Formation kinetics of Sr<Subscript>2</Subscript>FeMoO<Subscript>6</Subscript> double perovskite

The formation kinetics of Sr₂FeMoO₆ (SFMO) was diagnosed during the solid-state reaction of SrFeO_{3 - x} and SrMoO₄ at different temperatures and heating rates. The reaction kinetics was analyzed by X-ray diffractometer, and the extent of the reaction was quantified by the internal standard method. The non-isothermal kinetic empirical model was proposed to evaluate the activation energy and rate constant of SFMO and further justified to be valid to picture the reaction kinetics of SFMO.     

Novel method of synthesis for double-perovskite Sr<Subscript>2</Subscript>FEMoO<Subscript>6</Subscript>

Polycrystalline double perovskite Sr₂FeMoO₆ nanosized powders have been prepared using a wet chemical coprecipitation method of synthesis. The products were ignited in nitrogen at different temperatures in order to examine thermal evolution of precursors. Powders that were fired at an above 850°C for two hours have almost a single-phase structure. Some intermediate phases have been found at low temperatures. Powder X-ray diffraction linewith measurements show an average particle size in the order of 40 nm for samples annealed in 600–1000°C. It is suggested that the samples annealed at these temperatures were both simultaneously paramagnetic and ferromagnetic in nature. The advantages of this method are the low temperature involve in the sample preparation and the use of a non-reducing atmosphere.     

Structural,dielectric and impedance study of Bi and Li co-substituted Ba<Subscript>0.50</Subscript>Sr<Subscript>0.50</Subscript>TiO<Subscript>3</Subscript> ceramics for tunable microwave devices applications

In this article, the structural, dielectric and electrical properties of Bi and Li co-substituted (Ba, Sr) site in Ba_0.50Sr_0.50TiO₃ ceramics are presented. Four different compositions of Ba_0.50Sr_0.50TiO₃, (Ba_0.50Sr_0.50)_0.98(Bi, Li)_0.02TiO₃, (Ba_0.50Sr_0.50)_0.96(Bi, Li)_0.04TiO_3, and (Ba_0.50Sr_0.50)_0.92(Bi, Li)_0.08TiO₃ were synthesized using solid-state reaction with microwave heating of starting materials. Phase detection for all samples has been examined by XRD along with Rietveld refinement analyses, and the results show the formation of single phase without observation of any secondary phase. However, a decrease in crystallite size, lattice parameters, and unit cell volume has been observed with the increase of Bi and Li concentration. A Dense microstructure with different grains sizes and shapes has been obtained by scanning electron microscopy. Impedance spectroscopy in the temperature range of 30–300 °C and frequency range of 60 Hz–1 MHz has been used to study the dielectric properties. The result shows that the Bi and Li co-substituted Ba_0.5Sr_0.5TiO₃ ceramics exhibit very interesting features, such as enhanced dielectric constant with low loss which make it suitable for microwave tunable devices applications. An electric impedance analysis was carried out at different temperatures namely (400, 450, 500, and 550 °C). A single semicircular arc with single relaxation process has been observed in all studied samples which suggest that the grains contribute to the total resistance in these materials. The activation energy was obtained from the impedance analysis using Arrhenius plot of grain conductivity.     

Electrical properties of MREGeO<Subscript>4</Subscript> (M = Li,Na, K; RE = rare earth) ceramics

Fourteen kinds of alkali-metal rare earth germanate ceramics, MREGeO₄ (M = Li, Na, K; RE = rare earth), were synthesized and their electrical properties were investigated. The crystal structures of the major phases of MREGeO₄ could be classified into four groups; hexagonal (apatite type), tetragonal, orthorhombic (olivine type) and orthorhombic (-K₂SO₄ type). The tetragonal group exhibited higher ionic conductivity than the other groups in the range 300 to 700°C. The highest conductivity was achieved for the LiNdGeO₄ having a tetragonal structure (2.95 × 10^–2 S cm^–1 at 700°C). Analyses on the basis of the Nernsts equation indicated that the two electron reaction associated with carbon dioxide molecule takes place at the detection electrode above 400°C regardless of the kind of electrolyte. The electromotive force, EMF, of the potentiometric CO₂ gas sensors designed using MLaGeO₄ (M = Li, Na, K) increased linearly with an increase in the logarithmic value of CO₂ partial pressure, in accordance with the Nernst equation.     

X-ray diffraction study of the Tm<Subscript>2</Subscript>M<Stack><Subscript>3</Subscript><Superscript>I</Superscript></Stack>Fe<Subscript>5</Subscript>O<Subscript>12</Subscript> (M<Superscript>I</Superscript> = Li,Na, K) ferrites

The Tm₂M₃^IFe₅O₁₂ (M_I = Li, Na, K) compounds have been prepared for the first time by solid-state reactions between thulium(III) oxide, iron(III) oxide, and alkali metal carbonates, and their symmetry class and unit-cell parameters have been determined by X-ray diffraction.     

