Luminescence properties of La1−χTmχTa7O19

The luminescence properties of Tm³⁺ in La_1−χTm_χTa₇O₁₉ solid solutions were examined systematically. The substitution of Tm³⁺ for La³⁺ was carried out by a decomposition reaction of nitrates involving the corresponding constituents at 1200 °C in air. X-Ray diffraction patterns of the solid solutions indicated that the crystal structure consisted of a network of (La_1−χ³⁺Tm_χ^staggered|3+, Ta⁵⁺)—O²⁻ polyhedra interstratified with a double layer of Ta⁵⁺—O²⁻ polyhedra. According to the excitation and emission spectra, the most intense emission was found near 460 nm and quenched above χ=0.14 in La_1−χTm_χTa₇O₁₉. Also, lifetime results verified that the emission could be assigned not to the transition ¹G₄ å ³H₆, but to the transition ¹D₂ å³H₄. Upon cathode ray excitation some emissions of Tm³⁺ were superimposed by a broad emission due to the clusters of Ta⁵⁺—O²⁻ polyhedra. As a result, a low dimensional arrangement of Tm³⁺ was much more preferable for getting intense emission because it reduced the energy migration between Tm³⁺ ions.     

Synthesis and characterization of doped LaCrO3 perovskite prepared by EDTA–citrate complexing method

The La_0.85Sr_0.15Cr_0.95Ni_0.02Co_0.02O₃ (LSC) interconnect materials for solid oxide fuel cells (SOFCs) were synthesized by EDTA–citrate complexing method. Thermal decomposition behavior of the gel, phase formation and morphology of LSC powders were characterized by thermogravimetry/differential thermal (DSC/TG) analysis, X-ray diffraction (XRD) and scanning electron microscope (SEM), respectively. It appeared that lower pH value (pH 4.9) of the precursor solution resulted in a transient liquid phase, SrCrO₄, in the calcined LSC powder. The sintering characteristics, electrical conductivity and thermal expansion properties of sintered bars were investigated. La_0.85Sr_0.15Cr_0.95Ni_0.02Co_0.02O₃ prepared in the condition of pH 4.9 showed an electrical conductivity of 15.6 S cm⁻¹ at 800 °C and a thermal expansion coefficient (TEC) of 10.8 × 10⁻⁶ K⁻¹ (20–900 °C), which is suitable for use as interconnect materials for SOFCs.     

Luminescence based on energy transfer in xerogels doped with Ln2−xTbx(WO4)3

This paper presents preparation, optical absorption and photoluminescence properties of luminescent materials consisting of Ln_2−xTb_x(WO₄)₃ [where Ln = Gd(III) or La(III)] incorporated into silica xerogel. Photoluminescence behaviour of the salt in the rigid matrix was studied by the luminescence spectroscopy. The excitation spectra of the system Ln_2−xTb_x(WO₄)₃ show an intense broad band with a maximum placed at about 240 nm. This band is attributed to ligand–metal charge transfer (LMCT) inside the tungstate group. On the other hand, Tb³⁺ ion exhibits its characteristic emission in the material. Owing to energy transfer from the excited tungstate groups to the Tb³⁺ ions the emission intensity is improved. The energy transfer from WO₄²⁻ group to Tb(III) ion is particularly effective for such dopants as Gd_0.4Tb_1.6(WO₄)₃ or La_0.8Tb_1.2(WO₄)₃ incorporated into SiO₂ xerogel. Concentration of the emission quenchers such as water molecules and OH groups was reduced by thermal treatment. The high emission intensity and easy preparation of these systems make them potential candidates for application as luminescent materials.     

