analysed deeply by comparing the decay curves of nanoscale and bulk SrAl₂O₄ : Eu²⁺, Dy³⁺.     

Persistent luminescence of Eu and Dy doped barium aluminate (BaAl2O4:Eu,Dy) materials

Polycrystalline Eu²⁺ and Dy³⁺ doped barium aluminate materials, BaAl₂O₄:Eu²⁺,Dy³⁺, were prepared with solid state reactions at temperatures between 700 and 1500 °C. The influence of the thermal treatments on the stability, homogeneity and structure as well as to the UV-excited and persistent luminescence of the materials was investigated by X-ray powder diffraction, SEM imaging and infrared spectroscopies as well as by steady state luminescence spectroscopy and persistent luminescence decay curves, respectively. The IR spectra of the materials prepared at 250, 700, and 1500 °C follow the formation of BaAl₂O₄ composition whereas the X-ray powder diffraction of compounds revealed how the hexagonal structure was obtained. The morphology of the materials at high temperatures indicated important aggregation due to sintering. The luminescence decay of the quite narrow Eu²⁺ band at ca. 500 nm shows the presence of persistent luminescence after UV irradiation. The dopant (Eu²⁺) and co-dopant (Dy³⁺) concentrations affect the crystallinity and luminescence properties of the materials.     

Ru Doping of Perovskite Manganites: An Effective Route to Ferromagnetism, Metallicity, and CMR

The doping of Mn sites by Ru in manganites has been studied. It is shown that, whatever the nature of the antiferromagnetic data of the undoped perovskite, CE or pseudo-CE type, like for Ln_0.5Ca_0.5MnO₃ and Ln_0.4Ca_0.6MnO₃, or A type as for Pr_0.5Sr_0.5MnO₃, or C type as for Sm_0.2Ca_0.8MnO₃, or G type as for CaMnO₃, ferromagnetism and metallicity can be induced by Ru doping in a very effective manner, leading to colossal magnetoresistance (CMR) properties. Thus a dramatic modification of the magnetic phase diagrams can be obtained by Ru doping, as shown for the Sm_{1 – y} Ca_y Mn_0.88Ru_0.12O₃ system where the T_C of the ferromagnetic insulating (FMI) region (0.12 y 0.40) has been increased significantly by Ru doping. But more important, the charge-ordered state of the electron rich region (0.40 y < 0.80) has been spectacularly replaced by a ferromagnetic metallic (FMM) state with a very high T_C, up to 235 K by Ru substitution. The peculiar nature of the so obtained FMM state is shown to be related to the structural phase separation. This particular efficiency of ruthenium, compared to chromium, is discussed in terms of not only valency effect due to the presence of Ru⁵⁺ that increases the Mn³⁺ content, but also valency fluctuation between Ru⁵⁺ and Ru⁴⁺, and in terms of strong ferromagnetic superexchange interactions between ruthenium and manganese, due to orbital hybridization, so that Ru⁴⁺ and Ru⁵⁺ participate in the band formation.     

Atomic carbon in magnesium oxide Part VIII: General discussion and outlook

The presence of atomic carbon in MgO and the experimentally well documented fact that an internal equilibrium exists between defect-bound OH^? and H + O^? lead to a very complex pattern of defect equilibria and defect reactions in the bulk and at the surface of the MgO crystals. The most important extrinsic bulk defects are the bare and the OH-compensated cation vacancies, [V″_$\text{Mg}$]″, |OH^· V″_$\text{Mg}$|′, and [OH^· V″_$\text{Mg}$ HO^·]^$\text{x}$ of which the latter yields [O^· (H_$\text{2}$)″_$\text{Mg}$ O^·]^$\text{x}$, interstitial hydrogen molecules (H_$\text{2}$)^$\text{x}$, interstitial C atoms C^x_i, which may enter the available cation vacancies, and the positive hole, [O^·]^?. Reactions between C atoms entering the O V″ O and [O^· V″_$\text{Mg}$|′ at the surface lead to the formation of CO_$\text{2}$ and CO molecules and to the formation of double F and single F centers respectively. The free electrons in the F centers are held responsible for the formation of Mg-organic commpounds. Reaction between the C atoms and interstitial H₂ molecules lead to hydrocarbon formation. The strain fields associated with the interstitial C atoms and the positive holes provide the driving force for segregation of the carbon into the elastically relaxed subsurface zone and for fluctuating reaction kinetics.     

