Transport properties and thermal expansion of La2Mo2O9-based solid electrolytes

The oxygen ion transference numbers of La_1.7Bi_0.3Mo₂O₉, La₂Mo_1.7W_0.3O₉ and La₂Mo_1.95V_0.05O₉ ceramics, determined by modified faradaic efficiency and e.m.f. methods at 973-1173 K, vary in the range 0.995-0.977 in air, decreasing when temperature increases. The activation energies for the ionic and electronic transport are 61-71 kJ/mol and 123-141 kJ/mol, respectively. Reducing oxygen chemical potential leads to increasing n-type electronic contribution to the total conductivity, which remains, however, essentially p(O₂)-independent down to oxygen pressures of 10⁻⁴-10⁻³ atm and exhibits reversible drop on further reduction, probably due to phase decomposition. Doping La₂Mo₂O₉ with calcium results in segregation of a CaMoO₄-based phase, accompanied with increasing electronic transport. The average thermal expansion coefficients of La₂Mo₂O₉-based materials, calculated from dilatometric data in air, are (14.4-14.8) × 10⁻⁶ K⁻¹ at 300-700 K and (16.4-22.5) × 10⁻⁶ K⁻¹ at 700-1070 K.     

Hygroscopicity and bulk thermal expansion in Y2W3O12

Negative thermal expansion material, Y₂W₃O₁₂ has been synthesized by the solid-state method and bulk thermal expansion of the material has been investigated from 300 to 1100 K. The material reversibly forms a trihydrate composition whose X-ray diffraction pattern can be indexed to an orthorhombic unit cell with a = 10.098(1) Å, b = 13.315(3) Å, c = 9.691(4) Å. The cell volume of the hydrated pattern is 7% smaller than the unhydrated cell volume. According to the dilatometric studies, the material shows a 3-6% increase in the linear strain at about 400 K, which can be attributed to the removal of water. Sintering the material at 1473 K leads to large grain size of >100 μm, which results in a large hysteresis in the bulk thermal expansion behavior. Hot pressing at 1273 K under a uniaxial pressure of 25 MPa results in a fine-grained (2-5 μm) ceramic. Glazing the ceramic prevents moisture pick up and a linear thermal expansion over the entire temperature range 1100-300 K and an average linear thermal expansion co-efficient of −9.65 × 10⁻⁶/K is observed. The effect of water on the thermal expansion behavior of this system is discussed.     

Crystal growth and characterization of Yb-doped Gd3Ga5O12

Single crystals of (Yb_xGd_1−x)₃Ga₅O₁₂ (0.0 ≤ x ≤ 1.0) have been grown by the micro-pulling-down method. Formation of continuous solid solutions with a garnet structure was confirmed. Composition dependence of the lattice constant, thermal diffusivity, specific heat capacity and thermal conductivity was investigated. Assignment of the Yb³⁺-energy levels in Gd₃Ga₅O₁₂-host lattice has been performed by using absorption, emission and Raman spectroscopy measurements at both, room temperature and at 12 K.     

Phase relations, lattice thermal expansion in CeO2-Gd2O3 system, and stabilization of cubic gadolinia

The phase relation in CeO₂-Gd₂O₃ system has been established under slow cooled conditions from 1400 °C. Two phase regions, namely F-type cubic and C-type cubic, were observed in this system. A striking observation of this investigation is the stabilization of C-type gadolinia after Ce⁴⁺ substitution, which is attributed to decrease in average cationic size on Ce⁴⁺ substitution at Gd³⁺ site. The lattice thermal expansion behaviour of C-type gadolinia samples was investigated by High temperature-XRD. The lattice thermal expansion coefficient was found to gradually increase on increasing the Ce⁴⁺ content, within the C-type homogeneity range, in Ce_1−xGd_xO_2−x/2 series.     

