New scheelite-type Cd1−3x⌷xGd2x(MoO4)1−3x(WO4)3x ceramics – their structure,thermal and magnetic properties

Polycrystalline samples of scheelite-type Cd_1−3x⌷_xGd_2x(MoO₄)_1−3x(WO₄)_3x solid solution with limited homogeneity (0<x≤0.25) and cationic vacancies (denoted as ⌷) have successfully prepared by a high-temperature annealing of CdMoO₄/Gd₂(WO₄)₃ mixtures composed of 50.00 mol% and less of gadolinium tungstate. Initial reactants and obtained ceramic materials were characterized by XRD, simultaneous DTA–TG, and SEM techniques. A phase diagram of the pseudobinary CdMoO₄–Gd₂(WO₄)₃ system was constructed. The eutectic point corresponds to 1404±5 K and ~70.00 mol% of gadolinium tungstate in an initial CdMoO₄/Gd₂(WO₄)₃ mixture. With decreasing of Gd³⁺ content in a CdMoO₄ framework, the melting point of Cd_1−3x⌷_xGd_2x(MoO₄)_1−3x(WO₄)_3x increases from 1406 (x=0.25) to 1419 K (x=0.0833), and next decreases to 1408 K (x=0). EPR method was used to identify paramagnetic Gd³⁺ centers in Cd_1−3x⌷_xGd_2x(MoO₄)_1−3x(WO₄)_3x for different values of x parameter as well as to select biphasic samples containing both Cd_0.25⌷_0.25Gd_0.50(MoO₄)_0.25(WO₄)_0.75 and Gd₂(WO₄)₃.     

Thermal and magnetic properties of new scheelite type Cd1−3x□xGd2xMoO4 ceramic materials

The limited scheelite type Cd_1−3x□_xGd_2xMoO₄ solid solution, where 0 < x ≤ 0.25 and □ are cationic vacancies have been successfully synthesized by high-temperature annealing of CdMoO₄/Gd₂(MoO₄)₃ mixtures composed of 50.00 mol.% and less of Gd₂(MoO₄)₃. The obtained materials as well as CdMoO₄ and Gd₂(MoO₄)₃ were characterized by powder XRD, DTA–TG, DSC, and SEM techniques. A phase diagram of the pseudobinary CdMoO₄–Gd₂(MoO₄)₃ system was constructed. The eutectic point corresponds to 1350 ± 5 K and ∼70.00 mol.% of Gd₂(MoO₄)₃ in an initial CdMoO₄/Gd₂(MoO₄)₃ mixture. With decreasing of Gd³⁺ amount in the crystal lattice of CdMoO₄, a melting point of the Cd_1−3x□_xGd_2xMoO₄ solid solution increases from 1351 (x = 0.25) to 1408 K (x = 0). EPR method was used to identify the paramagnetic Gd³⁺centers in Cd_1−3x□_xGd_2xMoO₄ for different values of x parameter as well as to select biphasic samples containing both Cd_0.2500□_0.2500Gd_0.5000MoO₄ and Gd₂(MoO₄)₃.     

Influence of cobalt and sintering temperature on structure and electrical properties of BaZr0.05Ti0.95O3 ceramics

Pure and Co-modified BaZr_0.05Ti_0.95O₃ ceramics were fabricated by the traditional solid state reaction technique. The influence of cobalt and sintering temperature on structure, dielectric, ferroelectric properties and diffuse phase transition of BZT ceramics were investigated systematically. 1300 °C was the optimal sintering temperature for BZT ceramics. The solid solubility limit of Co ions in BZT matrix was determined to be 0.4 mol%. The introduction of a moderate amount of Co ions was believed to benefit the microstructure development and make the grain size more uniform. Compared with undoped counterparts, 0.4 mol% Co-modified ceramics showed equivalent ferroelectric properties with a high remnant polarization (P_r=9.6 μC/cm²) and a low coercive field (E_c=0.21 kV/mm). Besides these, a relative high dielectric coefficient (ε_r=2030) and a low dielectric loss (tan δ=1.85%) were also obtained on this composition. The degree of diffuse phase transition was enhanced by the addition of Co ions. The related mechanism of the diffused phase transition behavior was discussed.     

