Order-disorder phase transitions and high-temperature oxide ion conductivity of Er2+xTi2−xO7−δ (x = 0, 0.096)

The Er_2+xTi_2−xO_7−δ (x = 0.096; 35.5 mol% Er₂O₃) solid solution and the stoichiometric pyrochlore-structured compound Er₂Ti₂O₇ (x = 0; 33.3 mol% Er₂O₃) are characterized by X-ray diffraction (phase analysis and Rietveld method), thermal analysis and optical spectroscopy. Both oxides were synthesized by thermal sintering of co-precipitated powders. The synthesis study was performed in the temperature range 650-1690 °C. The amorphous phase exists below 700 °C. The crystallization of the ordered pyrochlore phase (P) in the range 800-1000 °C is accompanied by oxygen release. The ordered pyrochlore phase (P) exists in the range 1000−1200 °C. Heat-treatment at T ≥ 1600 °C leads to the formation of an oxide ion-conducting phase with a distorted pyrochlore structure (P2) and an ionic conductivity of about 10⁻³ S/cm at 740 °C. Complex impedance spectra are used to separately assess the bulk and grain-boundary conductivity of the samples. At 700 °C and oxygen pressures above 10⁻¹⁰ Pa, the Er_2+xTi_2−xO_7−δ (x = 0, 0.096) samples are purely ionic conductors.     

Luminescence properties of Ca0.65La0.35F2.35:Yb, Er with enhanced red emission via upconversion

Yb³⁺/Er³⁺ codoped Ca_0.65La_0.35F_2.35 materials with intense red emission via upconversion were prepared by a high temperature solid-state method. Based on the upconversion luminescence properties investigations, it was found that, under 980 nm excitation, Ca_0.65La_0.35F_2.35:20 mol.%Yb³⁺, xEr³⁺ showed intense red upconversion luminescence, which was ascribed to ⁴F_9/2 → ⁴I_15/2 transition of Er³⁺, although both green and red emissions could be detected. It was also found that the green and red emissions originated the two photon processes, and the ground-state absorption (GSA), excited-state absorption (ESA) and energy transfer (ET) processes between Er³⁺/Yb³⁺ ions and Er³⁺/Er³ ions were involved in the enhanced red emission mechanism.     

Synthesis and characterization of Ni1−xZnxFe2O4 spinel ferrites from tailored layered double hydroxide precursors

In this paper, a series of pure Ni_1 − xZn_xFe₂O₄ (0 ≤ x ≤ 1) spinel ferrites have been synthesized successfully using a novel route through calcination of tailored hydrotalcite-like layered double hydroxide molecular precursors of the type [(Ni + Zn)_{1 − x − y}Fe_y²⁺Fe_x³⁺(OH)₂]^x+(SO₄²⁻)_x/2·mH₂O at 900 °C for 2 h, in which the molar ratio of (Ni²⁺ + Zn²⁺)/(Fe²⁺ + Fe³⁺) was adjusted to the same value as that in single spinel ferrite itself. The physico-chemical characteristics of the LDHs and their resulting calcined products were investigated by powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and Mössbauer spectroscopy. The results indicate that calcination of the as-synthesized LDH precursor affords a pure single Ni_1 − xZn_xFe₂O₄ (0 ≤ x ≤ 1) spinel ferrite phase. Moreover, formation of pure ferrites starting from LDHs precursors requires a much lower temperature and shorter time, leading to a lower chance of side-reactions occurring, because all metal cations on the brucite-like layers of LDHs can be uniformly distributed at an atomic level.     

Fluorine-doped LiNi0.5Mn1.5O4 for 5 V cathode materials of lithium-ion battery

Fluorine-doped 5 V cathode materials LiNi_0.5Mn_1.5O_4−xF_x (0.05 ≤ x ≤ 0.2) have been prepared by sol-gel and post-annealing treatment method. The results from X-ray diffraction and scanning electron microscopy (SEM) indicate that the spinel structure changes little after fluorine doping, but the particle size varies with fluorine doping and the preparation conditions. The electrochemical measurements show that stable cycling performance can be obtained when the fluorine amount x is higher than 0.1, but the specific capacity is decreased and 4 V plateau capacity resulting from a conversion of Mn⁴⁺/Mn³⁺ remains. Moreover, influence of the particle size on the reversible capacity of the electrode, especially on the kinetic property, has been examined.     