Structural and electrical properties of La2−xBaxMo2O9−x/2 (x = 0-0.18)

La_2−xBa_xMo₂O_9−x/2 (x ≤ 0.18) have been prepared by solid state reaction method. The lattice parameter of La_2−xBa_xMo₂O_9−x/2 (x ≤ 0.18) determined by XRD data refinement shows a linear dependence on the dopant Ba content x. For the specimen with a La/Ba molar ratio of 0.18-0.2, additional reflection of secondary phase exists in the XRD pattern, so the value of solubility limit for Ba in La₂Mo₂O₉ is defined in range of 0.18 < x < 0.2. As the replacement degree of La³⁺ by Ba²⁺ increases, the bulk conductivity of La_2−xBa_xMo₂O_9−x/2 (x ≤ 0.18) decreases initially and then increases, a minimum value at La_1.9Ba_0.1Mo₂O_8.95 exists. Hebb-Wagner studies in argon atmosphere, which use an oxide-ion blocking electrode, show that La_2−xBa_xMo₂O_9−x/2 (x ≤ 0.18) are predominantly oxide-ion conducting in the temperature ranging from 773 to 1173 K. The average thermal expansion coefficient of La_1.84Ba_0.16Mo₂O_8.92 determined by high-temperature XRD was deduced as great as 17.5 × 10⁻⁶ K⁻¹ between 298 and 1173 K.     

Conductivity of reduced La2Mo2O9 based oxides: The effect of tungsten substitution

The electrical properties of reduced LAMOX-type oxides (La_1.9Y_0.1Mo_2−yW_yO_9−δ with y = 0, y = 0.5, y = 1.0) were investigated by complex impedance spectroscopy.When reduced at 605 °C in hydrogen, La_1.9Y_0.1Mo₂O_9−δ is 10 times and 3 × 10⁵ times more conductive at 605 and 180 °C, respectively, than in air at the same temperatures. The conductivity curve presents a low slope (0.37 eV versus 1.2 eV in air).Besides, the stabilising effect of tungsten against reduction is evidenced, in good agreement with previous reports.In low oxygen partial pressures however (PO₂ < 10⁻¹⁸ Pa), the decomposition of the materials is detected, whatever the tungsten content (0 ≤ y ≤ 1 in La_1.9Y_0.1Mo_2−yW_yO_9−δ). This observation points out the efficiency limit of Mo⁶⁺/W⁶⁺ substitution to stabilise the structure against reduction, and the limit for an application as IT-SOFC electrolyte.However, given the high electronic conductivity upon reduction, the application of these materials in IT-SOFC electrodes could be considered.     

Synthesis of solid solution Er2−xCexW3O12 and studies of their thermal expansion behavior

The syntheses and structures of Er_2−xCe_xW₃O₁₂ were studied. It was found that pure phases could form only for 0.0 ≦ x ≦ 0.4 and 1.5 ≦ x ≦ 2.0. Compounds with 0 ≦ x ≦ 0.4 have the hydrated orthorhombic structure at room temperature and transform to unhydrated orthorhombic one above 135 °C whereas samples with 1.5 ≦ x ≦ 2.0 crystallize in monoclinic structure. Thermal expansion properties of Er_2−xCe_xW₃O₁₂ were studied with high temperature X-ray powder diffraction. Samples with 0 ≦ x ≦ 0.4 exhibit negative thermal expansion in temperature range of 200-800 °C and higher cerium content leads to more negative thermal expansion coefficient. However, compounds with 1.5 ≦ x ≦ 2.0 show positive thermal expansion owing to the edge-sharing polyhedra.     

Synthesis and optical properties of (Gd1−x,Eux)2O2SO4 nano-phosphors by a novel co-precipitation method

(Gd_1−x,Eu_x)₂O₂SO₄ nano-phosphors were synthesized by a novel co-precipitation method from commercially available Gd₂O₃, Eu₂O₃, H₂SO₄ and NaOH starting materials. Composition of the precursor is greatly influenced by the molar ratio of NaOH to (Gd_1−x,Eu_x)₂(SO₄)₃ (the m value), and the optimal m value was found to be 4. Fourier transform infrared spectrum (FT-IR) and thermal analysis show that the precursor (m = 4) can be transformed into pure (Gd_1−x,Eu_x)₂O₂SO₄ nano-phosphor by calcining at 900 °C for 2 h in air. Transmission electron microscope (TEM) observation shows that the Gd₂O₂SO₄ phosphor particles (m = 4) are quasi-spherical in shape and well dispersed, with a mean particle size of about 30-50 nm. Photoluminescence (PL) spectroscopy reveals that the strongest emission peak is located at 617 nm under 271 nm light excitation, which corresponds to the ⁵D₀ → ⁷F₂ transition of Eu³⁺ ions. The quenching concentration of Eu³⁺ ions is 10 mol% and the concentration quenching mechanism is exchange interaction among the Eu³⁺ ions. Decay study reveals that the ⁵D₀ → ⁷F₂ transition of Eu³⁺ ions has a single exponential decay behavior.     

