Preparation,thermal stability and magnetic properties of new AgY1−xGdx(WO4)2 ceramic materials

Polycrystalline samples of α-AgY_1−xGd_x(WO₄)₂ with x=0, 0.005, 0.01, 0.025, 0.05, 0.1, 0.2, and 1 have been prepared by a solid state reaction method and the influence of Gd³⁺ substitution for Y³⁺ on microstructure, thermal and magnetic properties was investigated. The X-ray diffraction analysis showed the phases to crystallize in the monoclinic symmetry, space group C2/m. A reversible monoclinic to tetragonal phase transition occurs in AgY_1−xGd_x(WO₄)₂ and strongly depends on Gd³⁺ ion concentration. Electron paramagnetic resonance (EPR) spectra of Gd³⁺ ions showed non-monotonous dependence of interaction strength on gadolinium concentration. Magnetic measurements showed paramagnetic behavior and strong increase of magnetic moment as the yttrium content decreases.     

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.     

New vacancied and Gd3+-doped lead molybdato-tungstates and tungstates prepared via solid state and citrate-nitrate combustion method

New Pb_1-3x⌷_xGd_2x(MoO₄)_1-3x(WO₄)_3x (0<x≤0.1774) and Pb_1-3x⌷_xGd_2xWO₄ (0<x≤0.1154, ⌷ denotes vacancy) solid solutions were synthesized via solid state reaction route and citrate-nitrate combustion method. XRD and SEM data showed that as-prepared ceramics crystallize in the tetragonal scheelite type symmetry (space group I4₁/a) with the crystallite size varying between ~10 and ~40 µm (solid state method) or ~500 nm and 2 µm (combustion synthesis). A change in lattice constants (a and c), lattice parameter ratio c/a and progressive deformation of MoO₄/WO₄ tetrahedra with an increase of Gd content was observed. The melting point of each Gd-doped sample is lower than the melting point of adequate scheelite matrix. Ceramics under study are insulators with indirect band gap (E_g)>3 eV. EPR investigation revealed a difference among spectra obtained for varied gadolinium content, whereas synthesis method has no influence on EPR results.     

Spin-orbit coupling in manganese doped calcium molybdato-tungstates

The manganese doped calcium molybdato-tungstates with the formula of Ca_1-xMn_x(MoO₄)_0.50(WO₄)_0.50 (x = 0.01, 0.03, 0.05, 0.10, 0.125, and 0.15) were successfully obtained by two-step synthesis using in both steps a solid state reaction route. All ceramics show scheelite-type tetragonal structure with space group I4₁/a. The electrical and magnetic studies within the temperature range of 2–300 K showed a weak p-type electrical conductivity and the paramagnetic state of Mn-doped ceramic materials. With increasing Mn content in samples under study, a change in the short-range interactions from ferromagnetic to antiferromagnetic as well as an increase in the orbital contribution to the magnetic moment, resulting in a strong spin-orbit coupling, were observed. The Brillouin procedure was used to estimate the Landé factor.     

Perovskite structure and low frequency relaxor- like- dielectric response of (Sr,Ce)TiO3 solid solution

Phase composition, structure stability, cations valance state, and relaxor-like-dielectric behavior of Sr_(1–3/2x)Ce_xTiO₃ (SCT, x = 0.3, 0.4) solid solution were investigated systematically. The Sr_(1–3/2x)Ce_xTiO₃ samples appear to be single phase within the detection limits of the technique, whereas the solid solution exhibits the higher angle doublet and triplet peak splitting associated with (200) and (321). X-ray photoelectron spectroscopy (XPS) analysis showed that the Ce substitution induces change in the cations valance state upon oxygen vacancies formation. The system exhibits features of low-frequency dependent relaxor-like-dielectric behavior rather than sharp frequency-independent anomalies. Besides this, two kind of relaxations were detected in the temperature range ≥ 350 °C. According to the electric modulus and ac conductivity analysis, the relaxor-like-dielectric behavior results from the long-range conduction associated with ionized vacancies and mixed state of Ti^3+/4+ and Ce^3+/4+ cations.     

