Proton conductivity and mobility in Sr-doped LaScO3 perovskites

In this work, we set out to investigate the electrical conductivity of single-phase and high-density La_1-xSr_xScO_3-δ (x = 0.05; 0.1) ceramics depending on temperature and рО₂ and рН₂О. The crystal structure of materials was characterized by XRD method. The samples show the structure of an orthorhombic perovskite with a Pnma space group. The unit cell volume increases along with the Sr concentration. The microstructure features of samples were investigated by SEM analysis. The transference numbers of protons and oxygen-ions were determined by the EMF (electromotive force) measurements in a gas concentration cell. In addition, the proton, oxygen-ion and hole conductivities were evaluated from the рО₂-dependencies of electrical conductivity at different humidity. The results obtained using both methods showed a good level of agreement. It is found that the partial conductivity of each charge carrier in La_1-xSr_xScO_3-δ increases along with an increase in the concentration of the Sr dopant from x = 0.05 to x = 0.1. The highest proton conductivity about 3 × 10^?2 S cm^?1 is achieved for La_0·9Sr_0.1ScO_3-δ at 800 °C. The mobility of proton defects increases with Sr concentration and reaches 2.5 × 10^?4 cm² V^?1 s^?1 at 800 °C for La_0·9Sr_0.1ScO_3-δ. Thus, La_0·9Sr_0.1ScO_3-δ should be considered as a promising proton-conducting electrolyte for various electrochemical devices, such as protonic ceramic fuel cells.     

Enhanced negative thermal expansion of (KMg)3+-substituted Fe2Mo3O12 ceramics

Herein, we report the solid-state synthesis of (KMg)_xFe_2-xMo₃O₁₂ (0 = x ≤ 1.5) ceramics. Phase composition, crystal structure, morphology, phase transition and thermal expansion behavior of the (KMg)_xFe_2-xMo₃O₁₂ ceramics were investigated by XRD, Raman, XPS, HRTEM, EDX, SEM, TMA and high-temperature XRD. Results indicate that (KMg)³⁺ dual-cations have successfully replaced Fe³⁺ in Fe₂Mo₃O₁₂ ceramics and single-phase monoclinic (KMg)_xFe_2-xMo₃O₁₂ ceramics were prepared for 0.25 = x ≤ 1. (KMg)³⁺ introduction can increase the density of (KMg)_xFe_2-xMo₃O₁₂ ceramics and effectively improve their negative thermal expansion (NTE) performance. In addition, the phase transition temperature (Tc) of Fe₂Mo₃O₁₂ was reduced from 508.1 °C to room temperature with the increase of (KMg)³⁺-substitution. Monoclinic KMgFeMo₃O₁₂ ceramics was observed to show stronger NTE in a wider temperature range of 30–700 °C for the first time. Its corresponding coefficient of thermal expansion (CTE) is as high as ?17.21 × 10^?6 °C^?1. The distortion of [FeO₆/MgO₆] polyhedra in (KMg)_xFe_2-xMo₃O₁₂ caused by (KMg)³⁺-substitution contributed to the stronger NTE.     

Influence of post-deposition annealing on electrical and optical properties of (Ta2O5)1-x - (TiO2)x thin films,x ≤ 0.08