Electrical conductivity of 5 mol.% Yb2O3 and 5 mol.% Gd2O3 co-doped Sm2Zr2O7

Sm₂Zr₂O₇ co-doped with and without 5 mol.% Yb₂O₃ and 5 mol.% Gd₂O₃ were prepared by a pressureless-sintering method at 1973 K for 10 h in air. The relative density, structure and electrical conductivity were investigated by the Archimedes method, X-ray diffraction, scanning electron microscopy and impedance spectra measurements. Both Sm₂Zr₂O₇ and (Sm_0.9Gd_0.05Yb_0.05)₂Zr₂O₇ ceramics exhibit a single phase of pyrochlore-type structure. The grain conductivity, grain-boundary conductivity and total conductivity obey the Arrhenius relation, respectively, and gradually increase with increasing temperature from 723 to 1173 K. (Sm_0.9Gd_0.05Yb_0.05)₂Zr₂O₇ ceramic is the oxide-ion conductor in an oxygen partial pressure range of 1.0 × 10⁻⁴ to 1.0 atm at all test temperature levels. The grain conductivity, grain-boundary conductivity and total conductivity of (Sm_0.9Gd_0.05Yb_0.05)₂Zr₂O₇ with dual Yb³⁺ + Gd³⁺ doping are higher than those of undoped Sm₂Zr₂O₇ at identical temperature levels.     

Hydrothermal synthesis and luminescent properties of Y2O3:Tb and Gd2O3:Tb microrods

One-dimensional (1D) Y₂O₃:Tb³⁺ and Gd₂O₃:Tb³⁺ microrods have been successfully prepared through a large-scale and facile hydrothermal method followed by a subsequent calcination process in N₂/H₂ mixed atmosphere. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (IR), thermogravimetric analysis (TGA), energy-dispersive X-ray spectra (EDX), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), photoluminescence (PL) and cathodoluminescence (CL) spectra as well as kinetic decays were used to characterize the samples. The as-formed products via the hydrothermal process could transform to cubic Y₂O₃:Tb³⁺ and Gd₂O₃:Tb³⁺ with the same morphology and slight shrinking in size after a postannealing process. Both Y₂O₃:Tb³⁺ and Gd₂O₃:Tb³⁺ microrods exhibit strong green emission corresponding to ⁵D₄ → ⁷F₅ transition (542 nm) of Tb³⁺ under UV light excitation (307 and 258 nm, respectively), and low-voltage electron beam excitation (1.5 → 3.5 kV), which have potential applications in fluorescent lamps and field emission displays.     

Structure and lithium ion conductivity of garnet-like Li5La3Sb2O12 and Li6SrLa2Sb2O12

Oxides with the nominal chemical compositions Li₅La₃Sb₂O₁₂ and Li₆SrLa₂Sb₂O₁₂ were prepared by solid-state reaction. The structures were refined by the Rietveld method using powder X-ray diffraction data. The synthesis of Li₅La₃Sb₂O₁₂ resulted in the well known garnet-related structure plus 5 wt.% of La₂LiSbO₆ in the bulk. In contrast to that, Li₆SrLa₂Sb₂O₁₂ could be synthesised in single garnet-related type phase. Lithium ion conductivities of Li₅La₃Sb₂O₁₂ and Li₆SrLa₂Sb₂O₁₂ were studied by the ac impedance method. The grain-boundary contribution to the total (bulk + grain-boundary) resistance is very small and about 5 and 3% for Li₅La₃Sb₂O₁₂ and Li₆SrLa₂Sb₂O₁₂, respectively, at 24 °C and decreases further with increase in temperature. Among the investigated compounds, Li₅La₃Sb₂O₁₂ exhibits the highest total (bulk + grain-boundary) and bulk ionic conductivity of 7.8 × 10⁻⁶ and 8.2 × 10⁻⁶ S cm⁻¹, respectively, at 24 °C. The structural data indicate that the coupled substitution Li + Sr ⇒ La leads to a closure of the bottle neck like O-O distances of the shared edges of neighbouring Li octahedra and therefore reduces the mobility of Li ions in Li₆SrLa₂Sb₂O₁₂. Scanning electron microscope (SEM) images of the Li₆SrLa₂Sb₂O₁₂ compound revealed well crystallised large homogeneous grains (∼4.8 μm) and the grains were in good contact with the neighbouring grain, which leads to a smaller grain-boundary contribution to the total resistance.     

