Synthesis of high pressure Cd2V2O7 and related compositions

Cd₂V₂O₇1bII has been prepared at about 80 kbars pressure. This new compound has a structure related to that of pyrochlore, but the symmetry is distorted from cubic to monoclinic. Solid solutions of the types Cd₂V_2?xNb_xO₇ and Cd_2?xA_xV₂O₇ (A = In or T1) were prepared at 58 kbars. Attempts to confirm previous reports of A¹⁺A³⁺V₂O₇ pyrochlores were unsuccessful.     

The preparation,characterization and optical properties of Cd2V2O7 and CdCO3 compounds

Cadmium vanadium oxides (Cd₂V₂O₇) and Cadmium carbonates (CdCO₃) were synthesized via a facile hydrothermal method. X-ray diffraction (XRD), Raman spectroscopy, infrared spectrometer (IR), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS) were employed to characterize the structure, morphology and chemical state of the samples, respectively. The photoluminescence (PL) properties of the as-synthesized Cd₂V₂O₇ and CdCO₃ were measured at room temperature using an excitation wavelength of 325 nm. The Cd₂V₂O₇ shows two visible light emission centers located at 589 and 637 nm, which are supposed to be relevant to local defects in Cd₂V₂O₇. The CdCO₃ shows three emission centers located at 408, 530 and 708 nm, which are supposed to be relevant to the electron transition from the conduction band to valence band and defect related energy level.     

Pulsed laser deposition of (WO3)1 − x(Nb2O5)x thin films: Characterization and gasochromic studies

A series of (WO₃)_{1 − x}(Nb₂O₅)_x (x = 0, 0.05, 0.1 and 0.15) mixed oxide films were fabricated by pulsed laser deposition (PLD) at 27 Pa oxygen partial pressure on ITO glass substrates. The thickness of the (WO₃)_{1 − x}(Nb₂O₅)_x thin films is about 350 ± 30 nm and their surface has a uniform morphology. A layer of platinum (Pt) was then sputtered onto the surface of the film. The hydrogen gas sensing performance of Pt catalyst activated (WO₃)_{1 − x}(Nb₂O₅)_x thin films were investigated. The cycling of the coloration was obtained from UV–vis spectra. Gasochromic coloration of (WO₃)_{1 − x}(Nb₂O₅)_x thin films were investigated at room temperature in H₂–N₂ mixtures containing 1–100 mol% of H₂. The results show that the shortest response time of (WO₃)_{1 − x}(Nb₂O₅)_x/Pt hydrogen sensor is within 30 s and the highest transmittance change (ΔT) varies from 20% to 30%.     

Phase analysis studies on the system PbOPbF2Nb2O5NbO2F

Samples in the system PbOPbF₂Nb₂O₅NbO₂F prepared at 875 K and 1025 K were studied by X-ray single crystal and powder diffraction as well as electron diffraction methods. Six new compounds were characterized as Pb_0.74Nb_10.62(O,F)₃₀ (orth.), Pb_1.36Nb_10.91(O,F)₃₀ (tetr.), Pb_0.27Nb(O,F.)_3.17 (hex.), Pb₂Nb₃O₇F₅ (tetr.), Pb_0.75NbO₃F_0.5 (tetr.) and Pb_1?$\text{x}$Nb_$\text{x}$(O,F)_2+$\text{y}$ (cub.)     

Lithium insertion into Li2Ti3O7

Li₂Ti₃O₇ with the ramsdellite-type structure undergoes lithium insertion reactions with n-BuLi. Li_2+xTi₃O₇ phases form with x = 0.5 and 1.0 at room temperature and at 50°C, respectively. The ESR spectrum of Li₃Ti₃O₇ confirms the partial reduction of Ti⁴⁺ ions to Ti³⁺. The electrical conductivity of the fully lithiated phase is several orders of magnitude higher than that of the host compound, suggesting charge hopping in the mixed valent lithiated compound.     