Tantalum (Ta) and titanium (Ti) metal targets were direct current (DC) magnetron sputtered in the oxygen environment by varying its relative areas to deposit (Ta₂O₅)_1-x- (TiO₂)_x (TTO_x) thin films, with x = 0, 0.03, 0.06, and 0.08, onto the boron-doped p-silicon (1 0 0) and optically polished quartz substrates, at room temperature; and were annealed at 500, 600, 700, and 800 °C, for 1.5 h. The thin films annealed at and above 600 °C show the Ta₂O₅ structure. The leakage current density and capacitance-voltage (C–V) characteristics were measured for TTO_x, x ≤ 0.08, assisted Ag/TTO_x/p-Si metal– oxide– semiconductor (MOS) structures. The leakage current density was found minimum, for the films annealed at 800 °C, for all the prepared TTO_x films, x ≤ 0.08. The minimum leakage current density 1.6 × 10^?8 A/cm², at 3.5 × 10⁵ V/cm electric field, was observed for x = 0.03, annealed at 800 °C, among the prepared compositions. The prepared TTO_0.03 films, annealed at 700 °C show maximum dielectric constant 39, at 1 MHz. The optical parameters, viz., refractive index (n), extinction coefficient (k), and optical band gap (E_g) of the films, with x = 0.03, prepared on quartz substrates, were determined from their optical transmittance plots. The values of n and k of the crystalline films were observed increasing from 2.123 to 2.143, and 0.099 to 0.130, respectively, at 550 nm wavelength; and E_g decreasing from 3.95 to 3.89 eV with the increasing annealing temperature, from 600 to 800 °C. Ohmic emission, in the lower electric field; Schottky and space-charge- limited current conduction mechanisms, in the intermediate to higher electric fields, were generally envisaged from the current-voltage characteristics in the prepared Ag/TTO_0.03/p-Si structures.     

Colossal permittivity in Ta-doped SrTiO3 ceramics induced by interface effects and defect structure: An experimental and theoretical study

The lack of systematic research on the phase structure, defect structure, and polarization mechanism hinders the full comprehension of the colossal permittivity (CP) behavior for SrTiO₃-based ceramics. For this purpose, Ta-doped SrTiO₃-based ceramics were synthesized in an N₂ atmosphere with a traditional method. When the appropriate amount of Ta was doped, colossal permittivity (ԑ_r ∼ 62505), low dielectric loss (tanδ ∼ 0.07), as well as excellent temperature stability (−70 °C–180 °C, ΔC/C_25°C ≤ ±15%) were obtained in the Sr_0.996Ta_0.004TiO₃ ceramic. The relationship between Ta doping, polarization mechanism, and dielectric performance was systematically researched according to experimental analysis and theoretical calculations. The first-principle calculations indicate that the Ta⁵⁺ ion prefers to replace the Sr-site. The defect dipoles and oxygen vacancies formed by heterogeneous-ion doping play an active role in regulating the dielectric performance of ceramics. In addition, the interface barrier layer capacitance (IBLC) effect associated with semi-conductive grains and insulating grain boundaries is the primary origin of colossal permittivity for Sr_1-xTa_xTiO₃ ceramics. The polarization mechanism and defect structure proposed in the study can be extended to the research of SrTiO₃ CP ceramics. The results have a good development prospect in colossal permittivity (CP) materials.     

Dielectric performance of pyrochlore-type Bi2MgNb2-xTaxO9 ceramics: The effects of tantalum doping

The solid solutions based on the pyrochlore-type system Bi₂MgNb_2-xTa_xO₉ were formed in the compositional range х = 0–2.0 (Bi_1·6Mg_0·8Nb_1.6-tTa_tO_7.2, t = 0–1.6). The Rietveld method was used to refine the structure for Bi₂MgNb_2-xTa_xO₉ (x = 0, 1.0, 2.0). The increasing tantalum content led to the slight decrease in the cubic unit cell parameters from 10.56934 (4) Å for x = 0 and 10.54607 (3) Å for x = 2 (sp.gr. Fd-3m:2). At the same time, tantalum additions suppressed grain growth in the pyrochlore ceramics during sintering and made it possible to obtain materials with an average grain size of 1–2 μm (Bi_1·6Mg_0·8Ta_1·6O_7.2). The increase in the Ta⁵⁺ concentration led to the decrease in the dielectric permeability from 104 (Bi_1·6Mg_0·8Nb_1·6O_7.2) to 20 (Bi_1·6Mg_0·8Ta_1·6O_7.2) at room temperature, while the dielectric loss tangent remained lower than 0.002, which is due to the small grain size and the high porosity of the samples. An increase in temperature has practically no effect on the values of the dielectric permittivity in the entire frequency range. The samples have weak through conductivity. The activation energies of electrical conductivity varied in the range of 0.84–1.00 eV, and the less tantalum, the lower the activation energy. The electrical properties of the samples at 200 Hz to 1 MHz are described by the simplest parallel scheme.     

