Phase relations and high temperature-XRD studies of Gd2−xNdxTi2O7 solid solutions

A series of compositions with the general formula Gd_2−xNd_xTi₂O₇ (0.0 ≤ x ≤ 2.0) was prepared by ceramic sintering and characterized by powder XRD. Nd³⁺ has been used as a surrogate for Am³⁺, an actinide found in spent nuclear fuel. One end member, Gd₂Ti₂O₇, had the pyrochlore structure and formed solid solutions with Nd₂Ti₂O₇ up to the nominal composition Gd_1.2Nd_0.8Ti₂O₇. An increase in lattice parameter was observed as a function of x in the series Gd_2−xNd_xTi₂O₇ in the composition range 0.0 ≤ x ≤ 0.8. Compositions Gd_2−xNd_xTi₂O₇ in the range 0.8 ≤ x ≤ 1.4 were biphasic. From x ≥ 1.6, the solid solutions are monoclinic, isotypic with Nd₂Ti₂O_7. These results were explained based on the radius (r_A/r_B) ratio of the cations. High temperature-XRD studies on cubic pyrochlores showing thermal expansion are reported.     

Ionic conductivity of high-purity Gd-doped ceria solid solutions

The Ce_1−xGd_xO_2−δ (0.02 ≤ x ≤ 0.3) solid solutions with ∼30 ppm SiO₂ were prepared through the conventional mixed-oxide method from high-purity CeO₂ and Gd₂O₃ powders. The ionic conductivity (especially the grain boundary (GB) conduction) in this system was studied as a function of dopant content, over the temperature range of 250–750 °C in air, using an impedance spectroscopy. The GB impedance played an important role in the total conduction in the samples with low Gd content (usually x ≤ 0.1), but decreased sharply with increasing Gd content. Both the total and GB conductivities maximized at the composition x = 0.15, and this composition also had the lowest activation energies for the total and GB conduction. The maximum total conductivity at 700 °C, σ_t = 4.07 (Ω m)⁻¹ with the activation energy, E_t = 0.77 eV, was found for the composition x = 0.15.     

Influence of Ga3+ ions on spectroscopic and dielectric features of multi component lithium lead boro bismuth silicate glasses doped with manganese ions

Multi-component glasses of the chemical composition 19.5Li₂O–20PbO–20B₂O₃–30SiO–(10 − x)Bi₂O₃–0.5MnO:xGa₂O₃ with 0 ≤ x ≤ 5.0 have been synthesized. Spectroscopic (optical absorption, IR, Raman and ESR) and dielectric properties were investigated. Optical absorption and ESR spectral studies have indicated that managanese ions do exist in Mn³⁺ state in addition to Mn²⁺ state in the samples containing low concentration of Ga₂O₃. The IR and Raman studies indicated increasing degree of disorder in the glass network with the concentration of Ga₂O₃ up to 3.0 mol%. The dielectric constant, loss and ac conductivity are observed to increase with the concentration of Ga₂O₃ up to 3.0 mol%. The quantitative analysis of the results of dielectric properties has indicated an increase in the insulating strength of the glasses as the concentration of Ga₂O₃ is raised beyond 3.0 mol%. This has been attributed to adaption of gallium ions from octahedral to tetrahedral coordination.     

Processing by both classical and mechanosynthesis routes and characterization of a new solid solution of tungsten–bronze structure ceramics

A new family of ferroelectric compounds with Ba_2−xNa_1+xLi_xNb₅O₁₅ composition (0 ≤ x ≤ 1) and tetragonal tungsten–bronze structure is processed for the first time. This new family of materials derived from Ba₂NaNb₅O₁₅ compound was processed by classical solid-state reaction and by mechanosynthesis. The powders prepared by these two routes were characterized by X-ray diffraction (at room and high temperature), differential thermal analysis, thermogravimetry and scanning electron microscopy. The results confirm the formation of the solid solution in the whole range of composition. The influence of Li addition on the dielectric permittivity and losses and on the ferro-paraelectric transition temperature is also studied.     