Thermochemical studies on SrFe12O19(s)

The citrate-nitrate gel combustion route was used to prepare SrFe₁₂O₁₉(s) powder sample and the compound was characterized by X-ray diffraction analysis. A solid-state electrochemical cell of the type: (−)Pt, O₂(g)/{CaO(s) + CaF₂(s)}//CaF₂(s)//{SrFe₁₂O₁₉(s) + SrF₂(s) + Fe₂O₃(s)}/O₂(g), Pt(+) was used for the measurement of emf as a function of temperature from 984 to 1151 K. The standard molar Gibbs energy of formation of SrFe₁₂O₁₉(s) was calculated as a function of temperature from the emf data and is given by: (SrFe₁₂O₁₉, s, T)/kJ mol⁻¹ (±1.3) = −5453.5 + 1.5267 × (T/K). Standard molar heat capacity of SrFe₁₂O₁₉(s) was determined in two different temperature ranges 130-325 K and 310-820 K using a heat flux type differential scanning calorimeter (DSC). A heat capacity anomaly was observed at 732 K, which has been attributed to the magnetic order-disorder transition from ferrimagnetic state to paramagnetic state. The standard molar enthalpy of formation, (298.15 K) and the standard molar entropy, (298.15 K) of SrFe₁₂O₁₉(s) were calculated by second law method and the values are −5545.2 kJ mol⁻¹ and 633.1 J K⁻¹ mol⁻¹, respectively.     

Study on properties of CaO-SiO2-B2O3 system glass-ceramic

Low temperature co-fired ceramic (LTCC) is prepared by sintering a glass selected from CaO-SiO₂-B₂O₃ system, and its sintered bodies are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). It is found that the optimal sintering temperature for this glass-ceramic is 820 °C for 15 min, and the major phases of this material are CaSiO₃, CaB₂O₄ and SiO₂. The glass-ceramic possesses excellent dielectric properties: ?_r = 6.5, tan δ < 2 × 10⁻³ at 10 MHz, temperature coefficient of dielectric constant about −51 × 10⁻⁶ °C⁻¹ and coefficient of thermal expansion about 8 × 10⁻⁶ °C⁻¹ at 20-400 °C. Thus, this material is supposed to be suitable for the tape casting process and be compatible with Ag electrode, which could be used as the LTCC materials for the application in wireless communications.     

Synthesis, characterization and electrical conductivity studies on A3Bi2P3O12 (A = Na, K) materials

Crystalline Na₃Bi₂P₃O₁₂, K₃Bi₂P₃O₁₂ and glassy K₃Bi₂P₃O₁₂ compounds were prepared by solid-state reaction method. The prepared samples are characterized by X-ray powder diffraction, Fourier transform infrared spectroscopy and differential scanning calorimetry. The crystalline materials are found to be orthorhombic. The electrical conductivity measurements on the crystalline and glassy samples show that at ∼373 K, the σ_DC for crystalline K₃Bi₂P₃O₁₂ (0.81 × 10⁻⁸ S/cm) is about two orders of magnitude higher than the corresponding glassy phase (1.25 × 10⁻¹⁰ S/cm). The scaling results show that the conductivity relaxation mechanism is independent of temperature.     

Novel Dy-doped Ca2Gd8(SiO4)6O2 white light phosphors for Hg-free lamps application

The luminescent properties of Ca₂Gd_8(1−x)(SiO₄)₆O₂:xDy³⁺ (1% ≤ x ≤ 5%) powder crystals with oxyapatite structure were investigated under vacuum ultraviolet excitation. In the excitation spectrum, the peaks at 166 nm and 191 nm of the vacuum ultraviolet region can be assigned to the O²⁻ → Gd³⁺, and O²⁻ → Dy³⁺ charge transfer band respectively, which is consistent with the theoretical calculated value using Jφrgensen's empirical formula. While the peaks at 183 nm and 289 nm are attributed to the f-d spin-allowed transitions and the f-d spin-forbidden transitions of Dy³⁺ in the host lattice with Dorenbos's expression. According to the emission spectra, all the samples exhibited excellent white emission under 172 nm excitation and the best calculated chromaticity coordinate was 0.335, 0.338, which indicates that the Ca₂Gd₈(SiO₄)₆O₂:Dy³⁺ phosphor could be considered as a potential candidate for Hg-free lamps application.     