Study of polar relaxation processes in Sr(1−1.5x)LaxTiO3 ceramics by using field-induced thermally stimulated currents

Polar relaxation processes in Lanthanum doped SrTiO₃ (STO) ceramics, with general formulae Sr_(1−1.5x)La_xTiO₃, were studied by undertaking field-induced thermally stimulated currents measurements below room temperature.The experimental results obtained for doped ceramic (x = 0.0133) were analysed by using dipolar and space-charge relaxation thermally stimulated depolarization currents (TSDC) models in order to determine the nature of the relaxation processes involved.Our results reveal the existence of different relaxation processes in the temperature range 60–300 K. Whereas at low temperature, a relaxation mechanism of a dipolar type was disclosed within the temperature interval centred around 100 K, a space-charge relaxation process could be identified in the temperature range 120–300 K. The temperature dependence of the relaxation parameters will be also discussed in detail.     

Dielectric relaxations in fine-grained SrTiO3 ceramics with Cu and Nb co-doping

SrTi_1?3x(Cu_xNb_2x)O₃ (x?=?0.05, 0.1, 0.15 and 0.2) ceramics were synthesized using a solid-state reaction method at 1400?°C in air. Their structures, valence states, conduction mechanisms and dielectric properties were investigated in detail. Fine grains and vibration modes of the samples related to the doping effects were observed. A distorted pseudo-cubic structure was confirmed by XRD and Raman spectroscopy. All ceramics exhibited a colossal dielectric constant and dielectric relaxations. The dielectric constant in the low- and high-frequency ranges for x?=?0.05 and 0.1 was ascribed to the contribution of the grain boundary and grain, respectively, and the electrode polarization was significant at x?=?0.15 and 0.2. The dielectric relaxation peaks obeyed the T^?1/4 law at low temperature for all samples, confirming the polaron relaxation process. The electric modulus analysis confirmed that low-temperature dielectric relaxation was related to the grain response, and the electric conduction exhibited the same behavior with the dielectric relaxation. The variable-range-hopping conduction indicated a highly distorted structure and the localization of carriers in SrTi_1?3x(Cu_xNb_2x)O₃, which was consistent with the XRD and Raman results. The mixed-valence structure of Cu was identified by XPS, and the polaron hopping between the mixed-valence Cu ions was supposed to be responsible for the dielectric relaxation and electric conduction.     

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.     

Synthesis and lithium ionic conductivity of Li3−2x(In1−xZrx)2(PO4)3 (0≦x≦0·20)

Lithium ion conductors, Li_3−2x(In_1−xZr_x)₂(PO₄)₃ (0≦x≦0·20), were synthesized by a solid state reaction. A superionic conductive phase of Li₃In₂(PO₄)₃ was stabilized down to room temperature, assisted by the substitution of Zr⁴⁺ for In³⁺ sites of Li₃In₂(PO₄)₃. TG-DTA analysis indicated no phase transition in the samples with x superior to 0·05. The substituted samples showed much higher ionic conductivity by a couple of magnitude than that of Li₃In₂(PO₄)₃. In particular, the highest conductivity at room temperature was 1·42×10⁻⁵ Scm⁻¹ for Li_2·8(In_0·9Zr_0·1)₂(PO₄)₃. Thin films with the composition of Li_2·8(In_0·9Zr_0·1)₂(PO₄)₃ was prepared by a sol–gel method. A coating solution was made from lithium isopropoxide, indium isopropoxide, zirconium isopropoxide and diphosphorus pentaoxide. Well crystallized films were obtained on silicon dioxide and quartz glass substrates by dropping the coating solution, followed by firing over 873 K. In the temperatures above 473 K the lithium ionic conductivity of the film was slightly higher than that of sintered samples prepared by the solid state reaction at 1373 K.     

Effects of sintering method and BiFeO3 dopant on the dielectric and ferroelectric properties of BaTiO3–BiYbO3 based solid solution ceramics