Structural study of CdxNi1−xAl2O4 spinels

NiAl₂O₄ and several mixed spinels Cd_xNi_1?xAl₂O₄ have been prepared, in polycrystalline form, by solid-state reaction of mixtures of CuO, CdO and Al₂O₃ at 1273 K. X-ray diffraction analysis was used to determine the lattice parameter, a_O, oxygen parameter, u, and cation distribution for each spinel. The lattice parameter was found to increase continuously from 804.9 pm for x = 0 up to 824.6 pm for x = 0.65. The u-parameter also increases, along the same composition range, from 0.380 to 0.391. A solubility limit for Cd²⁺ in the spinel phase is reached at x = 0.68 (at 1273 K). Cd²⁺ was found to occupy almost exclusively tetrahedral sites in all cases, whilst Ni²⁺ showed a strong octahedral preference.     

Etude par diffraction X et microscopie electronique des composes inedits de formule AnBnO3n+2 dans les systems La2Ti2O7CaTiO3, Nd2Ti2O7CaTiO3 et Ca2Nb2O7CaTiO3

New compounds corresponding to the formula A_nB_nO_3n+2 with n = 4,5; 5 and 6 have been found in the two following systems Nd₂Ti₂O₇CaTiO₃ and Ca₂Nb₂O₇CaTiO₃. These compounds have been investigated by electron microscopy. The results obtained are in good agreement with our structural hypothesis.     

Large-scale fabrication of H2(H2O)Nb2O6 and Nb2O5 hollow microspheres

Hollow micro-sized H₂(H₂O)Nb₂O₆ spheres constructed by nanocrystallites have been successfully synthesized via a bubble-template assisted hydrothermal process. In the reaction process, H₂O₂ acts as a bubble generator and plays a key role in the formation of the hollow structure. An in situ bubble-template mechanism has been proposed for the possible formation of the hollow structure. The spherelike assemblies of these H₂(H₂O)Nb₂O₆ nanoparticles have been transformed into their corresponding pseudohexagonal phase Nb₂O₅ through a moderate annealing dehydration process without destroying the hierarchical structure. Optical properties of the as-prepared hollow spheres were investigated. It is exciting that the absorption edge of the hollow Nb₂O₅ microspheres shifts about 18 nm to the violet compared with bulk powders in the UV/vis spectra, indicating its superior optical properties.     

Mode de formation et stabilite des phases AnBnO3n+2 mises en evidence dans les systemes La2Ti2O7 - CaTiO3, Nd2Ti2O7 - CaTiO3 et Ca2Nb2O7 - CaTiO3

The stability of the A_nB_nO_3n+2 phases, whose structure is derived from the perovskite structure, was investigated through X-Ray diffraction, in the following systems: La₂Ti₂O₇ - CaTiO₃, Nd₂Ti₂O₇ - CaTiO₃ and Ca₂Nb₂O₇ - CaTiO₃. Some of the phases quenched from the liquid are metastable and include vacancies overA and B sites. A model of formation of these phases is proposed.     

Ba3ZnTa2−xNbxO9 and Ba3MgTa2−xNbxO9 (0≤x≤1): synthesis, structure and dielectric properties

Oxides belonging to the families Ba₃ZnTa_2−xNb_xO₉ and Ba₃MgTa_2−xNb_xO₉ were synthesized by the solid state reaction route. Sintering temperatures of 1300°C led to oxides with disordered (cubic) perovskite structure. However, on sintering at 1425°C hexagonally ordered structures were obtained for Ba₃MgTa_2−xNb_xO₉ over the entire range (0≤x≤1) of composition, while for Ba₃ZnTa_2−xNb_xO₉ the ordered structure exists in a limited range (0≤x≤0.5). The dielectric constant is close to 30 for the Ba₃ZnTa_2−xNb_xO₉ family of oxides while the Mg analogues have lower dielectric constant of ∼18 in the range 50 Hz to 500 kHz. At microwave frequencies (5-7 GHz) dielectric constant increases with increase in niobium concentration (22-26) for Ba₃ZnTa_2−xNb_xO₉; for Ba₃MgTa_2−xNb_xO₉ it varies between 12 and 14. The “Zn” compounds have much higher quality factors and lower temperature coefficient of resonant frequency compared to the “Mg” analogues.     

