Preparation and optical properties of Ce activated (Ca1 − x,Srx)Al2Si2O8 phosphors

(Ca_1 − x, Sr_x)Al₂Si₂O₈:0.06Ce³⁺, M⁺ (M⁺ = Li⁺, Na⁺, K⁺) phosphors have been prepared by conventional solid-state reaction method. The structural and optical properties of the phosphors were characterized by X-ray diffraction (XRD) technique and spectrophotometer, respectively. A regular variation was found among the XRD patterns of (Ca_1 − x, Sr_x)Al₂Si₂O₈:0.06Ce³⁺ phosphors based on the changing of Sr content. With the increase of Sr content, the maximum of emission band presented slight blue shifts (~ 15 nm). The luminescence intensity of CaAl₂Si₂O₈:0.06Ce³⁺ and SrAl₂Si₂O₈:0.06Ce³⁺ were significantly enhanced when K⁺ and Li⁺ were incorporated, respectively.     

Growth and characterization of Nd:Lu3ScxGa5 − xO12 series laser crystals

A new series Nd:Lu₃Sc_xGa_5 − xO₁₂ (x = 0.5, 0.8, 1, 1.2 and 1.5) laser crystals have been successfully grown by the optical floating zone method. Their absorption and luminescence spectra were measured at room temperature and spectral parameters were systemically calculated using Judd-Ofelt (JO) theory. The fluorescence τ_f lifetimes were experimentally measured and compared with the theoretical results. Diode-pumped continuous-wave (CW) laser performance at 1.06 μm with mixed crystals was demonstrated. The influence of different x values on laser performance and spectral parameters was also discussed. All the results show that Nd:Lu₃Sc_xGa_5 − xO₁₂ series crystals should be suitable for laser application.     

Influence of substitution on dielectric and impedance spectroscopy of Sr1 − xBi2 + yNb2O9 ferroelectric ceramics synthesized by chemical route

Nanocrystalline powders of Sr_1 − xBi_2 + yNb₂O₉ (SBN, x = 0.0, 0.1, 0.2, 0.3, and 0.4; y = 0, 0.066, 0.133, 0.200, and 0.266) were prepared by aqueous solution method using water-soluble Sr-EDTA, Bi-EDTA and Nb-tartarate as the starting materials. XRD showed that the samples were free from fluorite or pyrochlore phase within heat-treatment temperature from 550 to 600 °C. Average crystallite size and particle diameter were observed to be between 10 and 25 nm, which were analyzed through XRD and TEM, respectively. Bi-substitution has substantially improved the sinterability of SBN and enabled to achieve high density (96%), which was otherwise difficult in the case of pure SBN. The dielectric properties of SBN ceramics were significantly enhanced by the partial replacement of Sr²⁺ ions by the trivalent bismuth ions. The complex impedance diagrams of Bi-substituted SBN, x = 0.4 ceramics exhibited only one semicircle indicating a significant contribution from the grains. In contrast, the impedance plots for pure and other substituted SBN ceramics show an additional low-frequency semicircle, which was attributed to the blocker size effects. The dielectric behavior of pure and Bi-substituted SBN ceramics was rationalized using the impedance and modulus data.     

Adsorption of CO, NH3 and O2 on Fe site of La0.875Sr0.125FeO3 (0 1 0) surface by density function theory calculation

The adsorption of CO, NH₃ and O₂ gas molecules on Fe site of La_0.875Sr_0.125FeO₃ (0 1 0) surface has been investigated based on the density functional theory (DFT) with the spin polarized generalized gradient approximation (GGA). The optimal adsorption orientations as well as the adsorption energies for these molecules adsorption on Fe site of La_0.875Sr_0.125FeO₃ (0 1 0) surface are derived. Results show that adsorption configurations with C-down for CO, N-down for NH₃ are stable. For the O₂ molecule adsorption, the mode with an angle about 120° between the oxygen molecule and La_0.875Sr_0.125FeO₃ (0 1 0) surface is stable.     