Effects of SrTiO3 on dielectric and piezoelectric properties of K0.48Na0.48Li0.04Nb0.96Ta0.04O3-based piezoceramics

In this study, (100 ? x) K_0.48Na_0.48Li_0.04Nb_0.96Ta_0.04O₃ ? xSrTiO₃ (0 ≤ x ≤ 10) ceramics were fabricated via normal sintering of synthesized powder by using solid state reaction. All ceramics revealed pure perovskite structure, indicating formation of solid solution between KNNLT and ST up to 10%. With increasing x, the crystal structure of ceramics changed from orthorhombic to tetragonal and finally pseudocubic symmetry when x = 10. Ceramic containing 1% ST had orthorhombic and tetragonal symmetries, simultaneously. Investigation of the variation of dielectric constant of ceramics versus temperature revealed that for ceramic with x = 1, polymorphic phase transition (PPT) temperature between orthorhombic and tetragonal is less than room temperature. Thus coexistence of two different structures in this ceramic is due to vicinity of its composition to morphotropic phase boundary (MPB). As a result, the maximum piezoelectric constant was measured for this ceramic. Ceramics containing 5 and 7.5% ST tend to appear relaxor ferroelectric behavior which is because of chemical inhomogeneities in both A- and B-sites of the ABO₃ perovskite structure.     

Nanocrystalline spinel Ni0.6Zn0.4Fe2O4: A novel material for H2S sensing

Nanocrystalline powders of Ni_1?xZn_xFe₂O₄ (0 ≤ x ≤ 0.5) mixed ferrites, with cubic spinel structure and average crystallite size ranging from 28 to 42 nm, were synthesized by the ethylene glycol mediated citrate sol–gel method. The structure and crystal phase of the powders were characterized by X-ray diffraction (XRD) and microstructure by transmission electron microscopy (TEM). The response of prepared Ni_1?xZn_xFe₂O₄ mixed ferrites to different reducing gases (liquefied petroleum gas, hydrogen sulfide, ethanol gas and ammonia) was investigated. The sensor response largely depends on the composition, temperature and the test gas species. The Zn content has a significant influence on the gas-sensing properties of Ni_1?xZn_xFe₂O₄. Especially, Ni_0.6Zn_0.4Fe₂O₄ composition exhibited high response with better selectivity to 50 ppm H₂S gas at 225 °C. Incorporation of palladium (Pd) further improved the response, selectivity and response time of Ni_0.6Zn_0.4Fe₂O₄ to H₂S with the shift in the operating temperature towards lower value by 50 °C. The enhanced H₂S sensing properties can mainly be attributed to the selectivity to oxidation of H₂S and noble metal additive sensitization. Furthermore, the sensor exhibited a fast response and a good recovery.     

Magnetisation studies of phase co-existence in Gd1 − xCaxBaCo2O5.5

Magnetic properties of hole doped, oxygen deficient double perovskite compounds, Gd_1 ? xCa_xBaCo₂O_5.5, have been investigated. Ferromagnetic transition temperatures increase and the anti-ferromagnetic transition temperatures decrease with Ca substitution leading to stabilisation of ferromagnetisim for x ≥ 0.05. A detailed study of the ferromagnetic phase indicates the presence of double hysterisis loops for Ca fractions, 0.05 ≤ x ≤ 0.2 in the 50–200 K temperature range, suggestive of the co-existence of two ferromagnetic phases with different co-ercivities. Based on the magnetisation and transport measurements a phase diagram is proposed for Ca doped GdBaCo₂O_5.5.     

Hydrothermal synthesis of Sr–Ce–Sn–Mn–O mixed oxidic/stannate pyrochlore and its catalytic performance for NO reduction