Raman active phonons of RFe3(BO3)4, R=La or Nd, single crystals

Oriented single crystals of RFe₃(BO₃)₄, with R=La or Nd, have been studied by Raman spectroscopy. Spectra with the relevant polarization configurations have been recorded in order to obtain the symmetry of the observed phonons. The factor group analysis and the correlation with the free (BO₃)³⁻ ion are done in order to identify most of the phonons associated with the two different types of (BO₃)³⁻ ion present in the crystal. The number and symmetries of the optical Raman active modes are 7A₁+19E, among which 4A₁+8E can be assigned as mostly due to (BO₃)³⁻ vibrations. 7A₁+18E modes were observed.

The highest energy peaks have been assigned to the regular planar (BO₃)³⁻ and to the three irregular (BO₃)³⁻ groups. The two lowest energy peaks of A₁ symmetry (around 180 and 300 cm⁻¹) are very probably related to the BO₃ rotatory mode and to Fe displacements. R ions do not participate in A₁ symmetry modes. The E mode around 90 cm⁻¹ (the lowest frequency mode) is probably due to the R ions which have the longest bonds and are the heaviest ions.     

The characteristics of Li(CoxNi1 − x)O2 cathode powders formed from the fine-sized Co3O4/NiO precursor powders

Li(Co_xNi_1 − x)O₂ (0 ≤ x ≤ 1) cathode powders were prepared by solid state reaction method using Co₃O₄/NiO precursor powders obtained by spray pyrolysis. The effect of the ratios of cobalt and nickel components on the characteristics of Co₃O₄/NiO precursor and Li(Co_xNi_1 − x)O₂ cathode powders were investigated. The Co₃O₄/NiO precursor powders with the ratios of cobalt and nickel components as 1/0, 0.75/0.25 and 0.5/0.5 had submicron size and regular morphologies. On the other hand, the Co₃O₄/NiO powders with the high contents of nickel component had aggregated morphologies of submicron size primary powders. The fine-sized precursor powders formed the fine-sized LiCoO₂ and Li(Co_0.75Ni_0.25)O₂ cathode powders by solid state reaction with LiOH powders. However, the high contents of the nickel component of the Co₃O₄/NiO precursor powders formed the Li(Co_xNi_1 − x)O₂ (0 ≤ x ≤ 0.5) cathode powders with aggregated morphologies and large sizes. The discharge capacities of the powders increased with increasing the nickel content into the Li(Co_xNi_1 − x)O₂ cathode powders up to 188 mAh/g.     

Electronic absorption and vibrational IR and Raman studies of binary phosphate β-K4Ce2P4O15

The binary phosphate K₄Ce₂P₄O₁₅ was prepared in the polycrystalline state in the solid state reaction of cerium oxide and potassium phosphate KPO₃. The phosphate fragment of this compound appears in the form of two PO₄³⁻ and one P₂O₇⁴⁻ anions occupying the sites of low symmetry. Electronic absorption, emission as well as infrared and Raman spectroscopic methods have been applied to characterise the properties and structure of the compound studied. Its electronic spectra agree with the Ce³⁺ ion spectroscopic characteristics. The ²F_5/2→²F_7/2 transition appears in the typical for this ion region: about 2000 cm⁻¹. The multiplet structure of the spectrum suggests the existence of at least two crystallographic different sites of this ion in the unit cell. The absorption bands in the range 25000–45000 cm⁻¹ have been assigned to the 4f¹→5d¹ transitions of the cerium ion and CT transition of the phosphate ligands. The vibrational spectra were discussed on the basis of correlation diagrams and factor group analysis.

The radiation-less mechanism of the return from the excited state to the ground state via CT states in the system studied is proposed.     