Electrochemical properties of thin TiO2 electrode on Li1 + xAlxGe2 − x(PO4)3 solid electrolyte

A fabrication of all-solid-state thin-film rechargeable lithium ion batteries by sol-gel method is expected to achieve both the simplification and cost reduction for fabrication process. TiO₂ thin film electrode was prepared by PVP (polyvinylpyrrolidone) sol-gel method combined with spin-coating on Li_1 + xAl_xGe_2 − x(PO₄)₃ (LAGP) solid electrolyte which has wide electrochemical window. The thin film was composed of anatase TiO₂ that is the most active phase for Li insertion and extraction and contacted well with LAGP substrate. In the cyclic voltammogram, a redox couple was observed at 1.8 V vs. Li/Li⁺ assigned to Li insertion/extraction into/from anatase TiO₂, indicating that the thin film worked as electrode for lithium battery. The charge and discharge test in various charge and discharge rates revealed that the discharge process (delithiation) is thought to be faster than charge process (lithiation). It is attested that the sol-gel process, which derives both simplification and cost reduction for fabrication process, can be applied to thin film battery using LAGP solid electrolyte.     

Determination of the rate of H + O2 + M → HO2 + M (M = N2, Ar, H2O) from ignition of syngas at practical conditions

In H₂ and H₂/CO oxidation, the H + O₂ + M termination step is one of the most important reactions at elevated pressures. With the recent, increased interest in synthetic fuels, an accurate assessment of its rate coefficient becomes increasingly important, especially for real fuel/air mixtures. Ignition delay times in shock-tube experiments at the conditions selected in this study are only sensitive to the rates of the title reaction and the branching reaction H + O₂ = OH + O, the rate of which is known to a high level of accuracy. The rate coefficient of the title reaction for M = N₂, Ar, and H₂O was determined by adjusting its value in a detailed chemical kinetics model to match ignition delay times for H₂/CO/O₂/N₂, H₂/CO/O₂/Ar, and H₂/CO/O₂/N₂/H₂O mixtures with fuel/air equivalence ratios of ? = 0.5, 0.9, and 1.0. The rate of H + O₂ + N₂ = HO₂ + N₂ was measured to be 2.7 (−0.7/+0.8) × 10¹⁵ cm⁶/mol² s for T = 916-1265 K and P = 1-17 atm. The present determination agrees well with the recent study of Bates et al. [R.W. Bates, D.M. Golden, R.K. Hanson, C.T. Bowman, Phys. Chem. Chem. Phys. 3 (2001) 2337-2342], whose rate expressions are suggested herein for modeling the falloff regime. The rate of H + O₂ + Ar = HO₂ + Ar was measured to be 1.9 × 10¹⁵ cm⁶/mol² s for T = 932-965 K and P = 1.4 atm. The rate of H + O₂ + H₂O = HO₂ + H₂O was measured to be 3.3 × 10¹⁶ cm⁶/mol² s for T = 1071-1161 K and P = 1.3 atm. These are the first experimental measurements of the rates of the title reactions in practical combustion fuel/air mixtures.     

Phase and electrical behaviour in the Bi14W1 − xLaxO24 − 3x/2 system

The compositional and thermal dependencies of phase and electrical behaviour of compositions in the system Bi₁₄W_1 − xLa_xO_{24 − 3x/2} (0.00 < x < 1.00) have been studied by X-ray powder diffraction, differential thermal analysis and a.c. impedance spectroscopy. The system exhibits polymorphism and phase separation, which shows both compositional and thermal dependence. Compositions with x = 0.25 and x = 0.50 exhibit a single phase tetragonal structure at room temperature. In contrast, the x = 0.75 composition at room temperature shows a mixture of a cubic phase and a secondary β-Bi₂O₃ related tetragonal phase. A full solid solution is observed at high temperatures, corresponding to the occurrence of a δ-Bi₂O₃ type phase. The appearance of the various phases correlates well with the observed electrical behaviour. The x = 0.75 composition exhibits exceptionally high conductivity at high temperatures (σ₈₀₀ = 1.34 S cm^− 1), but also shows significant phase separation at lower temperatures.     