Sc- and W-substitution and tuning of phase transition in the Al2Mo3O12

Al₂Mo₃O₁₂ is a typical negative thermal expansion (NTE) material, whose thermal expansion behavior depends on its crystal phase. The thermal shock caused by temperature-induced phase transition limits its wide application. The two series of Al_{2. x}Sc_xMo₃O₁₂ (0 ≤ x ≤ 1) and Al₂Mo_3-xW_xO₁₂ (0 ≤ x ≤ 2.5) solid solutions with controllable phase transition temperature were synthesized via single cation substitution at the A or B position. The problem of thermal shock caused by the change of temperature is effectively solved in the synthesized Al_1.6Sc_0.4Mo₃O₁₂ and Al₂Mo_0.5W_2.5O₁₂, showing stable NTE performance above room temperature, and the coefficients of thermal expansion of which are ?2.19 × 10^?6 °C^?1 in 100–550 °C and ?4.25 × 10^?6 °C^?1 in 85–500 °C, respectively. A-site cation substitution is a more effective way to tune the thermal expansion properties of Al₂Mo₃O₁₂, which is attributed to the fact that the bond strength of A-O is weaker than that of B–O in the compound.     

Fabrication and characterization of In doped Li13.9Sr0.1Zn(GeO4+δ)4 electrolytes for proton-conducting solid oxide fuel cells

Li_13.9Sr_0.1Zn(GeO_4+δ)₄ (LSZG) materials can exhibit proton conduction by Li⁺/H⁺ ion exchange in hydrogen atmosphere. It can be used in solid oxide fuel cells (SOFCs) as an electrolyte. In this study, In³⁺ doped LSZG powders are synthesized by sol-gel method. X-ray diffraction, scanning electron microscopy, thermal gravimetric analyzer, and electrochemical impedance spectroscopy are used to investigate the effects of In doping on LSZG. All Li_13.9-xIn_xSr_0.1Zn(GeO_4+δ)₄ (LISZG, 0 ≤ x ≤ 0.3) ceramics exhibit the same phase with LSZG. The dopant of In promotes the sintering activity and Li⁺/H⁺ ion exchange rate of LSZG. The optimum doping of In is x = 0.2. At 600 °C, Li_13.7In_0.2Sr_0.1Zn(GeO_4+δ)₄ (0.2LISZG) shows a proton conductivity of 0.094 S/cm under 0.9 V direct current bias voltage. In addition, the single cell based on 0.2LISZG electrolyte is prepared, and it has been demonstrated that the practical utilization of 0.2LISZG in IT-SOFCs is feasible.     

A medium-entropy perovskite oxide La0.7Sr0.3Co0.25Fe0.25Ni0.25Mn0.25O3-δ as intermediate temperature solid oxide fuel cells cathode material

In this study, we report a novel medium-entropy perovskite oxide of La_0.7Sr_0.3Co_0.25Fe_0.25Ni_0.25Mn_0.25O_3-δ (LSCFNM73) with high constitutive entropy (S_config) as the cathode material of intermediate temperature solid oxide fuel cells (IT-SOFCs). The intrinsic properties of phase structure, electrical conductivity, thermal expansion and oxygen adsorption capacity of La_1-xSr_xCo_0.25Fe_0.25Ni_0.25Mn_0.25O_3-δ (LSCFNM, x = 0, 0.1, 0.2, 0.3) oxides are evaluated in detail. The LSCFNM73 oxide exhibits the maximum electrical conductivity of 464 S cm⁻¹ at 800 °C and a relatively lower thermal expansion coefficient (TEC) of 15.34 × 10⁻⁶ K⁻¹, which is selected as the propriate cathode composition. The B-site of LSCFNM73 contains four elements which can increase the configuration entropy. Additionally, NiO-Yttria stabilized zirconia (YSZ) supported fuel cell is fabricated by tape casting, hot pressing-lamination, co-sintering and screen printing technologies. The fuel cell demonstrates a maximum power density of 1088 mW cm² at 800 °C, and excellent stability at 750 °C under 0.75V in 120 h and 10 times thermal cycling between 750 °C and 400 °C. Therefore, the medium-entropy LSCFNM73 oxide can be applied in IT-SOFCs as a competitive cathode material.     