Molar enthalpies of formation of BaCmO3 and BaCfO3

The enthalpies of solution of BaCmO₃ and BaCfO₃ in 1.00 mol dm⁻³ HClO₄ were measured at 298.15 ± 0.05 K and p° = 101.325 kPa as −(345.3±4.7) and −(347.2 ± 1.9) kJ mol⁻¹, respectively. The resulting standard molar enthalpies of formation, Δ_fH_m°(BaCmO₃, cr) = −(1517.8 ± 7.1) kJ mol⁻¹ and Δ_fH_m°(BaCfO₃, cr) = −(1477.9 ± 5.6) kJ mol⁻¹, together with other corresponding experimental values for several lanthanide, actinide and transition metal complex oxides with barium and strontium, are used to estimate the molar enthalpies of formation of a number of homologous actinide compounds. The present results also provide additional information on the standard molar enthalpy of formation of CfO₂ and on the Cf⁴⁺/Cf³⁺ standard potential.     

New low thermal expansion ceramics: Sintering and thermal behavior of Ln1/3Zr2(PO4)3-based composites

Materials with the general formula M_xZr₂(PO₄)₃ are known to possess low coefficients of thermal expansion (CTE). The present work investigates the thermal properties of new composite materials issued from the decomposition at high temperature of Ln_1/3Zr₂(PO₄)₃ (Ln=La, Gd). The decomposition process was studied and showed that the resulting powder was a LnPO₄, Zr₂P₂O₉ and ZrO₂ mixture. Composite materials made of that mixture were sintered and characterized. The effect of sintering aids such as ZnO was considered. Final densities of the composites were about 90% of theoretical density and these materials presented low CTE in the 10⁻⁶ °C⁻¹ range.     

Synthesis and properties of lithium ion conductors Li3−2x(Sc1−xZrx)2(PO4)3

Lithium ion conductors, Li_3−2x(Sc_1−xZr_x)₂(PO₄)₃ (0 x 0.3), were prepared by a solid-state reaction. TG–DTA analysis indicated no phase transition in the samples with x superior to 0.05. X-ray powder diffraction analysis of these samples clearly showed the stabilization of a superionic conduction phase at room temperature with an orthorhombic system Pbcn. The highest conductivity was observed for the sample with x=0.05, and ascribed to the stabilization of the superionic conduction phase and the introduction of vacancies on the Li⁺ sites by substituting Zr⁴⁺ for Sc³.     

Red photoluminescence and morphology of Eu doped Ca3La3(BO3)5 phosphors

Europium doped phosphors Ca₃La₃(BO₃)₅ were first synthesized by a sol–gel process technique. The reaction temperature of the sol–gel process was 300 °C lower than that of the solid-state reaction and the reaction time of the sol–gel process was shorter. The photoluminescence properties of Eu³⁺ doped Ca₃La₃(BO₃)₅ indicated that the phosphors exhibited a strong luminescence of ⁵D₀–⁷F₂ transition at 612 nm under the excitation at 237 nm. The emission intensity of the phosphors prepared by the sol–gel process was higher than those prepared by the solid-state reaction. The relationship between optical properties and morphologies were studied. In particular, Li⁺ ion doping effectively enhanced the luminescent properties of the Eu³⁺ doped Ca₃La₃(BO₃)₅ phosphors. The highest brightness was observed in the phosphor Ca₃La_2.82Eu_0.1Li_0.08B₅O_15−δ prepared by the sol–gel process.     

Kinetic study on Li2.8(V0.9Ge0.1)2(PO4)3 by EIS measurement

Kinetics for lithium ion transfers in the fast ionic conductor Li_2.8(V_0.9Ge_0.1)₂(PO₄)₃ prepared by solid-state reaction method has been studied by electrochemical impedance spectroscopy (EIS) at various temperatures and the results were correlated with observed cathodic behavior. The specific conductivities of Li_x(V_0.9Ge_0.1)₂(PO₄)₃ (x = 0.9–2.8) versus temperatures were analyzed from blocking-electrodes by Wagner's polarization method and the activation energy was calculated. It was observed that electronic conductivities of Li_x(V_0.9Ge_0.1)₂(PO₄)₃ increased with lithium contents in the materials. The compounds show a reversible capacity of 131 mAh g⁻¹ at low current density (13 mA g⁻¹). Modeling the EIS data with equivalent circuit approach enabled the determination of charge transfer and surface film resistances. The Li ion diffusion coefficient (D_Li⁺) versus voltage plot shows three valleys during the first charge cycle coinciding with the irreversible plateau of the voltage versus lithium content profiles reflecting the irreversible phase change in the compound. The obtained D_Li⁺ from EIS varies within 10⁻⁸ to 10⁻⁷ cm² s⁻¹, so Li_2.8(V_0.9Ge_0.1)₂(PO₄)₃ shows excellent chemical diffusion performance.     