The ν1 + 3ν2 + 3ν3 and 4ν1 + ν2 + ν3 bands of ozone by CW-cavity ring down spectroscopy between 5900 and 5960 cm

The absorption spectrum of ozone, ¹⁶O₃, has been recorded in the 5903-5960 cm⁻¹ region by high sensitivity CW-cavity ring down spectroscopy (α_min ∼ 5 × 10⁻¹⁰ cm⁻¹). The ν₁ + 3ν₂ + 3ν₃ and 4ν₁ + ν₂ + ν₃ A-type bands centred at 5919.15 and 5947.07 cm⁻¹ were newly observed. A set of 173 and 168 energy levels could be experimentally determined for the (1 3 3) and (4 1 1) states, respectively. Except for a few K_a = 5 levels of the (4 1 1) state, the rotational structure of the two states was found mostly unperturbed. The spectroscopic parameters were determined from a fit of the corresponding line positions by considering the (1 3 3) and (4 1 1) states as isolated. The determined effective Hamiltonian and transition moment operators were used to generate a list of 785 transitions given as Supplementary Material.     

Photoluminescence investigation of ferromagnetic Ga1−xMnxN layers with GaN templates grown on sapphire (0 0 0 1) substrates

We have investigated the temperature and composition dependent photoluminescence (PL) spectra in Ga_1−xMn_xN layers (where x ≈ 0.1-0.8%) grown on sapphire (0 0 0 1) substrates using the plasma-enhanced molecular beam epitaxy technique. The efficient PL is peaked in the red (1.86 eV), yellow (2.34 eV), and blue (3.29 eV) spectral range. The band-gap energy of the Ga_1−xMn_xN layers decreased with increasing temperature and manganese composition. The band-gap energy of the Ga_1−xMn_xN layers was modeled by the Varshni equation and the parameters were determined to be α = 2.3 × 10⁻⁴, 2.7 × 10⁻⁴, 3.4 × 10⁻⁴ eV/K and β = 210, 210, and 230 K for the manganese composition x = 0.1%, 0.2%, and 0.8%, respectively. As the Mn concentration in the Ga_1−xMn_xN layers increased, the temperature dependence of the band-gap energy was clearly reduced.     

Hydroxylated α-Al2O3 (0 0 0 1) surfaces and metal/α-Al2O3 (0 0 0 1) interfaces

X-ray photoelectron spectroscopy was applied to study the hydroxylation of α-Al₂O₃ (0 0 0 1) surfaces and the stability of surface OH groups. The evolution of interfacial chemistry of the α-Al₂O₃ (0 0 0 1) surfaces and metal/α-Al₂O₃ (0 0 0 1) interfaces are well illustrated via modifications of the surface O1s spectra. Clean hydroxylated surfaces are obtained through water- and oxygen plasma treatment at room temperature. The surface OH groups of the hydroxylated surface are very sensitive to electron beam illumination, Ar⁺ sputtering, UHV heating, and adsorption of reactive metals. The transformation of a hydroxylated surface to an Al-terminated surface occurs by high temperature annealing or Al deposition.     

High-temperature measurements of the rates of the reactions CH2O + Ar → Products and CH2O + O2 → Products

The two-channel thermal decomposition of formaldehyde [CH₂O], (1a) CH₂O + Ar → HCO + H + Ar, and (1b) CH₂O + Ar → H₂ + CO + Ar, was studied in shock tube experiments in the 2258-2687 K temperature range, at an average total pressure of 1.6 atm. OH radicals, generated on shock heating trioxane-O₂-Ar mixtures, were monitored behind the reflected shock front using narrow-linewidth laser absorption. 1,3,5 trioxane [C₃H₆O₃] was used as the CH₂O precursor in the current experiments. H-atoms formed upon CH₂O and HCO decomposition rapidly react with O₂ to produce OH via H + O₂ → O + OH. The recorded OH time-histories show dominant sensitivity to the formaldehyde decomposition pathways. The second-order reaction rate coefficients were inferred by matching measured and modeled OH profiles behind the reflected shock. Two-parameter fits for k_1a and k_1b, applicable in this temperature range, are:

     

Growth of BaxSr1−xSO4 nano-steps on barite (0 0 1) face

Crystal growth has been promoted in the fluid cell of an Atomic Force Microscope (AFM) by passing Ba-Sr-SO₄ aqueous solutions over barite (0 0 1) cleavage surfaces. Steps advance in structural continuity with the original barite (0 0 1) surfaces and two-dimensional nucleation occurs preferentially on the newly-formed terraces. The terraces are, on average, 7.5% lower than pure barite terraces. Since the ionic radius of Sr²⁺ is smaller that the ionic radius of Ba²⁺, the reduction of terrace height is consistent with an extensive incorporation of Sr²⁺ into the barite structure. Therefore, it can be considered that the newly-formed terraces have compositions corresponding to terms of the Ba_xSr_1−xSO₄ solid solution. A non-linear dependence of step rate on [SrSO₄] concentration in the solution (and therefore on supersaturation) has been found. The growth behaviour has been discussed by considering both the physicochemical properties of the Ba_xSr_1−xSO₄ solid solution-aqueous solution (SS-AS) system and a kinetic-based step growth model.     

Effect of a LaSrCoO3 buffer layer on Pb1 − xLaxTi1 − x/4O3 films studied by polarized Raman spectroscopy

A confocal Raman investigation of Pb_1 − xLa_xTi_1 − x/4O₃ (PLT) thin films grown by RF magnetron sputtering on PbO_x/Pt/Ti/SiO₂/Si substrates with an intermediate LaSrCoO₃ (LSCO) layer was performed. The influence of the LaSrCoO₃ buffer layer was analyzed taking advantage of the observed Raman spectral band variation, which varied according to different manufacturing procedures. In the presence of a LSCO layer, the A1(1TO) Raman mode, which was indicative of tetragonal distortion, was pronouncedly enhanced, and a slight deviation from the (0 0 1) plane of the film was observed from the angular dependence of the polarized Raman spectral intensity. Furthermore, the spectral band variation as well as the residual stress along the in-depth direction was measured in the film from cross-sectional spectral line scans. This latter measurement showed a relaxation of the lattice mismatch in the presence of LSCO and PbO layers.     

Epitaxial MoOx nanostructures on TiO2(1 1 0) obtained using thermal decomposition of Mo(CO)6

Stoichiometric and highly-defective TiO₂(1 1 0) surfaces (called as yellow and blue, respectively) were exposed to Mo(CO)₆ vapours in UHV and in a reactive O₂ atmosphere. In the case of yellow-TiO₂, an O₂ reactive atmosphere was necessary to obtain the Mo(CO)₆ decomposition at 450 °C with deposition of MoO_x nanostructures where, according to core level photoemission data, the Mo⁺⁴ state is predominant. In the case of blue-TiO₂ it was possible to obtain Mo deposition both in UHV and in an O₂ atmosphere. A high dose of Mo(CO)₆ in UHV on blue-TiO₂ allowed the deposition of a thick metallic Mo layer. An air treatment of this sample at 580 °C led to the elimination of Mo as MoO₃ and to the formation of a transformed layer of stoichiometry of Ti_(1−x)Mo_xO₂ (where x is close to 0.1) which, according to photoelectron diffraction data, can be described as a substitutional near-surface alloy, where Mo⁺⁴ ions are embedded into the titania lattice. This embedding procedure results in a stabilization of the Mo⁺⁴ ions, which are capable to survive to air exposure for a rather long period of time. After exposure of the blue-TiO₂(1 1 0) substrate to Mo(CO)₆ vapours at 450 °C in an O₂ atmosphere it was possible to obtain a MoO₂ epitaxial ultrathin layer, whose photoelectron diffraction data demonstrate that is pseudomorphic to the substrate.     