Synthesis and characterization of Ca3-xLaxCo4-yCuyO9+δ cathodes for intermediate temperature solid oxide fuel cells

The calcium cobaltite Ca_3-xLa_xCo_4-yCu_yO_9+δ with x and y = 0 and 0.1 were synthesized and the electrical, thermal, and catalytic behaviors for the oxygen reduction reaction (ORR) for use as air electrodes in intermediate-temperature solid oxide fuel cells (IT-SOFCs) were evaluated. X?ray diffraction confirms the Ca_3-xLa_xCo_4-yCu_yO_9+δ samples were crystallized in a monoclinic structure and scanning electron microscopic image shows lamella-like grain formation. Introduction of dopants decreases slightly the loss of lattice oxygen and thermal expansion co-efficient. The Ca_3-xLa_xCo_4-yCu_yO_9+δ samples exhibit good phase stability for long-term operation, thermal expansion, and chemical compatibility with the Ce_0.8Gd_0.2O_2-δ electrolyte. Among the studied samples, Ca_2.9La_0.1Co₄O_9+δ shows a maximum conductivity of 176 Scm^?1 at 800 °C. Although the doped samples exhibit a higher total electrical conductivity, an improved symmetrical cell performance is displayed by the undoped sample. Comparing the sintering temperatures, the composite cathode Ca₃Co₄O_9+δ + Ce_0.8Gd_0.2O_2-δ sintered at 800 °C exhibit the lowest area specific resistance of 0.154 Ω cm² at 800 °C in air. In the Ca_3-xLa_xCo_4-yCu_yO_9+δ + GDC composite cathodes, the charge-transfer process at high frequencies presents a major rate limiting step for the oxygen reduction reaction.     

Valence state of europium and samarium in Ln2Hf2O7 (Ln = Eu,Sm) based oxygen ion conductors

Ln₂(Hf_2-xLn_x)O_7-x/2 (Ln = Sm, Eu; x = 0.1) pyrochlores have been prepared via mechanical activation of oxide mixtures, followed by heat treatment for 4h at 1450 and 1600 °C, respectively. According to the ESR data, the Eu cations on the Hf site in the Hf_1-xEu_xO₆ octahedra in pyrochlore Eu₂(Hf_2-xEu_x)O_7-x/2 (x = 0.1) are most readily oxidized and reduced. Oxidation at 840 °C for 24h in air reduces the total conductivity of the Ln₂(Hf_2-xLn_x)O_7-x/2 (Ln = Sm, Eu; x = 0.1) by a factor of 2.5–6, due to the decrease in the concentrations of oxygen vacancies and Ln²⁺ ions as a result of the oxidation. The anomalous low-frequency behavior of the permittivity of the Eu₂(Hf_2-xEu_x)O_7-x/2 (x = 0.1) at ~800 °C can be understood in terms of the changes in the oxygen sublattice of the pyrochlore structure as a result of the oxidation of divalent europium and partial filling of oxygen vacancies at this temperature.     

Synthesis and characterization of fibrous La0.8Sr0.2Fe1-xCuxO3-δ cathode for intermediate-temperature solid oxide fuel cells

Aiming to offer a high-performance Co-free cathode for intermediate-temperature solid oxide fuel cells (IT-SOFCs), a series of La_0.8Sr_0.2Fe_1-xCu_xO_3-δ (LSFCux, x = 0.0–0.3) nanofiber cathodes were synthesized by the electrospinning method. The effects of various Cu doping amounts on the crystal structure, fiber morphology, and electrochemical performance of LSF nanofiber cathode materials were investigated. The results indicate that after being calcined at 800 °C for 2 h, the perovskite structure samples with a high degree of crystallinity are obtained. The morphology of electrospun nanofibers is continuous, and the average diameter of nanofibers is about 110 nm. In addition, the La_0.8Sr_0.2Fe_0.8Cu_0.2O_3-δ (LSFCu2) fiber cathode displays the optimal electrochemical performance, and the polarization resistance (R_p) is 0.674 Ω cm² at 650 °C. The doping of Cu transforms the main control step of the low-frequency band from dissociation of oxygen molecules to charge transfer on the electrode, and the maximum power density (P_m) of the Ni-SDC/SDC/LSFCu2 single cell reaches 362 mW cm^-2 at 650 °C.     