(0.95–x) BaTiO₃–0.05 BiYbO₃–x BiFeO₃ (x?=?0, 0.01, 0.02, and 0.04) (abbreviated as (0.95–x) BT–0.05 BY–x BFO) ceramics were fabricated by conventional sintering (CS) and microwave sintering (WS) methods. Effects of sintering method and BFO dopant on the microstructure and electric properties of (0.95–x) BT–0.05 BY–x BFO ceramics were comparatively investigated. X-ray diffraction showed that all CS and WS samples presented a single perovskite phase. It was also found that WS ceramics possessed denser microstructure and finer grains compared to CS samples as indicated by the surface morphology characterization. Dielectric measurements revealed that all samples exhibited the weak relaxation behavior; however, the degree of relaxation behavior of BT–BY based ceramic could be strengthened by addition of BFO and by WS method. Moreover, the temperature and frequency stability could be improved with doped BFO. The density of 0.93BT–0.05BY–0.02BFO ceramic was found to be the largest while that of 0.94BT–0.05BY–0.01BFO ceramic was the smallest, thus, the dielectric constant of 0.93BT–0.05BY–0.02BFO was significantly larger than that of 0.94BT–0.05BY–0.01BFO and 0.94BT–0.05BY–0.04 BFO ceramics. minimum dielectric constant of (0.95–x) BT–0.05 BY–x BFO ceramic was obtained at x?=?0.01. Ferroelectric measurements indicated that all samples showed the slim hysteresis loop. The remnant polarization (P_r) and coercive field (E_C) of (0.95–x) BT–0.05 BY–x BFO ceramics first decreased and then increased with increasing x，the minimum values were obtained at x?=?0.01. Moreover, P_r and E_C of WS ceramics were slightly larger than those of CS ceramics, indicating that higher density and larger grain sizes contributed to enhancing the ferroelectric characteristic. These findings indicate that addition of moderate amount of BFO and use of WS technique can strengthen the degree of relaxation behavior and improve the ferroelectric properties of BT–BY based ceramics.     

Effect of Ca2+ site substitution on structural,optical, electrical and magnetic properties in Nd3+ and Mn2+-co-doped calcium molybdato-tungstates

Ca_1-3x-yMn_y[]_xNd_2x(MoO₄)_1-3x(WO₄)_3x molybdato-tungstates (? denotes vacant sites) were successfully synthesized by high-temperature solid-state reaction. New materials crystallize in scheelite-type structure within whole homogeneity range of solid solution (x ≤ 0.2000 and y = 0.0200). Morphological features and particle size distribution were investigated by SEM and laser diffraction methods, respectively. Spectroscopic measurements in the UV–vis range was carried out to determine optical direct band gap (E_g), Urbach energy (E_U) and confirmation of structural disorder. Refractive index (n) was calculated using four different models. Magnetic studies revealed paramagnetic behavior with long-range ferrimagnetic and short-range antiferromagnetic interactions. New materials showed weak n-type electrical conductivity and thermoelectric power factor (S²σ) that strongly depends on Nd³⁺ ions content. Dielectric parameters, i.e. relative permittivity (ε_r) and energy loss (tanδ) are insignificantly dependent on Nd³⁺ ions concentration. These effects were considered in terms of structural defects, thermal activation of charge carriers, and the Maxwell–Wagner polarization.     

A reddish orange stoichiometric phosphor LiEu(PO3)4 for white light-emitting diodes

Stoichiometric phosphors LiGd_1−xEu_x(PO₃)₄(x=0, 0.2, 0.4, 0.6, 0.8, 1.0) were synthesized via traditional solid state reactions. The X-ray powder diffraction measurements show that all prepared samples are isostructural with LiNd(PO₃)₄. Eu³⁺ doped phosphors can emit intense reddish orange light under the excitation of near ultraviolet light from 370 to 410 nm. The strongest two at 591 and 613 nm can be attributed to the transitions from excited state ⁵D₀ to ground states ⁷F₁ and ⁷F₂, respectively. The typical chromaticity coordinates (x=0.620, y=0.368) of Eu³⁺ doped phosphors are in red area. The recorded absorbance spectra indicate that there is effective absorbance in the near UV region for all Eu³⁺ doped samples. Present research indicates that LiGd_1–xEu_x(PO₃)₄ is a promising phosphor for white light-emitting diodes.     

Synthesis and characterization of quarternary Ti3Si(1−x)AlxC2 MAX phase materials

Ti₃Si_(1−x)Al_xC₂ (x=0–1) quarternary MAX phase materials were prepared by spark plasma sintering of TiC, Ti, Si and Al powder mixtures at 1200 °C. Effect of Al addition on lattice parameters, density and hardness were investigated. Impurities are limited to binary phases of TiC and Ti₅Si₃. No multinary compound other than Ti₃Si_(1−x)Al_xC₂ can be detected. TiC exists as impurity in all samples and trace amount of Ti₅Si₃ can be detected in Samples x=0.1–0.6. Oxidation of Al cannot be avoided although all sintering were performed under vacuum and trace amount of Al₂O₃ can be found in all samples with Al addition. Experimental results show that the lattice parameters a and c increase linearly with increasing Al content for x=0–1. The lattice variations are strongly anisotropic and follow Vegard׳s law. Both density and hardness decrease as Al content increases. The linear variation of lattice parameters, d spacings of crystalline faces and density against Al concentration suggest that continuous solid solutions of Ti₃Si_(1−x)Al_xC₂ (x=0–1) may have been formed between Ti₃SiC₂ and Ti₃AlC₂.     