Sol–gel synthesis of Nd-doped BiFeO3 multiferroic and its characterization

Neodymium doped bismuth ferrite (BiFeO₃, BFO) nanoparticles were successfully synthesized by a facile sol–gel route. The influence of annealing temperature, time, Bi content and solvent on the crystal structure of BFO was studied. Results indicated that the optimum processing condition of BFO products was 550–600 °C/1.5 h with excess 3–6% Bi and ethylene glycol as solvent. On the other hand, Nd³⁺ ion was introduced into the BFO system and the effect of Nd³⁺ concentration on the structure, magnetic and dielectric properties of BFO were investigated. It was found that the magnetization of BFO was enhanced significantly with Nd³⁺ substitution, being attributed to the suppression of the spiral cycloidal magnetic structure led by the crystal structure transition. Furthermore, with increasing Nd³⁺ content, the dielectric constant was found to decrease while the dielectric loss was enhanced, which was mainly due to the hoping conduction mechanism with the reduction of oxygen vacancies.     

New vacancied and Dy3+-doped molybdates – Their structure,thermal stability,electrical and magnetic properties

Microcrystalline samples of new Cd_1–3xDy_2x⌷_xMoO₄ solid solution with limited homogeneity (0<x≤0.2222) and cationic vacancies (denoted as ⌷) were prepared by a high-temperature solid state reaction. The XRD data and SEM analysis showed that as-prepared ceramics crystallize in the tetragonal scheelite type symmetry (space group I4₁/a) with the crystallite size varying between ~2 and ~20 µm. A systematic change in lattice constants, a and c, as well as in lattice parameter ratio c/a with an increase of Dy content was observed. Dy-doped molybdates are paramagnets with the antiferromagnetic short-range interaction and spin-orbit coupling. Optical and electrical investigations proved Cd_1–3xDy_2x⌷_xMoO₄ solid solution to be in the insulating state of E_g>3 eV at room temperature and the thermally activated conduction of the Arrhenius-type above 350 K. Moreover, the I-V characteristics provided the evidence of symmetrical and non-linear behavior typical of charge carrier emission weakly induced by the temperature. Relative dielectric permittivity ε_r below 10 as well as loss tangent tanδ below 0.15 do not substantially depend both on the temperature in the range of 76–400 K and the frequency in the range of 5·10²–1·10⁶ Hz. These results are interpreted in the framework of the acceptor and donor vacancy centers.     

Effect of Zr substitution on structural,magnetic, and optical properties of Bi0.9Dy0.1Fe1−xZrxO3 multiferroic ceramics prepared by rapid liquid phase sintering method

Zr substituted Bi_0.9Dy_0.1Fe_1−xZr_xO₃ (x=0.03, 0.06 and 0.10) multiferroic ceramics were synthesized by rapid liquid phase sintering technique to improve its multiferroic properties. Rietveld structural refinement of XRD patterns and Raman spectra revealed a partial structural phase transition from rhombohedral (R3c) to biphasic structure (R3c+P4mm) on codoping. The substitution of larger ionic radii and higher valence Zr⁴⁺ ions at Fe-site leads to decrease in the grain size as a result of charge compensation at Fe site. The weak ferromagnetic behavior were observed in all samples along with maximum M_r value of 0.159 emu/g for x=0.03 concentration, which is also endorsed by second order Raman modes. The distortion in FeO₆ octahedra due to Zr substitution leads to splitting of electronic bands of 3.2 eV into multiplets, which in turn reduced the optical band gap value in the range of 2.06–2.10 eV for all samples.     

