Phase transition and impedance spectroscopy analysis of Pb5(Cr1−xAlx)3F19 ceramics

Abstract

Analysis of the a.c. conductivity data of Pb₅Al₃F₁₉ within the complex permittivity formalism allowed, in a previous work, to show a new transition at 670 K corresponding most probably to the expected, but hitherto unobserved, transition from the paraelastic phase II to the paraelectric prototype phase I of Pb₅Al₃F₁₉. A new investigation of Pb₅(Cr_0.1Al_0.9)₃F₁₉ confirms this and allows us to complete the phase diagram of the Pb₅(Cr_1?x ;Al_x)3F₁₉ solid solution system for (0.80 ? x ? 1) in the high-temperature range. Ceramics with compositions corresponding to x = 0 and 0.5 were also characterized by this type of spectroscopy impedance method.     

Ionic conductivity properties and 19F NMR investigation of Pb1−xCrxF2+x solid solutions with fluorite-type structure

The F⁻ ion transport and diffusion properties inside the solid solution Pb_1−xCr_xF_2+x (0≤x≤0.12) with fluorite-type structure have been determined by impedance spectroscopy and ¹⁹F NMR investigation. The various electrical parameters of Pb_1−xCr_xF_2+x are compared with those of Pb_1−xAl_xF_2+x (0≤x≤0.12) whose structure derives also from the fluorite-type. The Cr³⁺ substitutional cation is of a slightly larger size than the Al³⁺ cation but both cations offer the same usual coordination (CN=6) in ternary fluorides. Comparison of ¹⁹F NMR spectra of some compositions Pb_1−xM_x′F_2+x (M′=Al, Cr) at different temperatures has been realized in order to discover whether the clustering process proposed in Pb_1−xAl_xF_2+x can be extended to Pb_1−xCr_xF_2+x.     

Etude des proprietes de conductivite ionique des phases appartenant au systeme PbF2-InF3

A detailed study of the transport properties of the disordered fluorite-type solid solution Pb_1-xIn_xF_2+x (0 < × < 0.25) and of the ordered fluorite-type related Pb₂InF₇ (x = 0.333) and Pb₇In₅F₂₉ (x = 0.417) has been carried out. For x = 0.25 the conductivity is shown to decrease when disordering diminishes as a consequence of annealing. The disordered Pb_0.88In_0.12F_2.12 phase is a fairly good anionic conductor.     

Electrical conduction mechanism in amorphous Se80In20−xPbx films

DC conductivity measurements on thin films of a-Se₈₀In_20−xPb_x (where x=0, 2, 6 and 10) are reported in the temperature range 200–400 K. The density of states near the Fermi level is calculated using the DC conductivity (Mott parameters). The conduction in the low-temperature region is found to be due to variable range hopping while that in the high-temperature region is due to thermally assisted tunneling of the carriers in the localized states near the band edge.     

Evolution des proprietes de transport des solutions solides M1−xM′xF2+x (M=Sr,Pb; M′=Sb,Bi) ET M1−xM″xF2+2x (M″=Zr,Th) pour les faibles taux de substitution

A detailed study of the transport properties of the solid solutions M_1?xM′_xF_2+x (M=Sr, Pb; M′=Sb, Bi) and M_1?xM″_xF_2+2x (M″=Zr, Th) has been carried out for x<0.10. Contrary to solid solutions of strontium, solid solutions of lead show for a very small value of x (x = 0.10 forPb_1?xM′_xF_2+x, x = 0.005 for Pb_1?xM″'_xF_2+2x) a minimum of conductivity associated with a maximum of activation energy. Hypotheses are proposed to explain the results.     