Effect of Ta5+ doping on the thermal physical properties of defective fluorite Y3NbO7 ceramics

The effects of substituting the B cation in A₃BO₇ ceramics on their thermal physical properties were investigated by applying a large mass difference. Y₃(Nb_1-xTa_x)O₇ (x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5) ceramics were synthesized, and their structural characteristics were determined. All the fabricated Y₃(Nb_1-xTa_x)O₇ ceramics showed defective fluorite structures and glass-like low thermal conductivity (1.18−2.04 W/m∙K at 25°C) because of the highly distorted crystal structure and significant mass difference. Substitution with Ta⁵⁺ enhanced the sintering resistance, leading to superior thermal-insulating performance via grain boundary scattering. Furthermore, the ceramics exhibited excellent coefficients of thermal expansion, implying the promising applicability of Y₃(Nb_1-xTa_x)O₇ as new thermal barrier materials. The effect of mass difference on the thermomechanical properties of the ceramics was examined, suggesting a simple strategy for engineering the chemical composition of new thermal barrier materials.     

Annealing temperature-dependent structural and electrical properties of (Ta2O5)1-x - (TiO2)x thin films,x ≤ 0.11

(Ta₂O₅)_1-x- (TiO₂)_x (TTO_x) thin films, with x = 0, 0.03, 0.06, 0.08, and 0.11, were deposited using magnetron direct current (DC) sputtering method onto the P/boron-silicon (1 0 0) substrates by varying areas of Tantalum and Titanium metallic targets, in oxygen environment at ambient temperature. The as-deposited thin films were annealed at temperatures ranging from 500 to 800 °C. Generally, the formation of the Ta₂O₅ structure was observed from the X-ray diffraction measurements of the annealed films. The capacitance of prepared metal– oxide– semiconductor (MOS) structures of Ag/TTO_x/p-Si was measured at 1 MHz. The dielectric constant of the deposited films was observed altering with varying composition and annealing temperature, showing the highest value 71, at 1 MHz, for the TTO_x films, x = 0.06, annealed at 700 °C. With increasing annealing temperature, from 700 to 800 °C, the leakage current density was observed, generally decreasing, from 10^?5 to 10^?8 A cm^?2, for the prepared compositions. Among the prepared compositions, films with x = 0.06, annealed at 800 °C, having the observed value of dielectric constant 48, at 1 MHz; and the leakage current density 2.7 × 10^?8 A cm^?2, at the electric field of 3.5 × 10⁵ V cm^?1, show preferred potential as a dielectric for high-density silicon memory devices.     

Achieving enhanced densification and superior ionic conductivity of garnet electrolytes via a co-doping strategy coupled with pressureless sintering

Lithium garnet oxides with 6.5 mol Li, such as Li_6.5La₃Zr_1.5(Ta/Nb)_0.5O₁₂, typically crystallise in cubic structure and exhibit excellent room-temperature ionic conductivity close to 1 mS cm^?1. However, it is challenging to densify garnet oxides. In this work, we investigated how the co-doping of tantalum (Ta) and niobium (Nb) affects the densification of pressureless sintered garnet electrolytes with compositions of Li_6.5La₃Zr_1.5Ta_(0.5?x)Nb_xO₁₂, where x = 0–0.5. The highest densification (94.5% of relative density) was achieved in Li_6.5La₃Zr_1.5Ta_0.1Nb_0.4O₁₂ (TN-LLZO) when it was sintered at 1150 °C for 6 h. This TN-LLZO garnet electrolyte delivers an ionic conductivity of 1.04 × 10^?3 S cm^?1 (at 22 °C) with a low activation energy of 0.41 eV. Our findings demonstrate that the content of dopants (Ta and Nb) plays a critical role in enhancing the sintering performance of garnet ceramics at ambient pressure.     