Synthesis of perovskite-type (La1−xCax)FeO3 (0 ≤ x ≤ 0.2) at low temperature

Gel formation was realized by adding citric acid to a solution of La(NO₃)₃·5H₂O, Ca(NO₃)₂·4H₂O, and Fe(NO₃)₂·9H₂O. Perovskite-type (La_1−xCa_x)FeO₃ (0 ≤ x ≤ 0.2) was synthesized by firing the gel at 500 °C in air for 1 h. The crystallite size (D_1 2 1) decreased with increasing x, while the specific surface area was 6.8-9.4 m²/g and independent of x. The XPS measurement of the (La_1−xCa_x)FeO₃ surface indicated that the Ca²⁺ ion content increased with increasing x, while the Fe ion content was independent of x. Catalytic activity for CO oxidation increased with increasing x.     

Different microstructure and dielectric properties of Ba1−xCaxTiO3 ceramics and pulsed-laser-ablated films

A comparative study of the microstructure and dielectric properties between Ba_1−xCa_xTiO₃ (BCT) ceramics and films were performed in the whole Ca concentration range of x = 0-1. The ceramics were prepared by conventional solid-state reaction technique and the films by the method of pulsed-laser deposition. X-ray diffraction (XRD) study of the BCT ceramics exhibited a pure tetragonal phase for x = 0-0.25, a tetragonal-orthorhombic diphase for x = 0.25-0.85 and a pure orthorhombic phase for x = 0.90-1.00. And the dielectric phase transition temperature from tetragonal to cubic was marginally affected by the Ca doping into BaTiO₃. However, BCT films deposited on Pt/Si/SiO₂/Si substrates showed a different microstructure and dielectric properties. Tetragonal-orthorhombic diphase was not found in the BCT films for x = 0.25-0.85, and a large decrease of the Curie point and diffuse phase transition were observed in the BCT films. Based on the compositional analysis, such phenomena were ascribed to the occupancy of some Ca²⁺ to the Ti⁴⁺ sites in the BCT films.     

Frequency upconversion fluorescence studies of Er/Yb-codoped KNbO3 phosphors

Different concentrations of Er³⁺ and Yb³⁺ ions-doped potassium niobate (K_0.9NbO₃:Yb_(x)Er_{(0.1 − x)} for x = 0, 0.01, 0.05, 0.09 and 0.1) polycrystalline powder phosphors were prepared by the conventional solid state reaction method and were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. Energy transfer and upconversion fluorescence properties of the Yb³⁺ and Er³⁺-codoped phosphors have been discussed. The XRD data has shown mono-phase for pure KNbO₃ while the doped samples represented additional phase formation. The SEM micrographs represented the rectangular crystal growth habit for the KNbO₃ phosphors when doped with 0.1 mol of Er³⁺ ions. An intense green emission at 557 nm along with a red emission at 674 nm was observed when the doped samples were excited with 975 nm IR radiation. The upconversion mechanism has been discussed based on the excited state absorption and energy transfer mechanisms.     

Effects of GeO2 on the thermal stability and optical properties of Er/Yb-codoped oxyfluoride tellurite glasses

The aim of this article was to study the influence of GeO₂ on the thermal stability and optical properties of Er³⁺/Yb³⁺ codoped (70 − x)TeO₂–xGeO₂–PbF₂–BaF₂ (TGEYx) glasses prepared by using a melting method. The properties of Er³⁺/Yb³⁺ codoped glasses were investigated by using differential scanning calorimetry, upconversion luminescence, Raman and optical absorption spectra. The results indicated that TGEY35 glass with the germanate–tellurite mixed network showed the best thermal stability and poor crystallization tendency. With increasing the GeO₂ content, the maximum phonon energy of oxyfluoride tellurite glass network increased, while the phonon density decreased. The upconversion emission intensities enhanced obviously based on the decreasing phonon density of glass hosts, while the increasing red emission (657 nm) with the increase of GeO₂ concentration was attributed to the relative larger maximum phonon energy which matching the energy gap between the pertinent ⁴S_3/2 and ⁴F_9/2 levels.     