Phase transition in Pb5Al3F19

A study of the PbF₂-AlF₃ system has resulted in a new material Pb₅Al₃F₁₉ with a tetragonal structure at room temperature closely related to that of Pb₅Cr₃F₁₉. A phase transition in Pb₅Al₃F₁₉ has been detected by means of X-ray diffraction, calorimetric and dielectric measurements. The coexistence of the high and low-temperature Pb₅Al₃F₁₉ phases over the range 270 to 300 K and the abrupt 0.7% increase in the a-axis and 1.5% decrease in the c-axis, on heating through the phase transition, are characteristic of a first-order phase change. The increase in symmetry from point group 4 below to point group 4/m above the phase transition is consistent with a ferroelectric to paraelectric transition, as found previously in Pb₅Cr₃F₁₉.     

Synthesis,characterization and photoluminescence of Eu3+, Ce3+ co-doped CaLaAl3O7 phosphors

Powder phosphors of CaLa_{1? x} Eu _x Al₃O₇, CaLa_{1? x} Ce _x Al₃O₇ and CaLa_{0.99? x} Eu _x Ce_0.01Al₃O₇, where x = 0.01, 0.03, 0.05 and 0.07, were prepared by a combustion method. The powders were well characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and photoluminescence techniques. Emission spectra of Eu³⁺-doped powder phosphors showed strong red emission at 613 nm (⁵D₀ → ⁷F₂); no concentration quenching was observed. Generally, Ce³⁺ acts as an efficient sensitizer when doped with other trivalent lanthanide ions. However, interestingly, in the CaLaAl₃O₇ powder phosphors, the addition of Ce³⁺ with Eu³⁺ exhibited an adverse effect–decreased photoluminescence intensity. The reasons for this behavior are discussed.     

Etude par diffraction de neutrons de la solution solide Pb1−1xThxF2+2x

Bragg neutron diffraction studies have been carried out on the fluorite type solid solutions Pb_1?xTh_xF_2+2x (0 < × < 0.25). The distribution of fluorine atoms between normal and interstitial sites as a function of the substitution rate is given. A volume of presence has been determined for interstitial anions. Electrical and structural data are correlated. Two conductivity mechanisms are proposed according to the fact that the pathway is free or blocked up. The best electrical performances are obtained for Pb_0.875Th_0.125F_2.25, which corresponds to a maximum of vacancies.     

High-temperature electrical transport in Al-doped calcium and strontium titanates

The electrical conductivity of perovskite-related oxides CaTi_1−xAl_xO_3−δ and SrTi_1−xAl_xO_3−δ (x=0−0.4) were investigated within the temperature range 900 to 1000 °C and the oxygen partial pressure range between 10⁻²⁰ and 0.21 atm using a dc four-point technique. The materials investigated show predominantly p-type electronic conductivity at high, n-type electronic conductivity at low, and ionic conductivity at intermediate oxygen partial pressures. The values of ionic conductivity in CaTi_1−xAl_xO_3−δ were found to be lower than those in CaTi_1−xFe_xO_3−δ. The effect of aluminium concentration on the high-temperature transport properties was examined. Paper presented at the 9th EuroConference on Ionics, Ixia, Rhodes, Greece, Sept. 15 – 21, 2002.     

Thermal conductivity of single crystals of Ca1 ? xErxF2 + x and Ca1 ? xTmxF2 + x solid solutions

The thermal conductivity of single crystals of Ca_{1 − x} Er _x F_{2 + x} (x = 0.01, 0.05, 0.07, and 0.10) and Ca_{1 − x} Tm _x F_{2 + x} (x = 0.02, 0.04, and 0.06) solid solutions is studied in the temperature ranges 50–300 and 298–673 K. With increasing content of rare-earth elements, the behavior of thermal conductivity in these solid solutions changes from the characteristic of defect single crystals to glasslike. The concentration dependences of thermal conductivity for the two systems differ insignificantly.     