Mixed oxides and pyrochlore-type materials based on the Sr, Ce, Sn, and Mn elements have been prepared by hydrothermal method using a mixture of nitrate salts at 200 °C for 40 h under N₂ atmosphere. Two groups of solids were synthesized: (i) (Sr_xCe_1?x)₂Sn₂O_7±δ (0.345 ≤ x ≤ 0.365) and (ii) (Sr_xCe_1?x)₂(Sn_yMn_1?y)₂O_7±δ (0.345 ≤ x ≤ 0.365, 0.385 ≤ y ≤ 0.395). The structure of the solids were studied by X-ray diffraction and the main crystal phases determined were SnO₂, (SrCe)₂Sn₂O₇ and a trace amount of CeO₂ in group (i) and only SnO₂ and CeO₂ were detected in group (ii). The mixed oxides/stannate pyrochlore have been tested as catalyst for NO reduction with hydrocarbons (CH₄, C₂H₄, and C₃H₆) in the presence of O₂. The CeO₂ containing stannate Sr_2xCe_2?2xSn₂O_7±δ pyrochlore coexist with SnO₂ (group (i)) was found to be the best for NO + C₃H₆ reaction giving very high N₂ production at 350 °C in the presence of O₂ and water vapor. SnO₂ as well as CeO₂ alone were also synthesized by the hydrothermal method and their NO reduction properties have been compared with those of the groups (i) and (ii).     

High dielectric constant observed in (1 − x)Ba(Zr0.07Ti0.93)O3–xBa(Fe0.5Nb0.5)O3 binary solid-solution

Binary solid-solutions of the (1 ? x)Ba(Zr_0.07Ti_0.93)O₃–xBa(Fe_0.5Nb_0.5O₃) system, with 0.1 ≤ x ≤ 0.9,were fabricated via a solid-state processing technique. X-ray diffraction analysis revealed that all samples exhibited a single perovskite phase. The BaFe_0.5Nb_0.5O₃ also promoted densification and grain growth of the system. Dielectric measurements showed that all samples displayed a relaxor like behavior. The x = 0.1 sample presented a dielectric-frequency and temperature with low loss tangent (<0.07 at 10 kHz). For x > 0.2 samples, the dielectric data showed a broad dielectric constant–temperature curve with a giant dielectric characteristic. In addition, a high dielectric constant > 50,000 (at 10 kHz and temperature > 150 °C) was observed for the x = 0.9 sample.     

Low-temperature sintering and microwave dielectric properties of CaTi1−x(Fe0.5Nb0.5)xO3 ceramics with B2O3 addition

CaTi_1−x(Fe_0.5Nb_0.5)_xO₃ (0 ≤ x ≤ 1) dielectrics were synthesized via the solid state reaction route and structure analysis was performed together with the dielectric characterization. The substitution of Ti⁴⁺ by Fe³⁺/Nb⁵⁺ and developed phase were studied by X-ray diffraction. The dielectric constant and temperature coefficient of resonant frequency decrease rapidly with an increase of x. The influence of 1–5 wt.% B₂O₃ as a sintering additive investigated at CaTi_0.5(Fe_0.5Nb_0.5)_0.5O₃ solid solutions. The dielectric properties were found to strongly depend on the sintering conditions and contents of B₂O₃ additions. ɛ_r = 52.3, Q × f_o = 2930 GHz and T_f = 13 ppm/°C were obtained for CaTi_0.5(Fe_0.5Nb_0.5)_0.5O₃ specimen 3 wt.% B₂O₃ sintered at 900 °C for 2 h.     

Structure and ionic transport studies of sodium borophosphate glassy system

Sodium borophosphate glasses of composition (mol%) 50Na₂O–50[xB₂O₃–(1−x)P₂O₅], 0 ≤ x ≤ 0.8 have been prepared by melt quenching method and characterized through XRD, DSC, FTIR and impedance spectroscopy techniques. The glass transition temperature increases with the substitution of B₂O₃ due to the cross-linking of the network and the FTIR study shows the presence of different structural units in the network. The ionic conductivity study as a function of composition of B₂O₃ shows increment in conductivity with two conductivity maxima at 10 and 30 mol% of B₂O₃ and conductivity variations with temperature follow an Arrhenius type behaviour. Transport numbers evaluated for ions and electrons show that Na⁺ ions are the mobile species in the investigated systems. The frequency dependence of the electric conductivity follows a simple power law feature. The analysis of various electrical parameters as a function of temperature in different complex planes shows that the charge transport occurs by the hopping mechanism.     