Electrical properties and light up conversion effects in Bi3.79Er0.03Yb0.18Ti3−xWxO12 ferroelectric ceramics

Modified ceramic compositions of Bi_3.79Er_0.03Yb_0.18Ti_3−xW_xO₁₂ with fixed Er and Yb content, and a varying W content (x=0.0, 0.01, 0.03, 0.06 and 0.10) are prepared. The site selectivity of Er³⁺, Yb³⁺, and W⁶⁺ cations is analyzed, and their influence on the electrical and light up conversion properties is studied. Formation of single phase orthorhombic structure is confirmed with enhanced grain growth up to x=0.03, and for (x≥0.04–0.10) the grain growth is inhibited, and the orthorhombic distortion is relaxed. Raman spectroscopy reveals W⁶⁺ cation substitutes preferentially at the B-site replacing Ti⁴⁺ ions in the Bi₄Ti₃O₁₂ lattice structure. Increasing W⁶⁺ donor concentration reduces the conductivity effects by lowering the oxygen vacancies. Reduced dielectric losses (tan δ=0.003) and dispersion with frequency in the range (10⁻²–10 Hz) are observed, and improvements in the remnant polarization (2P_r=28.86 μC/cm²) are seen up to an optimum content of x=0.03. At higher W content (x>0.03), the properties tend to degrade due to structural relaxation and microstructural changes. Up conversion photoluminescence (UC-PL) under 980 nm excitation shows strong emission in the green and red bands due to enhanced crystal field around the Er³⁺ ions for an optimum W content of x=0.06. A weak blue emission band around (~492 nm) is observed by cooperative emission (CE) due to radiative relaxation of an excited Yb–Yb pair from a virtual level. Variation of UC emission intensity with pump-power confirms a two-photon mechanism for the up conversion process.     

Separation and determination of cadmium in water by foam column prior to inductively coupled plasma optical emission spectrometry

The sorption profile of cadmium (II) ions from aqueous iodide media onto procaine hydrochloride (PQ⁺·Cl⁻) treated polyurethane foams (PUFs) solid sorbent was studied. PQ⁺·Cl⁻ treated PUFs solid sorbent was found suitable and fast for Cd²⁺ uptake as [CdI4]_aq²⁻. Thus, removal of Cd²⁺ at trace levels by the sorbent packed columns was achieved. The sorbed Cd²⁺ species onto packed column were recovered with HNO₃ (10.0 mL, 1.0 mol L⁻¹) prior determination by inductively coupled plasma-optical emission spectrometry (ICP-OES). Plot of Cd²⁺ ions concentration was linear in the range 0.05–15 μg L⁻¹. The limits of detection and quantification of Cd²⁺ were found 0.01 μg L⁻¹ and 0.033 μg L⁻¹, respectively. Such limits could be improved to lower values by retention of Cd²⁺ species from large sample volumes of the aqueous phase at the optimized conditions. The relative standard deviation of the packed column for the extraction and recovery of standard aqueous solutions (0.1 L) containing 1.0 and 5.0 μg L⁻¹ (n = 3) of Cd²⁺ ions at flow rate of 5.0 mL min⁻¹ were 1.98 and 2.9%, respectively. The method was validated by analysis of Cd in certified reference materials (CRMs) IAEA-Soil-7 and TMDW water and wastewater samples.     

Crystal structure and electrical properties of (Li,Ce, Nd)-multidoped CaBi2Nb2O9 high temperature ceramics

(Li, Ce, and Nd)-multidoped CaBi₂Nb₂O₉ (CBN) Aurivillius phase ceramics were prepared via a conventional solid-state sintering route. The crystal structure including bond lengths and bond angles, microstructure, dielectric constant, DC resistivity, and piezoelectric properties were systematically investigated. Rietveld-refinements of X-ray results indicated that small quantity of (Li, Ce, Nd) doping (< 2.5 mol%) increases orthorhombic distortion, because of the smaller ionic radii of doping ions. However, orthorhombic distortion obviously decreased with increasing (Li, Ce, Nd) doping concentration from 5 to 25 mol%. The replacement of asymmetric A-site Bi³⁺ with 6s2 lone pair electrons by symmetric Li⁺, Ce³⁺ and Nd³⁺ ions decreased the orthorhombic distortion. The morphologies and electrical properties of sintered ceramics were tailored by the introducing (Li, Ce, Nd) multi-dopants. The improvement of piezoelectric properties of modified-CBN ceramics were attributed to decreasing grain sizes and morphotropic phase boundary (MPB). Ca_0.85(Li_0.5Ce_0.25Nd_0.25)_0.15Bi₂Nb₂O₉ (CBNLCN-15) ceramics had optimum properties, and d₃₃ and T_c values were found to be ~ 13.1 pC/N and ~ 900 °C, respectively.     