Increase in the diamagnetic response from low-density Bi1.8Pb0.3Sr1.9Ca2Cu3Ox high-temperature superconductors and Bi1.8Pb0.3Sr1.9Ca2Cu3Ox + Ag composites

Low-density polycrystalline Bi_1.8Pb_0.3Sr_1.9Ca₂Cu₃O _x high-temperature superconductors with a foamlike microstructure and composites consisting of this superconductor and silver in an amount of 20, 25, and 30 vol % are synthesized. The microstructure, as well as the temperature and field dependences of the magnetization, M(T) and M(H), are studied. It is found that, in Bi_1.8Pb_0.3Sr_1.9Ca₂Cu₃O _x high-temperature superconductors and Bi_1.8Pb_0.3Sr_1.9Ca₂Cu₃O _x + Ag composites, the diamagnetic response is enhanced and the screening properties are improved compared with high-temperature polycrystalline superconductors with the same composition that are prepared by the standard technology. The observed effect is explained by the features of magnetic flux penetration into a porous medium.     

Potassium cation conduction in K3 − 2x Pb x PO4 solid solutions

New materials of the K_{3 ? 2x} Pb _x PO₄ system with high potassium-cation conductivity have been synthesized and studied. It has been found that the introduction of Pb²⁺ cations substantially increases the conductivity of K₃PO₄ due to the formation of potassium vacancies and the stabilization of the high-temperature cubic structure of the orthophosphate. At low temperatures, the maximum conductivity has been observed in the composition range x = 0.15–0.20 and varies from ～10^?2 S cm^?1 at 400°C to ～10^?1 S cm^?1 at 700°C. The factors influencing the transport properties of the materials under study have been discussed.     

Effect of Cu–Al double substitution on the electrical properties of Bi4V2O11

Bi₂Cu_0.1?xAl_xV_0.9O_5.35?x/2?δ, 0.02 ≤ x ≤ 0.08, were synthesized by standard solid-state reaction route. Structural and electrical properties of samples are characterized by X-ray diffraction (XRD), differential thermal analysis (DTA), Fourier transform infrared (FT-IR) and alternating current (AC) impedance spectroscopy. The tetragonal γ′ phase structure is preserved to room temperature with compound x = 0.02. The stabilization of β orthorhombic phase is observed for compositions 0.04 ≤ x ≤ 0.05. As the Al content increases, the monoclinic α phase is evidenced for materials 0.06 ≤ x ≤ 0.08. The electrical investigation of Bi₂Cu_0.1?xAl_xV_0.9O_5.35?x/2?δ system has been performed in the frequency range from 20 Hz to 1 MHz using AC impedance spectroscopy. The impedance spectra indicate the two semicircle arcs associated with the bulk and grain boundary resistances at temperature below ～450 ^○C. The conductivity generally changes when Al is substituted. The highest conductivity at 300 ^○C (σ = 2.55 × 10^?4 S cm^?1) is shown for x = 0.02.     

p型Ag0.5(Pb8－xSnx)In0.5Te10化合物的制备及其热电性能

采用高温熔融缓冷和放电等离子烧结工艺制备了p型Ag_0.5(Pb_8-xSn_x)In_0.5Te₁₀五元化合物.研究了Sn含量对化合物载流子传输特性及热电性能的影响规律.结果表明：在Ag_0.5(Pb_8-xSn_x)In_0.5Te₁₀(x 关键词： 0.5(Pb_8-xSn_x)In_0.5Te₁₀')" href="#">Ag_0.5(Pb_8-xSn_x)In_0.5Te₁₀ 合成 载流子 热电性能     

Ultrasonic studies of aluminium-substituted Bi(Pb)-2223 superconductors

The compositional dependence of elastic properties of Al³⁺-substituted Bi(Pb)-2223 superconducting system with the general formula Bi_1.7−x Al_xPb_0.3Sr₂Ca₂Cu₃ O_y (x = 0.0, 0.1, 0.2 and 0.3) have been studied by means of ultrasonic pulse transmission (UPT) technique at 1 MHz (300 K). The elastic moduli of the specimens are computed and corrected to zero porosity. The observed variation of elastic constants with aluminium substitution has been explained on the basis of the strength of interatomic bonding. The applicability of heterogeneous metal mixture rule for estimating elastic constants and transition temperature has been tested.     