Low-temperature firing and microwave dielectric properties of MgNb2-xVx/2O6-1.25x ceramics

MgNb_2-xV_x/2O_6-1.25x (0.1≤x≤0.6) ceramics with orthorhombic columbite structures were prepared at low-temperature by a solid-phase process. The phase component, microscopic morphology, low-temperature sintering mechanism and microwave dielectric performance of MgNb_2-xV_x/2O_6-1.25x ceramics were comprehensively investigated. Low-temperature sintering densification of dielectric ceramics was achieved via the nonstoichiometric substitution of vanadium (V) at the Nb-site. In contrast to pure MgNb₂O₆ ceramics, the sintering temperature of MgNb_2-xV_x/2O_6-1.25x (x = 0.2) ceramics was reduced by nearly 300 °C owing to the liquid-phase assisted sintering mechanism. The liquid phase arises from the autogenous low-melting-point phase. Meanwhile, MgNb_2-xV_x/2O_6-1.25x (x = 0.2) samples with nonstoichiometric substitution could achieve a more than 900% improvement in the Q × f value, compared with stoichiometrically MgNb_2-xV_xO₆ (x = 0.1, 0.2) ceramics. Finally, MgNb_2-xV_x/2O_6-1.25x dielectric ceramics possess outstanding microwave dielectric properties: ε_r = 20.5, Q × f = 91000, and τ_f = -65 ppm/°C when sintered at 1030 °C for x = 0.2, which provides an alternative material for LTCC technology and an effective approach for low-temperature sintering of Nb-based microwave dielectric ceramics.     

Effect of strontium substitution on the structure and dielectric properties of unfilled tungsten bronze Ba4-xSrxSmFe0.5Nb9.5O30 ceramics

The effects of Sr²⁺ substitution for Ba²⁺ on phase structure, microstructure, dielectric and electric properties for Ba_4-xSr_xSmFe_0.5Nb_9.5O₃₀ (x = 0, 1, 2, 3 and 4) ceramics were systematically researched. X-ray diffraction patterns show that Ba_4-xSr_xSmFe_0.5Nb_9.5O₃₀ (x = 0, 1, 2 and 3) ceramics are tetragonal tungsten bronze compound with a space group of P4bm, while the sample for x = 4 is an orthorhombic structure compound. The result can be corroborated by the analysis of Raman spectroscopy. As the Sr²⁺ contents increase from 0 to 3, the full width at half maximum of Raman lines of all samples increase gradually, indicating that the degree of lattice distortion increase. All tetragonal tungsten bronze ceramics exhibited a broad permittivity peaks, accompanied by frequency dispersion, indicating all samples are relaxor. The electrical properties of BSSFN ceramics were further studied by complex impedance spectroscopy. XPS spectrum shows that Fe²⁺ and Fe³⁺ coexist in Ba_4-xSr_xSmFe_0.5Nb_9.5O₃₀ ceramics, and their proportion varies with the concentration of Sr²⁺.     

Comprehensive properties and thermal stability of Ta2O5-doped PIN-PHT ceramics

Herein, high-performance 0.11 Pb(In_1/2Nb_1/2)O₃-0.89 Pb(Hf_0.47Ti_0.53)O₃-0.8Ta₂O₅ (PIN-PHT-0.8Ta) ceramics are successfully synthesized. In addition, performance improvement is comprehensively analyzed from viewpoints of microstructure, phase structure and electrical properties. Experimental results reveal that the addition of Ta₂O₅ changes phase structure of PIN-PHT ceramics from ferroelectric tetragonal phase to rhombohedral phase. This leads to the appearance of morphotropic phase boundaries (MPBs). At the same time, the addition of Ta₂O₅ reduces grain size and enhances grain uniformity. Also, Ta₂O₅ doping improves internal and external contribution of piezoelectric response, which greatly improves dielectric, piezoelectric and ferroelectric properties of PIN-PHT. Key performance parameters include d₃₃, k_p, T_C, ε_r and tanδ, which are found to be 630 pC/N, 0.73, 322.6 °C, 1917 and 1.55%, respectively. In particular, thermal stability of PIN-PHT-0.8Ta ceramics is found to be higher than PZT-based ceramics, as well as d₃₃ value and performance retention rate of PIN-PHT-0.8Ta are found to be 560 pC/N and 89% at 300 °C, respectively, which are far superior to commercial PZT-5 and PZT-8 ceramics. These properties indicate potential of PIN-PHT-0.8Ta ceramics in high-temperature applications.     