Growth and characterization of non-linear optical crystal Cd3Zn3B4O12

Cd₃Zn₃B₄O₁₂ polycrystals were synthesized by solid-state and wet chemical reaction methods. Cd₃Zn₃B₄O₁₂ single crystals with millimeter grade were grown from the self-flux B₂O₃ (Cd:Zn:B = 1:1:1.5); and larger crystals were obtained from the PbO-0.85PbF₂ fluxes easily. As-grown crystals were characterized by differential scanning calorimetry and thermogravimetric, X-ray diffraction, infrared and Raman spectral analysis, respectively. The non-linear optical coefficient of the Cd₃Zn₃B₄O₁₂ crystal is 2.6 times as large as that of KH₂PO₄ crystal. Chemical etching shows that this crystal is very stable in neutral solution and not hygroscopic in air at room temperature.     

Ionic conductivities of Na2Ti3O7, K2Ti4O9 and their related materials

Ionic conductivities were measured on the polycrystalline samples of layered titanates, Na₂Ti₃O₇ and K₂Ti₄O₉, and their derivatives. The activation energies and the prefactors of the conductions were 0.70 eV and 7.9 × 10 (Ωcm)^?1K for Na₂Ti₃O₇ and 0.81 eB and 1.1 × 10³ (Ωcm)^?1K for K₂Ti₄O₉. A small amount of Nb₂O₅ was doped to these titanates substituting TiO₂. Remarkable enhancements of ionic conductivities were observed with the doping. A new metastable phase, Li₂Ti₃O₇, was prepared by ion-exchange of Na₂Ti₃O₇ and its ionic conductivity was measured.     

Reaction chemistry and subsolidus phase equilibria in lead-based relaxor systems Part II The ternary system PbO-MgO-Nb2O5

Subsolidus compatibility relations in the lead-based relaxor system PbO-MgO-Nb₂O₅ were investigated by the solid-state reaction technique and the various phase assemblages that occur at 825°C in this system established. The existence of two previously reported ternary compounds, i.e. a cubic perovskite Pb₃MgNb₂O₉ [Pb(Mg_1/3Nb_2/3)O₃] and an oxygen- deficient cubic pyrochlore Pb₆MgNb₆O₂₂ [Pb_1.714(Mg_0.286Nb_1.714)O_6.286] was confirmed. A minor amount of PbO enters into the pyrochlore lattice and forms a narrow homogeneity range within the ternary system PbO-MgO-Nb₂O₅ which corresponds to a general formula Pb_{2 – x} (Mg_0.286Nb_1.714)O_{6.571 – x} , where 0 > x > 0.286. At subsolidus temperatures (<825°C), the perovskite Pb(Mg_1/3Nb_2/3)O₃ is compatible with the ternary pyrochlore solid solution and together they form a biphasic area within the system in which the perovskite and the pyrochlore phases coexist with one another. Both PbO and MgO are compatible with the perovskite Pb(Mg_1/3Nb_2/3)O₃ and these phases constitute a compatibility triangle with one another in the ternary system. However, the perovskite Pb(Mg_1/3Nb_2/3)O₃ is not compatible with Nb₂O₅ and these two phases react with one another to yield the pyrochlore Pb₆MgNb₆O₂₂ and MgO. The ternary pyrochlore solid solution is compatible with several binary lead niobates, i.e. Pb₃Nb₂O₈, Pb₅Nb₄O₁₅, Pb₂Nb₂O₇ and Pb₃Nb₄O₁₃ and forms pseudobinary tie-lines with these compounds in the ternary system PbO-MgO-Nb₂O₅. The pyrochlores Pb₃Nb₄O₁₃ [Pb_1.5Nb₂O_6.5] and Pb₆MgNb₆O₂₂ [Pb_1.714(Mg_0.286 Nb_1.714)O_6.286] are isostructural compounds and a series of continuous solid solution is formed between them.     