Luminescence of Pb<Superscript>2+</Superscript> in MAl<Subscript>2</Subscript>B<Subscript>2</Subscript>O<Subscript>7</Subscript> (M = Ca,Sr)

Pb²⁺ doped SrAl₂B₂O₇ was prepared by solution combustion synthesis. The synthesized material was determined by powder XRD. The photoluminescence properties of the synthesized phosphor were investigated at room temperature using a spectrofluorometer. The emission peak of Pb²⁺ doped SrAl₂B₂O₇ was observed at 420 nm upon excitation at 277 nm. The Stokes shift of SrAl₂B₂O₇:Pb²⁺ was calculated to be 12 292 cm⁻¹. The luminescence behavior of Pb²⁺ in both SrAl₂B₂O₇ and CaAl₂B₂O₇ was discussed. The text was submitted by the authors in English.     

Crystal structure of the perovskites Ba<Subscript>2</Subscript>A<Superscript>II</Superscript>UO<Subscript>6</Subscript> (A<Superscript>II</Superscript> = Zn,Cd, Pb)

The compounds Ba₂ZnUO₆, Ba₂CdUO₆, and Ba₂PbUO₆ were prepared by high-temperature solidphase reactions. Their structures (space group Fm \(\bar 3\) m) were refined by the Rietveld method. In the morphotropic series Ba₂A^IIUO₆, correlations were found between the A^II-O, U-O, and Ba-O bond lengths and the crystal-chemical radius of A^II.     

CaAl<Subscript>2</Subscript>Si<Subscript>2</Subscript>O<Subscript>8</Subscript>-B<Subscript>2</Subscript>Al<Subscript>2</Subscript>Si<Subscript>2</Subscript>O<Subscript>8</Subscript>-H<Subscript>2</Subscript>O (B = Na,K) zeolites

The structures of gismondine and amicite are analyzed in comparison with one another and with those of the zeolites whose compositions lie in the hypothetical plane “CaAl₂Si₃O₁₀”-Na₂Al₂Si₃O₁₀-H₂O. It is shown that the structures are similar to each other and may undergo mutual transformations.     

Synthesis of nanocomposites based on MO-Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>-B<Subscript>2</Subscript>O<Subscript>3</Subscript> (M = Ca,Sr, Ba) glasses

This work examines the effect of KBF₄ additions on the crystallization behavior of glasses based on the multicomponent systems MO-Bi₂O₃-B₂O₃ with M = Ca, Sr, and Ba. The glass-ceramic composites obtained contain a δ-Bi₂O₃-based crystalline phase with a crystallite size of ≃7 nm, evenly distributed over the glass matrix. The 400°C electrical conductivity of the nanocomposites reaches 2 × 10⁻⁴ S/cm, and the activation energy is 1.1 eV, typical of anion conduction. These values are comparable to those reported for δ-Bi₂O₃ ceramics.     

Gas-sensing properties of nanostructured M<Subscript><Emphasis Type=Italic>x</Emphasis></Subscript>V<Subscript>2</Subscript>O<Subscript>5</Subscript> (M = Na,K, Rb,Cs) oxides

The gas-sensing properties of nanostructured vanadium oxide doped with alkali-metal cations (M _x V₂O₅ with M = Na, K, Rb, and Cs) have been studied by measuring the resistance of films as a function of analyte content using an impedance meter. The results show that the doped vanadium oxide is sensitive to hydrogen, carbon monoxide, and ammonia in the concentration ranges 0.1–80, 0.1–5, and 0.1–5 vol %, respectively. The strongest gas response is offered by Rb_0.47V₂O₅, which is due to the smallest interlayer spacing in the structure of this nanomaterial and to its larger specific surface area.     

Formation and growth studies of the (Bi,Pb)<Subscript>2</Subscript>Sr<Subscript>2</Subscript>Ca<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>10</Subscript> phase in Ag sheathed tapes

The formation and the thickness growth of the (Bi,Pb)₂Sr₂Ca₂Cu₃O₁₀ phase in Ag-sheathed tapes have been investigated by scanning electron microscopy on samples sintered at 840°C in a flow of 8.5% O₂ (rest N₂) and quenched in air after sintering for 1 to 50 h. The thickness of the (Bi,Pb)₂Sr₂Ca₂Cu₃O₁₀ grains was measured statistically after different sintering times. The experimental results show that the average thickness of these grains increases during the entire sintering period, while the average thickness growth rate decreases exponentially with sintering time. The volume fractions of the various phases present during the heat treatment were also measured from micrographs. Detailed studies of the microstructure and phase formation kinetics support the view that the formation of the (Bi,Pb)₂Sr₂Ca₂Cu₃O₁₀ phase proceeds via a nucleation and growth process. Based on the present observations, a model describing the microstructure evolution is presented.     