Power factor of La1−xSrxFeO3 and LaFe1−yNiyO3

The electrical conductivity (σ), Seebeck coefficient (S), and power factor (σS²) of perovskite-type LaFeO₃, La_1−xSr_xFeO₃ [0.1 ≤ x ≤ 0.4] and LaFe_1−yNi_yO₃ [0.1 ≤ y ≤ 0.6] were investigated in the temperature range of 300–1100 K to explore their possibility as thermoelectric materials. The electrical conductivity of LaFeO₃ showed semiconducting behavior, and its Seebeck coefficient changed from positive to negative around 650 K with increasing temperature. The electrical conductivity of LaFeO₃ increased with the substitutions of Sr and Ni atoms, while its Seebeck coefficient decreased. The Seebeck coefficient of La_1−xSr_xFeO₃ was positive, whereas that of LaFe_1−yNi_yO₃ changed from positive to negative with increasing Ni content. The substitutions of Sr and Ni were effective in increasing the power factor of LaFeO₃; 0.0053 × 10⁻⁴ Wm⁻¹ K⁻² for LaFeO₃ (1050 K), 1.1 × 10⁻⁴ Wm⁻¹ K⁻² for La_1−xSr_xFeO₃ (x = 0.1 at 1100 K) and 0.63 × 10⁻⁴ Wm⁻¹ K⁻² for LaFe_1−yNi_yO₃ (y = 0.1 at 1100 K).     

Characterization and sintering of BaZrO3 nanoparticles synthesized through a single-step combustion process

Barium zirconate (BaZrO₃) nanoparticles synthesized by a self-sustained single-step combustion process is reported in this paper. In this process, a phase pure nanopowder of BaZrO₃ has been obtained by the combustion of an aqueous solution containing Ba and Zr ions by using citric acid as complexing agent and liquor ammonia as fuel, thus giving rise to phase pure BaZrO₃ nanopowder in a single-step combustion without any further calcination. The X-ray diffraction studies have shown that the as-prepared powder was single phase, crystalline, and has a cubic perovskite structure (ABO₃) with a lattice constant a = 4.19 Å. The average particle size calculated from FWHM is 30 nm. The phase purity of BaZrO₃ nanopowder has been examined using differential thermal analysis (DTA), thermo gravimetric analysis (TGA) and Fourier transform of infrared spectroscopy (FTIR). The transmission electron microscopic investigation has shown that the particle size of the as-prepared powder was in the range 30–50 nm with a mean size of 40 nm. The nano BaZrO₃ has been sintered to a density of 99% of the theoretical density at 1650 °C in 2 h without the use of any sintering aids. The morphology of the sintered pellets has been studied with scanning electron microscopy (SEM). The dielectric constant (_r) and loss factor (tan δ) values obtained at 10 MHz for a well-sintered barium zirconate pellet has been found to be 32.2 and 1 × 10⁻⁴, respectively, at room temperature.     