XPS and Mössbauer studies on BaSn1−xNbxO3 (x ≤ 0.100)

A few compositions of perovskite oxide BaSn_1?xNb_xO₃ (with x ≤ 0.10) system, prepared by solid state ceramic method, have been studied employing XPS and Mössbauer spectroscopy techniques. Mössbauer spectra of these compositions in the temperature range of 78–300 K reveal that the oxidation state of Sn is +4. In XPS measurements, compositions with x ≤ 0.010 show no evidence of Nb⁵⁺ signal whereas the compositions with x ≥ 0.050 show clear evidence of Nb⁵⁺ signals indicating some unreacted Nb₂O₅ component in the system. This confirms the earlier report where presence of small amount of unreacted Nb₂O₅ was predicted.     

Physical and optical properties of Co2+, Ni2+ doped 20ZnO + xLi2O + (30 − x)K2O + 50B2O3 (5 ≤ x ≤ 25) glasses: Observation of mixed alkali effect

Co²⁺ and Ni²⁺ ions doped 20ZnO + xLi₂O + (30 ? x) K₂O + 50B₂O₃ (5 ≤ x ≤ 25) mol% glasses are prepared using melt quenching technique. Structural changes of the prepared glasses by addition of transition metal oxides, CoO and NiO are investigated by UV–vis–NIR, FT-IR spectroscopy and XRD. The XRD pattern indicates the amorphous nature of prepared glasses. FT-IR measurements of the all glasses revealed that the network structure of the glasses are mainly based on BO₃ and BO₄ units placed in different structural groups in which the BO₃ units being dominant. The optical absorption spectra suggest the site symmetry of Co²⁺ and Ni²⁺ ions in the glasses are near octahedral. Crystal field and inter-electronic repulsion parameters are also evaluated. The optical band gap and Urbach energies exhibited the mixed alkali effect. Various physical parameters such as density, refractive index, optical dielectric constant, polaron radius, electronic polarizability and inter-ionic distance are also determined.     

Dielectric properties of lanthanum substituted barium titanate microwave ceramics

In this study, we report the high dielectric constant lanthanum substituted barium titanate ceramic material for its possible applications at microwave frequencies. The microwave dielectric characterization of Ba_6 − 3xLa_8 + 2xTi₁₈O₅₄ solid solutions with 0.0 ≤ x ≤ 0.7 prepared by conventional mixed oxide route method has been carried out. The lattice parameters were obtained from the X-ray diffraction patterns. It was observed that lattice parameters increased with respect to an increase in the ‘La’ content. The crystal symmetry investigated was orthorhombic with space group of Pbam. From the evaluation of microwave dielectric properties of lanthanum doped barium titanate ceramics, it was observed that a maximum value of dielectric constant (ε′) = 157 and a minimum tangent loss (tanδ) = 0.0572 was obtained. The minimum value of a.c. conductivity (σ_a.c.) was observed to be 1.76e − 07 S/m.     

Electrical conductivity studies of Bi2O3–Li2O–ZnO–B2O3 glasses

Ac conductivity measurements and its analysis has been performed on xBi₂O₃–(65?x)Li₂O–20ZnO–15B₂O₃ (0 ≤ x ≤ 20) glasses in the temperature range 30–300 °C and a frequency range of 100 Hz to 1 MHz. The dc conductivity increased and the activation energy decreased with lithium content. The frequency dependent conductivity has been analyzed employing conductivity and modulus formalisms. The onset of conductivity relaxation shifts towards higher frequencies with temperature. The Almond–West conductivity formalism is used to explain the scaling behavior, and the relaxation mechanism is independent of temperature.     

Microwave dielectric properties of new SrLa4−xNdxTi5O17 ceramics

New microwave dielectric ceramics in the SrLa_4?xNd_xTi₅O₁₇ (0 ≤ x ≤ 4) composition series were prepared through a solid state mixed oxide route. All the compositions formed highly dense (～95%) single phase ceramics upon sintering at 1500–1580 °C. The molar volume and theoretical density decreased due to the substitution of small (1.27 Å) Nd ions for large (1.36 Å) La ions. This decrease was associated with a decrease in the dielectric constant (?_r), temperature coefficient of resonant frequency (τ_f) and quality factor (Q_uf_o). An analysis of properties achieved in the present study indicated that ceramics exhibiting nearly zero τ_f corresponding to ?_r ～ 54 could be fabricated in the SrLa_4?xNd_xTi₅O₁₇ composition series at x ～ 1.6; however, further work is required to improve Q_uf_o (～6000 GHz) for possible practical applications.     