Effect of substituting Al3+ for Ti4+on the microwave dielectric performance of Mg2Ti1-xAl4/3xO4 (0.01 ≤ x ≤ 0.09) ceramics

In this paper, Mg₂Ti_1-xAl_4/3xO₄ ceramics (0.01 ≤ x ≤ 0.09) were synthesized through conventional solid-state ceramic route. The cubic spinel structure, microstructure and microwave properties of Mg₂Ti_1-xAl_4/3xO₄ (x = 0.01, 0.03, 0.05, 0.07, 0.09) ceramics were investigated by X-ray diffraction, Raman spectra, infrared spectra. Rietveld refinements confirm that a spinel structure phase with space group Fd-3m is formed. The variation of the permittivity was concerned with the ionic polarizability, and the value of τ_f was influenced by the bond valence. Both Q × f values and relative density showed an identical trend. Intrinsic properties of Mg₂Ti_1-xAl_4/3xO₄ ceramics were analyzed by infrared spectra and Raman spectra. In addition, the Mg₂Ti_1-xAl_4/3xO₄ ceramic sintered at 1420 °C for 4 h possessed optimal dielectric properties (ε_r = 14.65, Q × f = 182347 GHz, τ_f = −57.7 ppm/°C) when x = 0.09.     

Negative thermal expansion coefficient of Sc-doped indium tungstate ceramics synthesized by co-precipitation

Co-precipitation was successfully applied to synthesize the Sc³⁺ doped In_2-xSc_x (WO₄)₃ (x = 0, 0.3, 0.6, 0.9 and 1.2) compounds. The composition- and temperature-induced structural phase transition and thermal expansion behaviors of Sc³⁺ doped In₂(WO₄)₃ were investigated. Results indicate that In_2-xSc_x (WO₄)₃ crystalizes in a monoclinic structure at 300 °C for x ≤ 0.3 and changes into hexagonal structure for x ≥ 0.6. Hexagonal In_1.1Sc_0.9(WO₄)₃ displays negative thermal expansion (NTE) with an average linear coefficient of thermal expansion (CTE) of −1.85 × 10⁻⁶ °C ⁻¹. After sintering at 700 °C and above, a phase transition from hexagonal to orthorhombic phase was observed in In_2-xSc_x (WO₄)₃ (x ≥ 0.6). Sc³⁺ doping successfully reduce the temperature-induced phase transition temperature of In_2-xSc_x (WO₄)₃ ceramics from 250 °C (x = 0) to room temperature (x = 0.9). When x = 0.9 and 1.2, the average linear CTEs of In_2-xSc_x (WO₄)₃ ceramics are −5.45 × 10⁻⁶ °C⁻¹ and -4.43 × 10⁻⁶ °C⁻¹ in a wider temperature range of 25–700 °C, respectively.     

Effect of substitution of La3+ on structural and electrical properties of Sr(Fe0.5Nb0.5)O3 ceramic

In this work, we have mainly reported the effect of lanthanum substitution on structural, dielectric, impedance and transport properties of strontium iron niobate (i.e., Sr_1-xLa_x(Fe_0.5Nb_0.5)_1-x/4O₃ (x = 0, 0.05, 0.1, 0.15, 0.2)). The materials were synthesized using standard ceramic technology. The preliminary structural analysis was done by using the room temperature X-ray diffraction data. The samples of higher concentrations (x = 0.15 and x = 0.20) show the development of an additional phase (i.e., LaNbO₄ and Sr₃La₄O₉). Studies of frequency and temperature dependence of dielectric parameters exhibit an anomaly and relaxor behavior in the compounds. The electrical impedance and modulus analysis of frequency and temperature-dependent data show the contributions of grains and grain boundaries in the resistive and capacitive properties of the compounds. The study of transport properties of AC conductivity has provided the conduction and relaxation mechanism. The substitution of La³⁺ has significantly changed the dielectric constant, tangent loss, and transport properties of the material.