Effect of Zn and Li codoping ions on nanosized Gd2O3:Eu phosphor

Nanosized Gd_1.92−x−yZn_xLi_yEu_0.08O_3−δ phosphor was fabricated by combustion synthesis. The effect of Zn²⁺ and Li⁺ ions on the crystallization behavior, morphology, and luminescence property of Gd₂O₃:Eu³⁺ was investigated. The results indicated that incorporation of Zn²⁺ and Li⁺ ions into Gd₂O₃:Eu³⁺ nanoparticles (NPs) could lead to a remarkable increase of photoluminescence or X-ray excited luminescence, and the intensity at 612 nm was increased by a factor of 7.1 or 21.5 in comparison with that of undoped sample. The enhanced luminescence was regarded as the results of the creation of oxygen vacancies due to the Gd³⁺ sites occupied by Li⁺ ions, the alteration of the crystal field surrounding the activator Eu³⁺ ions owing to the incorporation Zn²⁺ ions into interstitial sites, and the flux effect of Zn²⁺ and Li⁺ ions. The Zn- and Li-codoped Gd₂O₃:Eu³⁺ phosphor with highly enhanced luminescence is very encouraging for applications in high-resolution display devices.     

Role of strontium addition on the phase transition of lanthanum copper oxide from K2NiF4 to perovskite structure

Without Sr addition, the sintered La₂O₃ and CuO powder mixture in a mole ratio of 1:2 formed K₂NiF₄-structured La₂CuO₄ with excess CuO. When 15% of strontium was added, La₂CuO₄ transformed into the single perovskite La_1−xSr_xCuO_2.5−δ phase with orthorhombic structure. As the strontium addition increased to 20%, the perovskite lattice changed from orthorhombic to tetragonal. These phase transitions may be attributed to the enhanced oxidation of the divalent cupper ions (Cu²⁺) to trivalent ones (Cu³⁺) by the strontium addition. Based on the electroneutrality in an ABO₃ perovskite lattice, a divalent cation is unstable in the B-site cation sub-lattice when the A-site is occupied by a trivalent cation such as La³⁺. As strontium was added into the A-site cation sub-lattice, the oxidation of Cu²⁺ ion into trivalent Cu³⁺ ion was enhanced. The increase of Cu³⁺ concentration strengthened the electrostatic bonding (ESB) of copper ions with their neighboring anions. Consequently, the symmetrical tetragonal Sr-doped lanthanum copper oxide was obtained.     

Synthesis and properties of Ba3NaRu2O9−δ and Ba3(Na, R)Ru2O9−δ (R=rare earth) crystals

Crystals of Ba₃NaRu₂O_9−δ (δ≈0.5) and Ba₃(Na, R)Ru₂O_9−δ (R=Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb) were grown by an electrochemical method, and their crystallographic, magnetic, and electric properties were studied. All crystals have a hexagonal structure of space group P6₃mmc. Ba₃NaRu₂O_9−δ and Ba₃(Na, R)Ru₂O_9−δ (except Ce) have a negative asymptotic Curie temperature suggesting the existence of an antiferromagnetic order; however, they are paramagnetic at temperatures above 1.7 K. Ba₃NaRu₂O_9−δ has an effective magnetic moment P_eff of 0.91 μ_B, while P_eff of Ba₃(Na, R)Ru₂O_9−δ (except Ce) reflects the large free-ion moment of the rare earth ions. Ba₃(Na, Ce)Ru₂O_9−δ shows peculiar magnetic behavior that differs from the magnetism of other Ba₃(Na, R)Ru₂O_9−δ crystals. The resistivity of all crystals exhibits an activation-type temperature dependence with an activation energy in the range of 0.10.2 eV.     

Electron paramagnetic resonance spectra of Pr ions doped in BaMO3 (M=Ce, Zr, Sn) and SrCeO3

The isotropic electron paramagnetic resonance (EPR) spectra of powders with Pr⁴⁺ doped in BaMO₃ (M=Ce, Zr, Sn) were measured at 4.2 K. A very large hyperfine interaction with the ¹⁴¹Pr nucleus was observed. The results were analyzed based on the weak field approximation, and the g values and hyperfine coupling constants A were obtained. The measured g values are much smaller than |−10/7|, showing that the crystal field effect on the behavior of a 4f electron is large. The value of |g| decreases from 0.741 (Pr⁴⁺–BaCeO₃) to 0.583 (Pr⁴⁺–BaSnO₃), which is caused by the increase of the crystal field due to the shrinking of the lattice. In contrast, the hyperfine coupling constants are almost constant: A=0.060(1) cm⁻¹. The anisotropic EPR spectra were measured for the case of Pr⁴⁺–SrCeO₃, which indicates that the octahedral array of oxygen ions about Pr⁴⁺ is considerably distorted for this system.     