Synthesis and upconversion luminescence properties of Yb/Er codoped BaGd2(MoO4)4 powder

Synthesis and upconversion luminescence properties of the new BaGd₂(MoO₄)₄:Yb³⁺,Er³⁺ phosphor were reported in this paper. The phosphor powder was obtained by the traditional high temperature solid-state method, and its phase structure was characterized by the XRD pattern. Based on the upconversion luminescence properties studies, it is found that, under 980 nm semiconductor laser excitation, BaGd₂(MoO₄)₄:Yb³⁺,Er³⁺ phosphor exhibits intense green upconversion luminescence, which is ascribed to ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transition of Er³⁺. While the observed much weaker red emission is due to the non-radiative relaxation process of ⁴S_3/2 → ⁴F_9/2 and ⁴F_9/2 → ⁴I_15/2 transition originating from the same Er³⁺. The concentration quenching effects for both Yb³⁺ and Er³⁺ were found, and the optimum doping concentrations of 0.5 mol% Yb³⁺ and 0.08 mol% Er³⁺ in the new BaGd₂(MoO₄)₄ Gd³⁺ host were established.     

X-ray Rietveld analysis and Fourier transform infrared spectra of the solid solutions [Eu2−xMx][Sn2−xMx]O7−3x/2 (M = Mg or Zn)

Two series of mixed oxides with formula [Eu_2−xM_x][Sn_2−xM_x]O_7−3x/2 (M = Mg or Zn) have been synthesized. The study by X-ray diffraction and Fourier transform infrared spectroscopy shows that the solids obtained are new non-stoichiometric solid solutions with the pyrochlore type structure. For both series a decrease of the cell parameter is observed when the degree of substitution, x, increases. The structural refinements (X-ray studies) were achieved in the space group Fd-3m, no. 227 (origin at center -3m) by using the Fullprof software. The Rietveld refinements show that the divalent cations M²⁺ (Mg²⁺, Zn²⁺) substitute isomorphically for Eu³⁺ and Sn⁴⁺ ions producing vacancies in the anionic sublattice.     

Structural and transport properties of stoichiometric Mn-doped magnetite: Fe3−xMnxO4

The polycrystalline samples of Fe_3−xMn_xO₄ (0.10 ≤ x ≤ 0.50) were prepared by a solid-state route reaction method. X-ray diffraction pattern shows that Mn²⁺ doped magnetites are in single phase and possess cubic inverse spinel structure. The resistivity measurements (10 < T < 300 K) for x = 0.0 and 0.01 confirms the first order phase transition at the Verwey transition T_V = 123 K and 117 K, respectively. No first order phase transition was evidenced for Fe_3−xMn_xO₄ (0.10 ≤ x ≤ 0.50). Small polaron model has been used to fit the semiconducting resistivity behavior and the activation energy ?_a, for samples x = 0.10 and 0.50 is about 72.41 meV and 77.39 meV, respectively. The Raman spectra of Fe_3−xMn_xO₄ at room temperature reveal five phonons modes for Fe_3−xMn_xO₄ (0.01 ≤ x ≤ 0.50) as expected for the magnetite (Fe₃O₄). Increased Mn²⁺ doping at Fe site leads to a gradual changes in phonon modes. The Raman active mode for Fe_3−xMn_xO₄ (x = 0.50) at ≅641.5 cm⁻¹ is shifted as compared to parent Fe₃O₄ at ≅669.7 cm⁻¹, inferring that Mn⁺² ions are located mostly on the octahedral sites. The laser power is fixed to 5 mW causes the bands to broaden and to undergo a small shift to lower wave numbers as well as increase in the full width half maxima for A_1g phonon mode with the enhancement of Mn²⁺ doping. Mössbauer spectroscopy probes the site preference of the substitutions and their effect on the hyperfine magnetic fields confirms that Mn⁺² ions are located mostly on the octahedral sites of the Fe_3−xMn_xO₄ spinel structure.     

Preparation, characterization, and stability of calcium zinc hydrophosphate

Calcium zinc hydrophosphate phases with different Zn/(Zn + Ca) molar ratios (x_Zn, from 0 to 1) were synthesized using co-precipitation method at pH 10, 25 °C. X-ray diffraction, FT-IR spectroscopy and field emission scanning electron microscopy (FE-SEM) were used to characterize the synthesized products. Thermal behavior of the products was examined by thermal analytical instruments (TG-DSC-MS), while the chemical stability of the products was tested by toxicity characteristic leaching procedure (TCLP).The results showed that the phase constituents of formed calcium zinc hydrophosphate phases were related to the molar contents of Zn²⁺. With the increase of x_Zn, it formed calcium-deficient hydorxyapatite (CaHap), calcium zinc hydorxyapatite (CaZnHap), CaZn₂(PO₄)₂·2H₂O, and Zn₃(PO₄)₂·4H₂O, respectively. All the calcium zinc hydrophosphates were thermally stable up to 600 °C, and less Zn²⁺ leached in a wide pH range of 2-11, which indicated that calcium zinc hydrophosphate could effectively hold Zn²⁺ in their crystal phases with stabilization ratios of over 99.99%.     