Atomic pair distribution function at the Brazilian Synchrotron Light Laboratory: application to the Pb1–xLaxZr0.40Ti0.60O3 ferroelectric system

This work reports the setting up of the X‐ray diffraction and spectroscopy beamline at the Brazilian Synchrotron Light Laboratory for performing total scattering experiments to be analyzed by atomic pair distribution function (PDF) studies. The results of a PDF refinement for Al₂O₃ standard are presented and compared with data acquired at a beamline of the Advanced Photon Source, where it is common to perform this type of experiment. A preliminary characterization of the Pb_1–xLa_xZr_0.40Ti_0.60O₃ ferroelectric system, with x = 0.11, 0.12 and 0.15, is also shown.     

Luminescence and laser transition studies of Dy:K-Mg-Al fluorophosphate glasses

Dysprosium ion doped fluorophosphate glasses with compositions of PKMAFDy: (56−x/2) P₂O₅+17K₂O+8Al₂O₃+(15−x/2)MgO+4AlF₃+xDy₂O₃ (x=0.01, 0.05, 0.1, 1.0 and 2.0 mol%) have been prepared by melt quenching technique and are characterized by optical absorption, emission spectra and fluorescence lifetime measurements. The observed bands in the absorption spectrum are analysed by using free-ion Hamiltonian (H_FI) model. The Judd-Ofelt (JO) analysis has been performed and the intensity parameters (Ω_λ, λ=2, 4, 6) have been evaluated that are used to predict radiative properties. From emission spectra, the effective bandwidth (Δλ_eff) and the stimulated emission cross-section (σ(λ_p)) were evaluated. The fluorescence decay from the ⁴F_9/2 level of Dy³⁺ ions have been measured by monitoring the intense ⁴F_9/2→⁶H_13/2 transition (573 nm). The lifetimes (τ) are found to decrease with increasing concentration due to concentration quenching. The decay curves are single exponential for lower concentrations and gradually changes to non-exponential for higher concentrations. The non-exponential decay curves are well fitted to the Inokuti-Hirayama (IH) model for S=6 which indicates that the energy transfer between the donor and acceptor is of dipole-dipole type. The systematic analysis on decay measurements reveals that the energy transfer mechanism strongly depends on concentration as well as glass composition.     

Thermal conductivity of single-crystal ZrO2-Y2O3 solid solutions in the temperature range 50–300 K

The thermal conductivity of ZrO_2−x Y₂O₃ single crystals (x = 0.5, 1.5, 2.0, 2.5, 3.0, 8.0 mol %) has been studied experimentally in the temperature range 50–300 K. The influence of high-temperature annealings on the thermal conductivity has been analyzed.     

Etude des proprietes structurales et electriques de la solution solide Pb1−xBixOxF2−x

An investigation of the PbF₂BiOF system at 600°C has allowed to isolate a Pb_1-xBi_xO_xF_2-x solid solution for x ? 0.67 the structure is of fluorite type and for 0.67 < × ? 0.80 it presents a fluorite derived rhombohedral distortion. The ionic conductivity has been examined. For × ? 0.50 Pb_1-xBi_xO_xF_2-x has electrical performances of the same order than those of the best so far known fluorides. Bragg neutron diffraction investigations have been carried out in order to account for the electrical behavior of the cubic solid solution (0 < x ? 0.67). The distribution of the anions between normal and interstitial sites is given as a function of substitution rate. The structure has been described on the basis of local BiOF-type clusters. The formation of isolated antiprisms BiX₈ (X = O,F) allows to understand the evolution of the electrical properties.     

Pb1－xMnxSe薄膜的光学特性

采用分子束外延的方法在BaF₂(111)衬底上制备出了高质量的Pb_1-xMn_xSe(0≤x≤0.0681)薄膜.X射线衍射结果表明，Pb₁-xMn_xSe薄膜为立方相NaCl型结构，没有观察到MnSe相分离现象，薄膜的取向为平行于衬底(111)晶面.晶格常数随着Mn含量的增加逐渐减小，Mn含量由Vegard公式得到.通 关键词： 1-xMn_xSe外延薄膜')" href="#">Pb_1-xMn_xSe外延薄膜 透射光谱 带隙 折射率