Effect of different acid oxidation on morphology,dispersion and optical band-gap of multi-walled carbon nanotubes

In this paper the effect of different acid oxidation on morphology, dispersion and optical band gap of multi-walled carbon nanotubes (CNTs) is reported. Oxidation of CVD synthesized MWCNTs were carried out in 8M HNO₃, 8M H₂SO₄, 8M HNO₃/H₂O₂ and 8M H₂SO₄/HNO₃. Oxidized nanotubes sample were characterized by XRD, SEM, FT-IR, TGA, UV-Vis spectroscopy and Raman analysis. Oxidized-nanotubes show a lower shift in XRD peak and an increase in d-spacing, which implies deterioration of MWNTs. New peaks in FT-IR spectra of oxidized-nanotube samples at around 1742 cm^?1 confirms the presence of carbonyl groups. Optical bandgap, molar absorptivity coefficient is estimated for pristine and oxidized nanotubes sample using UV-Vis spectroscopy data and Tauc plot. It is observed that optical bandgap decreases on oxidation and lowest bandgap 2.92 eV is observed for H₂SO₄ treated MWNTs. The information on the optical bandgap of the MWNTs is of great importance for the development of optoelectronic devices.     

Effect of carbon monoxide and sulphur dioxide on the oxidation state of copper in zeolite

A study is made of the reduction and oxidation of Cu²⁺ in CuNaY zeolites by CO and SO₂, using electron spin resonance. It is observed that the Cu⁺ or Cu^o formed by the reduction of Cu²⁺ with CO can be reoxidized to Cu²⁺ by SO₂ at 400°C. SO₂ can also reduce a fraction of the Cu²⁺ sites in the Cu-exchanged zeolites. SO₂ decomposes and forms elemental S under experimental conditions on the zeolite.     

Electronic surface state of TiO2 electrode doped with transition metals, studied with cluster model and DV-Xα method

The surface electronic structure of rutile TiO₂ used for a photo-catalyst and an electrode of the dye-sensitized solar cell, was calculated by the DV-Xα method. The electrical properties of mesoporous TiO₂ electrode are dependent on the surface levels introduced by a high surface area. In this study, the electronic state of bulk TiO₂ structure was calculated using the (Ti₁₅O₅₆)⁵²⁻ model and the surface state of (1 1 0) plane was calculated using the (Ti₁₁O₃₄)²⁴⁻–O^* (O^*=surface oxygen) model, respectively. In addition to the surface state of pure TiO₂, the surface dopant levels introduced by the doping of transition metals in Ti⁴⁺ ion site were also calculated using the MTi₁₁O₃₄–O^* (M=transition metal) model. With calculated results, the variation of surface levels including dopant levels was discussed in association with electrode properties.     

Crystal structure, magnetic and thermal properties of LaFeTeO6

LaFeTeO₆ was prepared by solid state reaction of La₂O₃, Fe₂O₃ and TeO₂ in 1:1:2 molar ratios and characterized by powder X-ray diffraction, thermogravimetry and magnetometry. The detailed crystal structure analysis was carried out by Rietveld refinement. LaFeTeO₆ crystallizes in a trigonal lattice with unit cell parameters: a = 5.2049(1) Å and c = 10.3457(2) Å, V = 242.73(2) Å³. The crystal structure is built from sheets of the edge shared FeO₆ and TeO₆ octahedra stacked along the c-axis. These sheets are connected together by La³⁺ ions. Thermogravimetric analysis of the compound showed it to be thermally stable up to 1323 K and continuous loss of TeO₂ was observed above 1323 K leading to the formation of LaFeO₃. High temperature XRD studies revealed a normal expansion behavior of the compound. Temperature and field dependent magnetization of LaFeTeO₆ showed paramagnetic behavior in the temperature range of 3-300 K. The effective magnetic moment per Fe³⁺ ion (5.14 μB) indicates the high spin d⁵ state of Fe³⁺ ion.