Luminescent properties of Gd2Ti2O7: Eu phosphor films prepared by sol-gel process

Gd₂Ti₂O₇: Eu³⁺ thin film phosphors were fabricated by a sol-gel process. X-ray diffraction (XRD), atomic force microscopy (AFM) and photoluminescence (PL) spectra as well as lifetimes were used to characterize the resulting films. The results of XRD indicated that the films began to crystallize at 800 °C and the crystallinity increased with the elevation of annealing temperatures. Uniform and crack free phosphor films were obtained, which mainly consisted of grains with an average size of 70 nm. The doped Eu³⁺ showed orange-red emission in crystalline Gd₂Ti₂O₇ phosphor films due to an energy transfer from Gd₂Ti₂O₇ host to them. Both the lifetimes and PL intensity of the Eu³⁺ increased with increasing the annealing temperature from 800 to 1000 °C, and the optimum concentrations for Eu³⁺ were determined to be 9 at.%. of Gd³⁺ in Gd₂Ti₂O₇ film host.     

Pressureless sintering of negative thermal expansion ZrW2O8/Zr2WP2O12 composites

Negative thermal expansion material ZrW₂O₈/Zr₂WP₂O₁₂ composite was prepared by liquid phase sintering. The apparent density of ZrW₂O₈ without any sintering additive was about 3.7 g/cm³, corresponding to about 73% of its theoretical density. However, the relative density of the samples, sintered with more than 5 mol% P₂O₅ was about 90%. The identified phases were mainly ZrW₂O₈ with small amounts of WO₃, ZrO₂ and Zr₂WP₂O₁₂ by XRD. The intensity of Zr₂WP₂O₁₂ peaks increased with increasing P₂O₅ content. It was surmised that the melting of ZrO₂-P₂O₅ resulted in liquid phase formation, which is then converted to Zr₂WP₂O₁₂ on the final stage of sintering. Therefore, Zr₂WP₂O₁₂ phase was observed at the gap between the ZrW₂O₈ grains and at the triple junctions. The ceramics sintered with 20 mol% P₂O₅ showed a negative thermal expansion coefficient of − 4.0 × 10^− 6 °C^− 1.     

Phase relations of the Li2O-Ta2O5-B2O3 and Li2O-WO3-B2O3 systems and promising nonlinear optical compounds in K2O-Ta2O5-B2O3 system

The subsolidus phase equilibria of the Li₂O-Ta₂O₅-B₂O₃, K₂O-Ta₂O₅-B₂O₃ and Li₂O-WO₃-B₂O₃ systems have been investigated mainly by means of the powder X-ray diffraction method. Two ternary compounds, KTaB₂O₆ and K₃Ta₃B₂O₁₂ were confirmed in the system K₂O-Ta₂O₅-B₂O₃. Crystal structure of compound KTaB₂O₆ has been refined from X-ray powder diffraction data using the Rietveld method. The compound crystallizes in the orthorhombic, space group Pmn2₁ (No. 31), with lattice parameters a = 7.3253(4) Å, b = 3.8402(2) Å, c = 9.3040(5) Å, z = 2 and D_calc = 4.283 g/cm³. The powder second harmonic generation (SHG) coefficients of KTaB₂O₆ and K₃Ta₃B₂O₁₂ were five times and two times as large as that of KH₂PO₄ (KDP), respectively.     

Investigation of Na3GdP2O8:Tb as a potential green-emitting phosphor for plasma display panels

The photoluminescent properties of a series of Tb³⁺-doped Na₃GdP₂O₈ phosphors excitable by vacuum ultraviolet and ultraviolet light are reported. The host related absorption, f-f and f-d transitions of Gd³⁺ and Tb³⁺, and charge transfer of O²⁻ → Gd³⁺ and O²⁻ → Tb³⁺ are assigned. Under 147 nm light excitation, Na₃GdP₂O₈:Tb³⁺ phosphors show efficient green emissions with a dominant peak at 545 nm. The optimal sample Na₃Gd_0.4Tb_0.6P₂O₈ shows a shorter decay time and a comparable brightness when compared with the commercial Zn₂SiO₄:Mn²⁺ green phosphor. These results demonstrate that it is a potential candidate for plasma display panels application.