Synthesis and structural characterization of (Bi2O3)1?x (Y2O3)x and (Bi2O3)1?x (Gd2O3)x solid solutions

Solid solution series, (Bi₂O₃)_1−x (Y₂O₃)_x and (Bi₂O₃)_1−x (Gd₂O₃)_x, forx = 0.10, 0.20, 0.30 and 0.40 were synthesized by standard ceramic technique. The structural phase characterization was carried out using X-ray powder diffraction technique. It was found that the solid solution containing 20–40 mole% of Y₂O₃ had face-centred cubic structure. All samples of the solid solution series, (Bi₂O₃)_1−x (Gd₂O₃)_x, had rhombohedral single phase in the concentration range 0.10 ≤x ≤ 0.40. Lattice parameters offcc phase of Y₂O₃ doped samples were calculated from the X-ray diffraction data. The lattice constant ‘a’ gradually decreases with increasing content of dopant concentration (x) for the Y₂O₃ doped system and obeys Vegard’s rule. The unit cell parameters for the (Bi₂O₃)_1−x (Gd₂O₃)_x doped samples showing rhombohedral phase were obtained on hexagonal setting.     

Improved dielectric properties in pyrochlore type oxides: Ca3Sm3?xBixTi7Nb2O26.5 (x = 1.0, 2.0 or 3.0) by Bi substitution

Improved dielectric properties are observed in pyrochlore type oxides, Ca₃Sm_3−x Bi_xTi₇Nb₂O_26.5 (x = 1.0, 2.0 or 3.0) by Bi substitution. The dielectric constant increased with increasing Bi concentration. The dielectric constant obtained for Ca₃Bi₃Ti₇Nb₂O_26.5 is 110, whereas, without Bi (Ca₃Sm₃Ti₇Nb₂O_26.5) it is 62 at 100 kHz. The Powder X-ray diffraction analysis reveals that cubic pyrochlore type phase is formed for all the compositions. The experimental results further show that the formation of Bi substituted compounds is complete at a lower temperature than the compounds without Bi. Microstructure studies reveal that the grains formed are acicular when Bi is present in large amounts compared to cuboid grains in samples having no Bi.     

Structural and dielectric properties of the phase Pb1−2xKxM3+xNb2O6, M = La or Bi

The Pb_1?2xK_xM³⁺_xNb₂O₆, M = La or Bi, solid solutions have been prepared by solid state reaction and their structural and ferroelectric properties have been established. Complete crystalline solid solubility exists in both systems; in addition, three structurally related phases, namely, the ferroelectric orthorhombic and tetragonal tungsten bronze phases and the paraelectric tetragonal K.₅La.₅Nb₂O₆ type phase, have been identified at room temperature. The composition ranges for three phases are 0.0 ≤ x ≤ 0.47, 0.48 ≤ x ≤ 0.85, 0.86 ≤ x ≤ 1.0, respectively. The ferroelectric phase transition temperature, T_c, decreases with increasing concentration of K⁺ and La³⁺ or Bi³⁺ in the orthorhombic tungsten bronze phase. A few compositions from each system exhibit excellent dielectric and piezoelectric characteristics, indicating that they could be future materials for piezoelectric and high frequency dielectric studies.     

Effect of Na2W2O7 addition on low-temperature sintering and microwave dielectric properties of CaWO4

Effects of Na₂W₂O₇ addition on low-temperature sintering, microstructure and microwave dielectric properties of CaWO₄ were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and microwave dielectric resonator methods in this paper. The CaWO₄ material could be sintered into a dense ceramic (~ 96% theoretical density) at 850 °C/2 h without affecting its microwave dielectric properties greatly by adding appropriate amount of Na₂W₂O₇. The XRD results showed that CaWO₄ main phase with trace amount of Na₂W₂O₇ second phase was observed in (1 − x) CaWO4-xNa₂W₂O₇ (0 < x ≤ 0.04) ceramics. The chemical compatibility of 0.96CaWO₄-0.04Na₂W₂O₇ ceramic with silver (Ag) powders was also investigated. The Ag showed inert behavior with 0.96CaWO₄-0.04Na₂W₂O₇ ceramic when co-fired at 875 °C for 2 h.     