Local structures and transport properties of K ions in a hollandite superstructure KxMg(8 + x)/3Sb(16 − x)/3O16

Possible local structures of K ions in the tunnel of a hollandite superstructure K_xMg_(8 + x)/3Sb_{(16 − x)/3}O₁₆ (x ≈ 1.76) were presented, and the validity of the models was confirmed by the structure refinement using single-crystal X-ray diffraction data. Additional constraint conditions were introduced in refinements so that the average structure obtained from the refinement is consistent with assumed microscopic pictures. Joint-probability density functions were calculated for specific K ions concerning the hopping process between neighboring cavities, and converted to the one-particle potentials. Three types of the energy barrier for the hopping process were seen, which were uniformly reduced applying the anharmonic atomic displacement parameters.     

Superconductivity recurrence and charge carrier compensation effect in La2−xSrxCu0.94M0.06O4 (M = Fe,Co and Ni) system

In this paper, the substitution effect of Sr for La in three non-superconducting system La_2−xSr_xCu_0.94M_0.06O₄ (M = Fe, Co and Ni) have been systematically studied. As the concentration of Sr increases to around 0.20, the superconductivity is recovered. The peak at 695 cm⁻¹ in the infrared spectra which is assigned to the stretching mode of oxygen atoms in plane induced by M³⁺ doping is suppressed by Sr increasing, demonstrating that the holes introduced by the Sr substitution have been injected into the CuO₂ plane and compensate the localized electrons. These results suggest that the superconductivity recurrence is caused by the carrier compensation. The main origin of superconductivity suppression by the magnetic ions Fe/Co/Ni is the carrier localization effect.     

Effect of Fe substitution on magnetocaloric effect in La0.7Sr0.3Mn1−xFexO3 (0.05≤x≤0.20)

We have studied the effect of Fe substitution on magnetic and magnetocaloric properties in La_0.7Sr_0.3Mn_1−xFe_xO₃ (x=0.05, 0.07, 0.10, 0.15, and 0.20) over a wide temperature range (T=10-400 K). It is shown that substitution by Fe gradually decreases the ferromagnetic Curie temperature (T_C) and saturation magnetization up to x=0.15 but a dramatic change occurs for x=0.2. The x=0.2 sample can be considered as a phase separated compound in which both short-range ordered ferromagnetic and antiferromagnetic phases coexist. The magnetic entropy change (−ΔS_m) was estimated from isothermal magnetization curves and it decreases with increase of Fe content from 4.4 J kg⁻¹ K⁻¹ at 343 K (x=0.05) to 1.3 J kg⁻¹ K⁻¹ at 105 K (x=0.2), under ΔH=5 T. The La_0.7Sr_0.3Mn_0.93Fe_0.07O₃ sample shows negligible hysteresis loss, operating temperature range over 60 K around room temperature with refrigerant capacity of 225 J kg⁻¹, and magnetic entropy of 4 J kg⁻¹ K⁻¹ which will be an interesting compound for application in room temperature refrigeration.     

Reaction of water with Ce-Au(1 1 1) and CeOx/Au(1 1 1) surfaces: Photoemission and STM studies