Crystal structure and magnetic properties of Pr- and Ti-substituted La2RuO5

Polycrystalline samples of Pr- and Ti-substituted La₂RuO₅ were prepared applying a soft-chemistry route based on the thermal decomposition of citrate-stabilized precursors. The simultaneous substitution on the La-sites by Pr and on the Ru-sites by Ti results in samples of the composition La_2−xPr_xRu_1−yTi_yO₅ with 0 ≤ x ≤ 0.75 and 0 ≤ y ≤ 0.4. The crystal structures of these compounds were analyzed by Rietveld refinement of powder X-ray diffraction patterns. For pure La₂RuO₅ a structural transition from a monoclinic room-temperature modification to a triclinic low-temperature structure was found at 161 K. This structural change is linked to a low-temperature long-range ordered spin-singlet ground state formed by Ru⁴⁺ spin-moments. Both the structural transition and the formation of the singlet ground state become progressively suppressed with higher Ti contents, while the Pr substitution has only a minor influence on the dimerization. The behavior of the Curie–Weiss temperatures can be explained assuming two almost independent magnetic sublattices corresponding to the ruthenium and the rare-earth ions, respectively. For all investigated properties, i.e. crystal structure, magnetic susceptibilities, and dimerization temperature T_d, a completely additive behavior of the effects of Pr-substitution and Ti-substitution is observed.     

Preparation,characterization and dielectric properties of Ba3−xSrxM4Ti4O21 (M = Nb,Ta and 0 ≤ x ≤ 3) ceramics

Ba_3−xSr_xM₄Ti₄O₂₁ (M = Nb, Ta and 0 ≤ x ≤ 3) ceramics have been prepared through solid state ceramic route. The structure and microstructure of the compositions have been studied using powder X-ray diffraction and Scanning Electron Microscopic studies. In the present study, both Ba₃Nb₄Ti₄O₂₁ and Ba₃Ta₄Ti₄O₂₁ are obtained as single phase compositions whereas strontium rich compositions exhibit multiphase in nature. Energy dispersive X-ray analysis has been performed to identify the nature of additional phases present in the strontium rich compositions. The dielectric constant, dielectric loss and temperature variation of dielectric constant of the well sintered ceramic specimens have been studied in the low frequency region (< 13 MHz) using an impedance analyzer. The present study reveals that high dielectric compositions can be realized by Sr-substitution in the BaO–TiO₂–Nb₂O₅/Ta₂O₅ ternary systems.     

Elastic properties of Ca5−xAxNb2TiO12 (A = Mg,Zn) microwave ceramics

Microwave dielectrics in Ca_5−xA_xNb₂TiO₁₂ (A = Mg, Zn) system have been prepared for different values of x by conventional solid-state ceramic route. The elastic properties of the ceramics were investigated as a function of composition by ultrasonic techniques. X-ray diffraction studies revealed that the ceramics form phase pure compounds for 0 ≤ x ≤ 1 and form mixture phases beyond x = 1. The elastic properties were strongly affected by structural transformations. Magnesium-based compositions have better elastic properties compared with zinc-based system.     

Crystal structure and electrical properties of CaxWO3 (0.01 ≤ x ≤ 0.15) prepared by hybrid microwave synthesis

Calcium tungsten bronzes Ca_xWO₃ (0.01 ≤ x ≤ 0.15) were synthesized by hybrid microwave method from mixtures of CaO, WO₃ and tungsten powder. Single-phased samples can be obtained by microwave heating within 40 min. With the increase of calcium content, the crystal structure of Ca_xWO₃ transforms from orthorhombic (0.01 ≤ x ≤ 0.02) to tetragonal (0.03 ≤ x ≤ 0.11) and then to cubic (0.12 ≤ x ≤ 0.15). The average size of crystallites is in the range 1–5 μm. All samples show semiconductor behaviour in their temperature dependence of resistivity. The electrical conduction mechanism changes from variable-range hopping to the thermally activated mechanism when x > 0.12.