Electrical properties of CaBi4Ti4O15–Bi4Ti3O12 piezoelectric ceramics

CaBi₄Ti₄O₁₅–Bi₄Ti₃O₁₂ (CBT–BIT) ceramics were synthesized using a solid state reaction method. The X-ray diffraction (XRD) analysis revealed the existence of bismuth layered perovskite phase with orthorhombic crystal structure. High-resolution transmission electron microscopy (HRTEM) confirmed the alternate arrangement of CBT part and BIT part along c axis in the intergrowth structure. CBT–BIT ceramics showed excellent thermal stability of the dielectric loss (tan δ), but the relaxation of dielectric loss in the 100 Hz to 1 MHz frequency range had been observed. Meanwhile, an enhanced piezoelectric constant (d₃₃) value of 15 pC/N was observed without degradation even the temperature up to 650 °C. The dc resistivity (ρ_dc) of CBT–BIT performed a high value of 5.68×10¹⁴ (Ω cm) at room temperature (RT). In addition, the ρ_dc values of CBT–BIT within the temperature range of 100–450 °C were close to those of CBT and kept almost one hundred times higher than those of BIT.     

Preferential occupancy of metal ions in the hydroxyapatite solid solutions synthesized by hydrothermal method

The solid solutions in the systems of Ca-Cd HAp [Ca_10−xCd_xHAp (x = 0–10)], Ca-Sr HAp [Ca_10−xSr_xHAp (x = 0–10)] and Ca-Pb HAp [Ca_10−xPb_xHAp (x = 0–10)], were successfully synthesized at 200 °C for 12 h under hydrothermal conditions. The site of the metal ions in the solid solutions was analyzed by the Rietveld method. The results of the Rietveld analysis indicated that the metal ions of Pb²⁺, Sr²⁺, and Cd²⁺ all preferentially occupied M (2) sites in the apatite structure. The preferential occupancy of the metal ions in M (2) sites were explained mainly by their ionic radius and electronegativity.     

Microstructure effect on dielectric Properties of MgO-doped BaTiO3–BiYO3 ceramics

MgO-doped 0.97BaTiO₃–0.03BiYO₃ (0.97BT–0.03BY) polycrystalline ceramics were prepared by the solid-state sintering method. Then the structural, dielectric and resistant properties were investigated as functions of MgO addition. Microstructure was studied using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive spectrometry (EDS). The results show that Bi³⁺, Y³⁺ and Mg²⁺ ions exhibit nonuniform distribution behavior in BT–BY ceramics, demonstrating the existence of a “core–shell” structure, which plays important roles in the capacitance-temperature characteristics, where 0.97BT–0.03BY with the addition 2.2–2.8 at% MgO meets the Electronic Industries Association (EIA) X8R (−55 to 150 °C, ΔC/C_25 °C=±15% or less) specification. Moreover, the fine-grained samples with core–shell structure show much higher bulk resistance than the coarse-grained samples over the studied temperature range, which is attributed to the higher proportion of grain boundaries and the lower concentration of the effective acceptor.     

Cd-composition induced effects on structure,optical and electrical properties of sputtered Zn1−xCdxO films

Zn_1−xCd_xO films with different Cd contents (0≤x≤1) were successfully deposited on quartz substrates by the direct current reactive magnetron sputtering and post-annealing techniques. It was found that structures, band gaps and electrical properties of the films can be tuned by changing Cd contents x. The Zn_1−xCd_xO film consists of wurtzite phase with highly (002)-preferred orientation at x from 0 to 0.2, mixture of wurtzite and cubic phases at x=0.5, and cubic phase with highly (200)-preferred orientation at x≥0.8. The band gap decreases from 3.25 eV at x=0 to 2.75 eV at x=0.2 for the wurtzite Zn_1−xCd_xO, and decreases from 2.52 eV at x=0.8 to 2.42 eV at x=1, which has a little change for cubic Zn_1−xCd_xO. In addition, Hall measurement results indicate that the influence of Cd content on the conduction behavior of Zn_1−xCd_xO films is significant. The chemical compositions and the bonding states of Zn_1−xCd_xO films were examined by X-ray photoelectron spectroscopy analysis.     