The electrical properties of (Ba,Ca, Ce,Ti, Zr)O3 thermistor for wide-temperature sensor architecture

A series of new negative temperature coefficient (NTC) thermal materials based on (Ba_0.85Ca_0.15)_1-xCe_x/2(Zr_0.1Ti_0.9)O₃ (0.00 ≤ x ≤ 0.20) ceramics were synthesized by a solid-state method. X-ray diffraction, scanning electron microscope and X-ray photoelectron spectroscopy were used to demonstrate the crystal structure, morphology, and composition of the (Ba_0.85Ca_0.15)_1-xCe_x/2(Zr_0.1Ti_0.9)O₃ ceramics, which were composed of solid solution based on the BaTiO₃ phase. The average grain size of doped ceramic samples experienced the process of first decreasing and then increasing. The doping of Ce has reduced the sintering temperature. The temperature-dependent resistance analysis revealed that with the change of doping amount x, the thermal constant B_300/1200 (1.21 × 10⁴–1.13 × 10⁴ K) and the activation energy E_a300/1200 (0.9777–1.0471eV) was initially increased to maximum values at x = 0.05, followed by the decreasing when x > 0.05. It has been established that the concentration of oxygen vacancies is affected by the transition between Ce⁴⁺ and Ce³⁺ provided by high levels of Ce doping. (Ba_0.85Ca_0.15)_1-xCe_x/2(Zr_0.1Ti_0.9)O₃ ceramics exhibited excellent negative temperature characteristics in the range of 300–1200 °C. Moreover, the temperature resistance linearity was improved after samples were aged. Hence, the (Ba_0.85Ca_0.15)_1-xCe_x/2(Zr_0.1Ti_0.9)O₃ ceramics were regarded as a promising material for high-temperature NTC thermistors in a wide temperature range.     

Temperature stability and phase transition of Li0.02(KxNa1−x)0.98NbO3 ceramics

Li_0.02(K_xNa_1?x)_0.98NbO₃(x = 0.35–0.55) ceramics were prepared using the conventional solid state sintering method. The thermal behaviors of Li-modified (K_xNa_1?x)NbO₃ ceramics were investigated from ?30 to 150 °C, and the effect of Na/K ratio in (K_xNa_1?x)NbO₃ ceramics on thermal behavior and electrical properties was also studied. In the case of Li_0.02(K_xNa_1?x)_0.98NbO₃ ceramics with 0.5 wt.% ZnO, the transition temperature was sharply decreased because of a phase transition as the composition range of x was 0.425–0.475. From the results of the temperature dependence of piezoelectric properties, it is assumed that the Na-rich phase is less stable than the K-rich phase for temperature change.     

Mechanochemical synthesis of nanoparticulate ZnO–ZnWO4 powders and their photocatalytic activity

In this study, mechanochemical reaction systems with H₂WO₄ as a precursor were investigated for the synthesis of nanoparticulate powders of WO₃, ZnWO₄, and dual-phase (ZnWO₄)_x(ZnO)_1–x. The objective was to establish whether mechanochemical processing can be used to manufacture high activity photocatalysts in the ZnO–WO₃ system. Milling and heat treatment of H₂WO₄ + 12NaCl was found to result in the formation of irregularly shaped platelets of a sodium tungstate rather than nanoparticles of WO₃. Powders of single-phase ZnWO₄ and dual-phase (ZnWO₄)_x(ZnO)_1–x were successfully synthesised by incorporating H₂WO₄ into the ZnCl₂ + Na₂CO₃ + 4NaCl reactant mixture. The photocatalytic activity of these powders was evaluated using the spin-trapping technique with electron paramagnetic resonance spectroscopy. It was found that the photocatalytic activity decreased with the ZnWO₄ content. This decrease in activity was attributed to the larger average particle size of the ZnWO₄ component compared to the ZnO, which reduced the surface area available for interfacial transfer of the photogenerated charge carriers.     