Spectral parameters of Nd<Superscript>3+</Superscript> ion in Sr<Subscript>6</Subscript>NdSc(BO<Subscript>3</Subscript>)<Subscript>6</Subscript> crystal

The spectroscopic study of Nd³⁺ in Sr₆NdSc(BO₃)₆ crystal had been performed. Based on the Judd-Ofelt theory to analyze the optical strengths measured in absorption spectra, the following spectral parameters were obtained: intensity parameters are ₂ = 1.108 × 10^–20 cm², ₄ = 2.884 × 10^–20 cm², ₆ = 3.085 × 10^–20 cm², the radiative lifetime is 385 s, the quantum efficiency is 12.5%. The fluorescence branch ratios were calculated: ₁ = 0.423, ₂ = 0.482, ₃ = 0.092, ₄ = 0.005.     

Structure of the double formate KNpO<Subscript>2</Subscript>(OOCH)<Subscript>2</Subscript>

KNpO₂(OOCH)₂ was isolated from neutral Np(V) solutions with a high concentration of potassium formate. The crystal structure of this compound was determined. The structure consists of infinite anionic chains [NpO₂(OOCH)₂] _n ^n? . Potassium cations are located between these chains. The Np coordination polyhedron is a hexagonal bipyramid whose equatorial plane is formed by the oxygen atoms of four HCOO^? ions. The Np bipyramids in the chains are bound via common equatorial edges. The anionic chains in the structures of KNpO₂(OOCH)₂ and NH₄NpO₂(OOCH)₂ studied previously have similar composition but different structure.     

Thermodynamic study of the BiCl<Subscript>3</Subscript>-MCl (M = Li,Na, K,Cs) systems in the ideal associated solution model

The thermal stability of the congruently melting compounds LiCl · BiCl₃, LiCl · 4BiCl₃, NaCl · BiCl₃, 3KCl · BiCl₃, and 3CsCl · BiCl₃ is assessed from the curvature of the liquidus at their melting points, and the applicability of the ideal associated solution model to the BiCl₃-LiCl, BiCl₃-NaCl, BiCl₃-KCl, and BiCl₃-CsCl systems is analyzed.     

Microstructure and properties of epitaxial Sr<Subscript>2</Subscript>FeMoO<Subscript>6</Subscript> films containing SrMoO<Subscript>4</Subscript> precipitates

Thin films of Sr₂FeMoO₆ (SFMO) were grown by pulsed laser deposition in non-optimized argon ambient pressures. The films were found to contain a high number of precipitates of foreign phases. The nature and microstructure of these phases were investigated in detail by high-resolution scanning transmission electron microscopy (STEM) and X-ray diffractometry (XRD). We found out that the dominant foreign phase embedded in the SFMO film matrix was SrMoO₄ (SMO). Through STEM and XRD analysis, we determined that the SMO phase grows epitaxially with respect to the surrounding SFMO matrix and has a fairly good crystallinity. Although the SFMO films include many foreign precipitates, they still exhibit good conducting properties and moderate magnetization values. Tuning the growth of the SMO phase on top of SFMO films to obtain a natural tunnel barrier might pave the way for future applications of SFMO in spintronic devices.     

Prediction of formation of AB<Subscript>3</Subscript>X<Subscript>3</Subscript> (X = S,Se, Te)

For the first time, we consider the possibility of formation of AB₃X₃ compounds in the A₂X₃-B₂X (A and B are different elements; X = S, Se, Te) systems under standard conditions. For the prediction, we use an ad hoc information analysis system that combines the subsystem of databases on properties of inorganic substances and materials and programs for data analysis on the basis of computer training algorithms.     

Dielectric relaxation in double perovskite oxide,Ho<Subscript>2</Subscript>CdTiO<Subscript>6</Subscript>

A new double perovskite oxide holmium cadmium titanate, Ho₂CdTiO₆ (HCT), prepared by solid state reaction technique is investigated by impedance spectroscopy in a temperature range 50–400°C and a frequency range 75 Hz–1 MHz. The crystal structure has been determined by powder X-ray diffraction which shows monoclinic phase at room temperature. An analysis of complex permittivity with frequency was carried out assuming a distribution of relaxation times as confirmed by Cole–Cole plot. The frequency dependent electrical data are analysed in the framework of conductivity and electric modulus formalisms. The frequencies corresponding to the maxima of the imaginary electric modulus at various temperatures are found to obey an Arrhenius law with an activation energy of 0·13 eV. The scaling behaviour of imaginary part of electric modulus suggests that the relaxation describes the same mechanism at various temperatures. Nyquist plots are drawn to identify an equivalent circuit and to know the bulk and interface contributions.