Crystal structures of KBa2Nb5O15 and LaK2Nb5O15 with the TTB-type structure

Single crystals of two niobates, KBa₂Nb₅O₁₅ and LaK₂Nb₅O₁₅, were synthesized by high-temperature reaction and the crystal structures were determined by single crystal X-ray diffraction data. Although the space groups for these compounds were different (the non-centrosymmetrical space group P4bm (#100) for KBa₂Nb₅O₁₅ and the centrosymmetrical one P4/mbm (#127) for LaK₂Nb₅O₁₅), both compounds had the same tetragonal tungsten bronze-type (hereafter TTB-type) structure. The lattice parameters and R-factors of KBa₂Nb₅O₁₅ (LaK₂Nb₅O₁₅) were a = 12.533(2) (12.563(2)) and c = 4.0074(9) (3.9179(9)) Å, and R₁ = 0.040 (0.047) and wR₂=0.131 (0.120), respectively. From the crystal structural analysis, it was clarified that distribution of two large cations was different from each other in the way that K and Ba atoms in KBa₂Nb₅O₁₅ were distributed statistically at two crystallographic sites and K and La atoms in LaK₂Nb₅O₁₅ were ordered.     

Preparation and crystal chemistry of some rare earth vanadium (IV) pyrochlores (RE)2V2O7; RE = Lu,Yb, Tm,Lu1−xYx, Lu1−xScx

The preparation of the phases (RE)₂V₂O₇ where RE = Lu, Yb, Tm, and the solid solutions Sc_xLu_1?x (x = .10 → .50) and Y_xLu_1?x (x = .20 and .40), using a $\text{CO}\text{CO}{}_2$ buffer system are described. Precision lattice constants were determined and compared with previous work, finding good agreement with (1) but not with (2). The solid solution (Sc_xLu_1?x)₂V₂O₇ exists as a pyrochlore to x = .30, compared with x = .55 for the similar (Sc_xLu_1?x)₂ Ti₂O₇ system. Chemical analysis on the members x = .40 and x = .50 indicate significant amounts of V(III) and X-ray data show a diminishing intensity for the “ordering” reflections (h, k, l all odd). These results suggest that Lu(III), Sc(III), V(III) and V(IV) are partially disordered over both cation sites tending toward a C-type (RE)₂O₃ structure or a defect fluorite structure with increasing x.     

Synthesis and Li ion conductivity of Li2O-Nb2O5-P2O5 glasses and glass-ceramics

Glasses with the compositions of xLi₂O-(70 − x)Nb₂O₅-30P₂O₅, x = 30-60, and their glass-ceramics are synthesized using a conventional melt-quenching method and heat treatments in an electric furnace, and Li⁺ ion conductivities of glasses and glass-ceramics are examined to clarify whether the glasses and glass-ceramics prepared have a potential as Li⁺ conductive electrolytes or not. The electrical conductivity (σ) of the glasses increases monotonously with increasing Li₂O content, and the glass of 60Li₂O-10Nb₂O₅-30P₂O₅ shows the value of σ = 2.35 × 10⁻⁶ S/cm at room temperature and the activation energy (E_a) of 0.48 eV for Li⁺ ion mobility in the temperature range of 25-200 °C. It is found that two kinds of the crystalline phases of Li₃PO₄ and NbPO₅ are formed in the crystallization of the glasses and the crystallization results in the decrease in Li⁺ ion conductivity in all samples, indicating that any high Li⁺ ion conducting crystalline phases have not been formed in the present glasses. 60Li₂O-10Nb₂O₅-30P₂O₅ glass shows a bulk nanocrystallization (Li₃PO₄ nanocrystals with a diameter of ∼70 nm) and the glass-ceramic obtained by a heat treatment at 544 °C for 3 h in air exhibits the values of σ = 1.23 × 10⁻⁷ S/cm at room temperature and E_a = 0.49 eV.