A comparative study of the magnetic and microwave properties of Al3+ and In3+ substituted Mg–Mn ferrites

The effect of substitution of diamagnetic Al³⁺ and In³⁺ ions for partial Fe³⁺ ions in a spinel lattice on the magnetic and microwave properties of magnesium–manganese (Mg–Mn) ferrites has been studied. Three kinds of Mg–Mn based ferrites with compositions of Mg_0.9Mn_0.1Fe₂O₄, Mg_0.9Mn_0.1Al_0.1Fe_1.9O₄, and Mg_0.9Mn_0.1In_0.1Fe_1.9O₄ were prepared by the solid-state reaction route. Each mixture of high-purity starting materials (oxide powders) in stoichiometric amounts was calcined at 1100 °C for 4 h, and the debinded green compacts were sintered at 1350 °C for 4 h. XRD examination confirmed that the sintered ferrite samples had a single-phase cubic spinel structure. The incorporation of Al³⁺ or In³⁺ ions in place of Fe³⁺ ions in Mg–Mn ferrites increased the average particle size, decreased the Curie temperature, and resulted in a broader resonance linewidth as compared to un-substituted Mg–Mn ferrites in the X-band. In this study, the In³⁺ substituted Mg–Mn ferrites exhibited the highest saturation magnetization of 35.7 emu/g, the lowest coercivity of 4.1 Oe, and the highest Q×f value of 1050 GHz at a frequency of 6.5 GHz.     

Molten salt synthesis and optical properties of Eu3+, Dy3+or Nd3+ doped NiNb2O6 columbite-type phosphors

Pure, Eu³⁺, Dy³⁺ or Nd³⁺-doped NiNb₂O₆ powders have been prepared by a molten salt synthesis method by using Li₂SO₄–Na₂SO₄ salt mixture as a flux at relatively low temperatures as compared to the solid state reaction method. X-ray diffraction patterns of pure NiNb₂O₆ samples indicated an orthorhombic single phase. For Eu³⁺-doped NiNb₂O₆ samples, the luminescence of Eu³⁺ was observed at 615 nm as red emission while Dy³⁺-doped NiNb₂O₆ showed yellow emission at 577 nm and Nd³⁺ doped sample exhibited a typical emission at 1064 nm varying with the Eu³⁺ or Nd³⁺ doping concentrations. These luminescence characteristics of the doped samples may be attributed to the energy transfer between rare earth ions and NiO₆ octahedral groups in the columbite structure.     

Investigation on the anti-reduction mechanism of Ti4+ in high dielectric constant system Ca0.9La0.067TiO3 by doping with Al2O3

Ca_0.9La_0.067TiO₃ (abbreviated as CLT) ceramics doped with different amount of Al₂O₃ were prepared via the solid state reaction method. The anti-reduction mechanism of Ti⁴⁺ in CLT ceramics was carefully investigated. X-ray diffraction (XRD) was used to analyze the phase composition and lattice structure. Meanwhile, the Rietveld method was taken to calculate the lattice parameters. X-ray photoelectron spectroscopy (XPS) was employed to study the valence variation of Ti ions in CLT ceramics without and with Al₂O₃. The results showed that Al³⁺ substituted for Ti⁴⁺ to form solid solution and the solid solubility limit of Al³⁺ is near 1.11 mol%. Furthermore, the reduction of Ti⁴⁺ in CLT ceramics was restrained by acceptor doping process and the Q × f values of CLT ceramics were improved significantly. The CLT ceramic doped with 1.11 mol% Al₂O₃ exhibited good microwave dielectric properties: ε_r = 141, Q × f = 6848 GHz, τ_f = 576 ppm/°C.     