This article reports photoemission and STM studies for the adsorption and dissociation of water on Ce-Au(1 1 1) alloys and CeO_x/Au(1 1 1) surfaces. In general, the adsorption of water at 300 K on disordered Ce-Au(1 1 1) alloys led to O-H bond breaking and the formation of Ce(OH)_x species. Heating to 500-600 K induced the decomposition or disproportionation of the adsorbed OH groups, with the evolution of H₂ and H₂O into gas phase and the formation of Ce₂O₃ islands on the gold substrate. The intrinsic Ce ↔ H₂O interactions were explored by depositing Ce atoms on water multilayers supported on Au(1 1 1). After adsorbing Ce on ice layers at 100 K, the admetal was oxidized immediately to yield Ce³⁺. Heating to room temperature produced finger-like islands of Ce(OH)_x on the gold substrate. The hydroxyl groups dissociated upon additional heating to 500-600 K, leaving Ce₂O₃ particles over the surface. On these systems, water was not able to fully oxidize Ce into CeO₂ under UHV conditions. A complete Ce₂O₃ → CeO₂ transformation was seen upon reaction with O₂. The particles of CeO₂ dispersed on Au(1 1 1) did not interact with water at 300 K or higher temperatures. In this respect, they exhibited the same reactivity as does a periodic CeO₂(1 1 1) surface. On the other hand, the Ce₂O₃/Au(1 1 1) and CeO_2−x/Au(1 1 1) surfaces readily dissociated H₂O at 300-500 K. These systems showed an interesting reactivity for H₂O decomposition. Water decomposed into OH groups on Ce₂O₃/Au(1 1 1) or CeO_2−x/Au(1 1 1) without completely oxidizing Ce³⁺ into Ce⁴⁺. Annealing over 500 K removed the hydroxyl groups leaving behind CeO_2−x/Au(1 1 1) surfaces. In other words, the activity of CeO_x/Au(1 1 1) for water dissociation can be easily recovered. The behavior of gold-ceria catalysts during the water-gas shift reaction is discussed in light of these results.     

Controlling metal/oxide interactions in bifunctional nanostructured model catalysts: Pd and BaO on Al2O3/NiAl(1 1 0)

We have investigated the structure and growth of Pd and BaO containing nanoparticles sequentially co-deposited on an ordered Al₂O₃/NiAl(1 1 0) by scanning tunneling microscopy (STM), and their interaction with CO and NO₂ by infrared reflection absorption spectroscopy (IRAS).Ba deposition and subsequent oxidation result in Ba_xAl_2yO_x+3y nanoparticles being formed which are characterized by high particle densities and nucleation both on regular terraces and at oxide defects. In contrast, Pd interaction with the model support is weaker and preferential nucleation occurs mainly at rotational domain boundaries and to a lesser extent at anti-phase domain boundaries. For subsequent co-deposition of Pd on preformed Ba_xAl_2yO_x+3y/Al₂O₃/NiAl(1 1 0), Pd nucleates at the Ba_xAl_2yO_x+3y nanoparticles and covers them. The reverse deposition sequence, i.e. subsequent Ba co-deposition and oxidation on preformed Pd/Al₂O₃/NiAl(1 1 0), leads to formation of small isolated Ba_xAl_2yO_x+3y nanoparticles without contact to Pd, together with large Pd crystallites modified by Ba_xAl_2yO_x+3y. The present results provide control over the degree of interaction between metal nanoparticles and oxide nanoparticles on a well-defined model catalyst and thus allow us to study effects related effects on the reactivity and catalytic behavior.     

Sr2CeO4:Eu and Sr2CeO4:5 mol% Eu, 3 mol% Dy microphosphors: Wet chemistry synthesis from hybrid precursor and photoluminescence properties

In this article, Sr₂CeO₄:x mol% Eu³⁺ and Sr₂CeO₄:5 mol%Eu³⁺, 3 mol% Dy³⁺ phosphors were synthesized from assembling hybrid precursors by wet chemical method. As-prepared samples present uniform grain-like morphology and the particle size is about 0.2 μm. The luminescence spectra of Sr₂CeO₄:x mol% Eu³⁺ have been measured to examine the influence of the intensity of red emission lines for Eu³⁺ on the concentration of Eu³⁺, showing that the intensity of the red emission increases with an increase of the concentration from 1 to 5 mol%. Additionally, from the emission spectra of Sr₂CeO₄:5 mol%Eu³⁺, 3 mol% Dy³⁺ phosphors, the characteristic lines of Dy³⁺ have also been observed. This result indicates that there also exists an energy transfer process between Sr₂CeO₄ and Dy³⁺.