Preparation, characterization and luminescence of La2TeO6 phosphor doped with Eu

Phosphors of La₂TeO₆ doped with Eu³⁺ ions have been synthesized by the oxidation of the corresponding rare-earths oxytellurides of formula La_2−xEu_xO₂Te (x = 0.02, 0.06, and 0.1) at 1050 K. Powder X-ray diffraction confirms that the as prepared materials consist of the orthorhombic La₂TeO₆ as main phase. The photoluminescence (PL) of red-emitting La_2−xEu_xTeO₆ powder phosphors is reported. The emission spectrum, exhibits an intense emission peak due to ⁵D₀ → ⁷F₂ transition at 616 nm, which indicates that the Eu³⁺ ion occupies a non-centrosymmetric site in the host lattice. These materials could find application for use as lamp phosphors in the red region.     

Erbium-activated silica-zirconia planar waveguides prepared by sol-gel route

Er³⁺ doped (100 − x)SiO₂ − xZrO₂ planar waveguides were prepared by the sol-gel route, with x ranging from 10 up to 30 mol%. Multilayer films doped with 0.3 mol% Er³⁺ ions were deposited on fused quartz substrates by the dip-coating technique. The thickness and refractive index were measured by m-line spectroscopy at different wavelengths. The fabrication protocol was optimized in order to confine one propagating mode at 1.5 μm. Photoluminescence in the near and visible region indicated a crystalline local environment for the Er³⁺ ion.     

Electrical properties of MgAl2-2xZrxMxO4 (M = Co, Ni and x = 0.00-0.20) synthesized by coprecipitation technique using urea

This paper presents the results of a study concerning the structural and electrical properties of MgAl_2-2xZr_xM_xO₄ (x = 0.00-0.20 and M = Co²⁺ and Ni²⁺) prepared by a coprecipitation technique using urea as a precipitating agent. The X-ray diffraction data for the pure and its doped samples are consistent with the single-phase spinel and their crystallite sizes are in the range 7-20 ± 4 nm. The DC resistivity increases from 3.09 × 10⁹ Ω cm to 6.73 × 10⁹ and 8.06 × 10⁹ Ω cm whereas dielectric constant decreases from 5.80 to 5.11 and 4.95 on doping with Zr-Co and Zr-Ni, respectively. The electrical resistivity variations with increase in the dopant contents indicate two types of conduction mechanisms in operation. Several parameters such as, hopping energy (W), metal-semiconductor transition temperature (T_MS) and Debye temperature (θ_D) have also been determined. The increase in DC resistivity and decrease in dielectric constant suggest that the synthesized materials can be considered for application as an insulating and structural material in fusion reactors.     

Pechini sol-gel deposition and luminescent properties of SrLa1−xRExGa3O7 (RE = Eu, Tb) phosphor films

SrLa_1−xRE_xGa₃O₇ (RE = Eu³⁺, Tb³⁺) phosphor films were deposited on quartz glass substrates by a simple Pechini sol-gel method. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), atomic force microscopy, field-emission scanning electron microscopy, photoluminescence spectra, and lifetimes were used to characterize the resulting films. The results of XRD indicated that the films began to crystallize at 700 °C and crystallized fully at 900 °C. The results of FT-IR spectra were in agreement with those of XRD. Uniform and crack-free films annealed at 900 °C were obtained with average grain size of 80 nm, root mean square roughness of 46 nm and thickness of 130 nm. The RE ions showed their characteristic emission in crystalline SrLa_1−xRE_xGa₃O₇ films, i.e., Eu³⁺⁵D₀-⁷F_J (J = 0, 1, 2, 3, 4), Tb³⁺⁵D₄-⁷F_J (J = 6, 5, 4, 3) emissions, respectively. The optimum concentrations (x) of Eu³⁺ and Tb³⁺ were determined to be 50, and 80 mol% in SrLa_1−xRE_xGa₃O₇ films, respectively.     