Synthesis and characterization of proton conducting Sr(Ce1−xZrx)0.95Yb0.05O3−δ by the citrate method

The citrate method was used to synthesize Sr(Ce_1−xZr_x)_0.95Yb_0.05O_3−δ (x = 0.1, 0.2, 0.3, 0.4) and to avoid the drawbacks of the conventional solid state reaction method. The products were characterized by thermal analysis (TG-DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM) and electron probe X-ray microanalyzer (EPMA). The results indicate that the citrate method is an advantageous route in producing Sr(Ce_1−xZr_x)_0.95Yb_0.05O_3−δ materials. Sr(Ce_0.9Zr_0.1)_0.95Yb_0.05O_3−δ powders are composed of nanoscaled crystallites with the average grain size in the range of 60–70 nm. Single phase is confirmed over the whole x range. In addition, chemical stability against CO₂ and electrical conduction behavior of the sintered Sr(Ce_1−xZr_x)_0.95Yb_0.05O_3−δ ceramics were investigated. The chemical stability of the ceramics against CO₂ is certified to increase with the increase in zirconium content. Impedance spectroscopy was used to study the electrical conduction behavior of Sr(Ce_0.9Zr_0.1)_0.95Yb_0.05O_3−δ ceramic.     

H-NMR-NMR study of magnetic anomaly in (BEDT-TTF)3CuBr4

The H-NMR¹-NMR measurements were carried out on the organic conductor, (BEDT-TTF)₃CuBr₄, where strong coupling between π and 3d electrons are expected. The H¹ spin-lattice relaxation rate, T₁⁻¹, was large about 20 sec⁻¹ and almost temperature independent above 60 K indicating the effect of localized magnetic moments. Anomalies at the structural transition (60 K) was small. An antiferromagnetic transition was confirmed: The magnitude of the moment at Cu site was estimated as 0.6-0.7 μ_B. The moment at the Cu site was found to survive in the metallic state under pressure.     

Study of extraction of the ndcl3-smcl3-hcl-h2o-di-(2-ethylhexyl)-phosphoric-acid-kerosine system

The extraction equilibrium relations of neodymium and samarium were studied in the system HCl (1.0 mol dm⁻³)-di-(2-ethylhexyl) phosphorie acid (HDEHP), for a wide range of neodymium and samarium concentration (0.02-1.20 mol dm⁻³), mole fraction (X_i=0.005-0.995) and acidity (C_H+=0.27-1.06 mol dm⁻³). A relatively simple, high precision model for the distribution ratio D_i was established by correlating the experimental data with a progressive regression program D_i=exp(a₁+a₂I+a₃C_H++a₄X_i)I^a5X_i^{a6 In C_H+ + a7 In(1-X_i})

The extraction behaviour of the neodymium-samarium binary system was studied using the above model. The study of acid equilibrium shows that the extraction proceeds according to the cation exchange reaction within this range of experimental parameters. The neodymium and samarium concentrations in the organic phase increase rapidly and reach a maximum, and then decrease as the aqueous concentration increases; the maximum concentration appears at about 0.3-0.4 mol dm⁻³ of aqueous neodymium and samarium concentration. The variation of separation factors, β_Sm/Nd, was also studied with the change in aqueous acidity, rare earth concentration and mole fraction, β_Sm/Nd vary between 6.5 and 10.2 and the average value is 8.77.