Dielectric Properties and Impedance Analysis of K0.5Na0.5NbO3–Ba2NaNb5O15 Ceramics with Good Dielectric Temperature Stability

(1?x)(K_0.5Na_0.5)NbO₃–xBa₂NaNb₅O₁₅ [(1?x)KNN–xBNN, 0 ≤ x ≤0.1] ceramics were prepared by solid‐state reaction method. X‐ray diffraction analysis of the ceramics revealed that the crystal structure changed from orthorhombic to rhombohedral with increasing BNN content. Dielectric measurement showed that the ceramics exhibited good dielectric temperature stability over a wide temperature range. Basic mechanisms of the conduction and relaxation processes have been investigated using impedance spectroscopy analyses. It was concluded that the conduction and relaxation processes were thermally activated and oxygen vacancies were the possible ionic charge carriers at higher temperatures.     

Grain size control of lead-free Li0.06(Na0.5K0.5)0.94NbO3 piezoelectric ceramics by Ba and Ti doping

In this study, Ba- and Ti-doped Li_0.06(Na_0.5K_0.5)_0.94NbO₃ [(1 ? x)Li_0.06(Na_0.5K_0.5)_0.94NbO₃–xBaTiO₃ (x = 0–0.07)] ceramics were prepared by using conventional solid state reaction method, and the microstructure and electric properties of these samples were investigated. The grain size distribution of non-doped Li_0.06(Na_0.5K_0.5)_0.94NbO₃ ceramics was relatively wide. The microstructure was composed of grains ranging 1.1–5.0 μm in size. However, with increasing Ba and Ti content, the grain size distribution became narrow and the average grain size decreased from 2.0 to 0.9 μm in size. In particular, the microstructure of x = 0.07 sample was composed of grains ranging 0.5–2.2 μm in size. As a result, the frequency dispersion of dielectric constant for the (1 ? x)Li_0.06(Na_0.5K_0.5)_0.94NbO₃–xBaTiO₃ (x = 0–0.07) ceramics was reduced and the mechanical quality factor Q_m was enhanced with increasing Ba and Ti content.     

Bond characteristics,sintering behavior and microwave dielectric properties of Ce2[Zr1−x(Ca1/3Sb2/3)x]3(MoO4)9 ceramics

Ce₂[Zr_1?x(Ca_1/3Sb_2/3)_x]₃(MoO₄)₉ (CZ_1?x(CS)_xM) (x = 0.02–0.10) ceramics were prepared by the conventional solid-state reaction method. The correlations between the chemical bond parameters and microwave dielectric properties were calculated and analyzed by using the Phillips–Van Vechten–Levine (P–V–L) theory. Phase composition and microstructures were evaluated by scanning electron microscopy and X-ray diffraction patterns. Lattice parameters were obtained by Rietveld refinements based on XRD data. Excellent properties for Ce₂[Zr_0.96(Ca_1/3Sb_2/3)_0.04]₃(MoO₄)₉ ceramic sintered at 775 °C: ε_r = 10.68, Q×f = 85,336 GHz and τ_f = ?7.58 ppm/°C were achieved.     

Magnetic properties of ferrites-cobaltites Bi1 − x La x Fe1 − x Co x O3 (1.0 ≥ x ≥ 0.7) with a perovskite structure

The molar magnetic susceptibility (χ_mol) of Bi_{1 ? x} La _x Fe_{1 ? x} Co _x O₃ solid solutions (x = 1.0, 0.9, 0.8, or 0.7) with a crystal structure of rhombohedrally distorted perovskite (R $\bar 3$ c) has been investigated in the temperature range of 5–300 K in a 0.86 T magnetic field. In the temperature range where χ_mol depends on temperature T according to the Curie-Weiss law, the resulting effective magnetic moments of Fe³⁺ and Co³⁺ ions ( $\mu _{eff,Fe^{3 + } ,Co^{3 + } ,} \mu _{eff,Fe^{3 + } } $ and $\mu _{eff,Co^{3 + } } $ ) have been determined for the solid solutions under study. Fe³⁺ ions in the solid solutions have been found to be in the mixed intermediate spin (IS) and high spin (HS) states ( $\mu _{eff,Fe^{3 + } } $ is 4.26μ_B and 4.68μ_B for the temperature range of 5–100 and 150–300 K, respectively). It is shown that 8% Co³⁺ ions in LaCoO₃ at 5–19 K are in the paramagnetic IS state and they determine to a great extent the magnetic susceptibility. It is established that only 9% and 18% Co³⁺ ions in Bi_{1 ? x} La _x Fe_{1 ? x} Co _x O₃ solid solutions (x = 0.9 or 0.8) are in the paramagnetic IS state in the temperature ranges of 5–30 and 5–110 K, respectively, while the other ions are diamagnetic.