Dy3+ doped LiCl–CaO–Bi2O3–B2O3 glasses for WLED applications

Dy³⁺ doped calcium bismuth borate glasses were synthesized in the composition range of xLiCl-(30 − x)CaO-20Bi₂O₃-50B₂O₃ + 1 mol% Dy₂O₃ (x = 0, 2, 5, 7, 10 and 15 mol%, LC0, LC2, LC5, LC7, LC10 and LC15 respectively) using conventional melt-quench technique. Broad XRD profiles confirmed non-crystalline nature of synthesized compositions. The compositional dependencies of structural changes (using FTIR spectra), thermal behavior (using DSC thermographs) and optical band gap (using UV–Vis–NIR spectra) were discussed. Photoluminescence (PL) excitation spectra recorded at 577 nm yielded six different excitation peaks belonging to Dy³⁺ ions. The PL emission spectra recorded at 451 nm were analyzed to extract different light emission parameters viz. Y/B ratio, color coordinates, correlated color temperature (CCT) following CIE 1931 chromaticity diagram. The emission colors were found to lie in white light region and lies very close to standard white light emission. The CCT of sample LC10 (5335 K) is closest to CCT of standard white light (5615 K) which depicted the optimized concentration of LiCl for application of these glasses in WLED application.     

Induced anisotropic behavior and enhanced electrical properties on hot-pressed strontium barium niobate ceramics

Sr_0.30Ba_0.70Nb₂O₆ (SBN30) ferroelectric ceramics were fabricated by conventional sintering (CS) and by hot-pressing sintering (HP) and their crystal structure, microstructure, dielectric, ferroelectric and pyroelectric properties were studied and compared. Preferred orientation of the HP ceramics was detected through X-ray diffraction. Dense microstructure virtually free of pores has been achieved in HP samples. Moreover, the HP samples manifested prominent anisotropy in electrical properties. Besides, the relative permittivity (ε_r), saturated polarization (P_s), pyroelectric coefficient (p) in the direction perpendicular to the pressing axis were much higher than those of the randomly oriented CS samples. The HP samples sintered under 200 MPa show excellent pyroelectric properties in the direction perpendicular to the pressing axis, with pyroelectric coefficient of 2.38×10⁻⁸C/cm²K and pyroelectric figure of merit of F_i=1.13 pm/V, F_v=1.89 m²/C and F_d=0.63 μPa^−1/2,which roughly triple the values obtained in CS samples. These results indicate that hot-pressing is a viable option for accessing single-crystal-like anisotropy as well as enhanced electrical properties in polycrystalline ceramics, thus unveiling the distinctive potential of HP SBN30 ceramics for infrared detector applications.     

Study of physical properties of cobalt substituted Pr0.7Ca0.3MnO3 ceramics

An investigation on single phase semiconducting polycrystalline Pr_0.7Ca_0.3MnO₃ and Pr_0.7Ca_0.3MnCo_0.1O₃ crystallized in the orthorhombic system with Pnma space group is reported. We found that σ_DC increases when introducing Co for T<110 K but for T>110 K, it decreases. Also, the contribution of hopping process in conduction mechanism was in agreement with the Jonscher law and Mott theory. Capacitance was extensively dependent on temperature and frequency. A dielectric transition was observed at T=150 K for the doped compound. The temperature dependence of dielectric permittivity is well described by Curie–Weiss law. The parameter of deviation from Curie–Weiss behavior to modified Curie–Weiss law is found to be ΔT_m=30 K. The substitution of Mn by Co was found to destroy the charge order state observed in the parent compound and to induce a ferromagnetic phase at low temperature. The cobalt-substituted sample exhibits a maximum value of magnetic entropy change |∆S^max|=3.2 J kg⁻¹ K⁻¹and a large relative cooling power with a maximum value of 301 J/kg under an applied field of 5 T. Technically, these large values make the prepared material very promising for magnetic refrigeration.     

A new helical coordination polymer constructed on flexible dicarboxylate ligand and CdII center: Structure and luminescence

A novel cadmium coordination polymer [Cd₂L₂(bpp)₂]_n (1) (H₂L = 1,3-adamantanediacetic acid and bpp = 1,3-di-4-pyridylpropane) was prepared under hydrothermal condition. The three-dimensional (3D) structure is built from the interlinking lamellars. The Cd^II ions are six- and seven-coordinated and linked by L^2? ligands, which results in the right- and left-handed helical chains. The layers are stacked with each other via bpp to generate a 3D architecture. At room temperature, the complex 1 shows the strong fluorescent emission bands in the solid state.