Intense red-emitting phosphors for LED solid-state lighting

The phosphors Gd_2−xEu_x(MoO₄)₃ (x = 0.20, 0.40, 0.60, 0.80, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0), Gd_0.8−xY_xEu_1.2(MoO₄)₃ (x = 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8) and Gd_0.2Y_0.6−xEu_1.2Sm_x(MoO₄)₃ (x = 0.02, 0.024, 0.028, 0.032, 0.036, 0.04) were prepared by solid-state reaction technique at 950 °C. The presence of the Y³⁺ and Sm³⁺ ions strengthen and broaden the absorption of the phosphors at ∼400 nm. The intense red-emitting phosphor Gd_0.2Y_0.572Eu_1.2Sm_0.028(MoO₄)₃ with orthorhombic structure was obtained. Both Eu³⁺ and Sm³⁺ f-f transition absorptions are observed in the excitation spectra, the main emission line is at 616 nm (⁵D₀ → ⁷F₂ transition of Eu³⁺) and the chromaticity coordinates (x = 0.66, y = 0.33) is very close to the NTSC standard values (x = 0.67, y = 0.33). It is considered to be an efficient red-emitting phosphor for GaN-based light emitting diode (LED).     

Dielectric properties and substitution mechanism of samarium-doped Ba0.68Sr0.32TiO3 ceramics

Ba_0.68Sr_0.32TiO₃ ceramics of perovskite structure are prepared by solid state reaction method with addition of x mol% Sm₂O₃, and their dielectric properties are investigated. It is found that, integrating with the lattice parameters and tolerance factor t, there is an alternation of substitution preference of Sm³⁺ for the host cations in perovskite lattice. Owing to the replacement of Sm³⁺ ions for Ba²⁺ ions in the A site, T_c rises with the increase of Sm₂O₃ doping when the doping content is below 0.1 mol%; meanwhile, when the content is more than 0.1 mol%, Sm³⁺ ions tend to occupy the B-site, causing a drop of T_c. Owing to the modifications of Sm³⁺ doping, dielectric constant, dissipation factor and temperature stability of dissipation factor are influenced remarkably, making it a superior candidate for environment-friendly applications. Moreover, the creation of oxygen vacancies controls the dielectric constant when the addition is above 0.1 mol%, so the dielectric constant decreases with increasing of samarium.     

Oxygen vacancy ordering in strontium doped rare earth cobaltate perovskites Ln1−xSrxCoO3−δ (Ln = La, Pr and Nd; x > 0.60)

A family of Sr-doped perovskite compounds Ln_1−xSr_xCoO_3−δ (Ln = La³⁺, Pr³⁺ and Nd³⁺; x > 0.60), were prepared by sol-gel chemistry and reaction at 1100 °C under 1 atm of oxygen. This structural family has been shown to be present only for rare earth ions larger than Sm³⁺ and an upper limit of Sr²⁺ solubility in these phases was found to exist between x = 0.90 and 0.95. X-ray diffraction shows oxygen-deficient, simple cubic (Pm-3m) perovskite crystal structures. The combination of electron and powder neutron diffraction reveals that oxygen vacancy ordering occurs, leading to a tetragonal (P4/mmm) superstructure and a doubling of the basic perovskite unit along the crystallographic c-axis. No additional Ln³⁺/Sr²⁺ cation ordering was observed.     

Electrical properties of Pb(Mg1/3Nb2/3)O3-PbTiO3 ceramics modified with WO3

Ferroelectrics 0.67Pb (Mg_1/3Nb_2/3)O₃-0.33PbTiO₃ (PMN-PT) + x mol% WO₃ (x=0.1, 0.5, 1, 2) were prepared by columbite precursor method. Electrical properties of WO₃-modified ferroelectrics were investigated. X-ray diffraction (XRD) was used to identify crystal structure, and pyrochlore phase were observed in 0.67Pb (Mg_1/3Nb_2/3)O₃-0.33PbTiO₃+2 mol% WO₃. Dielectric peak temperature decreased with WO₃ doping, indicating that W⁶⁺ incorporated into PMN-PT lattice. Lattice constant, pyrochlore phase and grain size contribute to the variation of K_max. Both piezoelectric constant (d₃₃) and electromechanical coupling factors (k_p) were enhanced by doping 0.1 mol% WO₃, which results from the introduction of “soft” characteristics into PMN-PT, while further WO₃ addition was detrimental. We consider that the two factors, introduction of “soft” characteristics and the formation of pyrochlore phase, appear to act together to cause the variation of piezoelectric properties of 0.67PMN-0.33PT ceramics doping with WO₃.