Structural, magnetic and dielectric properties of Bi1−ySryFe(1−y)(1−x)Sc(1−y)xTiyO3 (x = 0-0.2, y = 0.1-0.3) ceramics

In this study, bulk ceramics with general formula Bi_1−ySr_yFe_{(1−y)(1−x)}Sc_(1−y)xTi_yO₃ (x = 0-0.2, y = 0.1-0.3 mol%) were prepared by traditional solid-state reaction method. As a comparison, bulk BiFeO₃ (BF) was also sintered by rapid sintering method. Their structural, magnetic, dielectric properties were investigated. X-ray diffraction analysis indicated that apart from a small amount of secondary phase detected in BF, all other samples crystallized in pure perovskite structure and maintained original R3c space group. The room temperature M-H curves were obtained. While BF had a coercive magnetic field (H_c) of 150 Oe, Bi_1−ySr_yFe_1−yTi_yO₃ solid solutions had a much larger value (for y = 0.1, 0.2, 0.3, H_c were 4537, 5230 and 3578 Oe, respectively). Sc³⁺ substitution decreased the H_c values of these solid solutions remarkably, and resulted in soft magnetic properties, as well as a decrease of the dielectric loss. At 1 MHz, the tan δ of Bi_0.7Sr_0.3Fe_0.7(1−x)Sc_0.7xTi_0.3O₃ with x = 0.05, 0.1, 0.15, 0.2 were 0.1545, 0.1078, 0.1046 and 0.1701, respectively.     

Effect of Pb(Fe1/2Nb1/2)O3 modification on dielectric and piezoelectric properties of Pb(Mg1/3Nb2/3)O3-PbZr0.52Ti0.48O3 ceramics

10 mol% Pb(Fe_1/2Nb_1/2)O₃ (PFN) modified Pb(Mg_1/3Nb_2/3)O₃-PbZr_0.52Ti_0.48O₃ (PMN-PZT) relaxor ferroelectric ceramics with compositions of (0.9 − x)PMN-0.1PFN-xPZT (x = 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8 and 0.9) were prepared. X-ray diffraction investigations indicated that as-prepared ceramics were of pure perovskite phase and the sample with composition of x = 0.8 was close to morphotropic phase boundary (MPB) between rhombohedral and tetragonal phase. Dielectric properties of the as-prepared ceramics were measured, and the Curie temperature (T_c) increased sharply with increasing PZT content and could be higher than 300 °C around morphotropic phase boundary (MPB) area. At 1 kHz, the sample with composition of x = 0.1 had the largest room temperature dielectric constant ?_r = 3519 and maximum dielectric constant ?_m = 20,475 at T_m, while the sample with composition of x = 0.3 possessed the maximum dielectric relaxor factor of γ = 1.94. The largest d₃₃ = 318 pC/N could be obtained from as-prepared ceramics at x = 0.9. The maximum remnant polarization (P_r = 28.3 μC/cm²) was obtained from as-prepared ceramics at x = 0.4.     

Effect of the Mn ion on electrical and magnetic properties of orthorhombic perovskite-type Ca(Mn1−xTix)O3−δ

Orthorhombic perovskite-type Ca(Mn_1−xTi_x)O_3−δ (0 ≤ x ≤ 0.7) was synthesized at 1173 K for 12 h in a flow of oxygen from a precursor gel prepared using citric acid and ethylene glycol. The Mn³⁺ ion was generated by substituting a Ti⁴⁺ ion in CaMnO₃. The average particle size was 100-300 nm and did not depend on x. The lattice constants and the (Mn, Ti)-O distance increased linearly with increasing x. The variation in global instability index (GII) indicated that the instability of the structure increases monotonically with increasing x. Ca(Mn_1−xTi_x)O_3−δ was an n-type semiconductor that had its minimum values of electrical resistivity (ρ) and activation energy (E_a) at x = 0.1. Ca(Mn_1−xTi_x)O_3−δ (x = 0 and 0.1) exhibited a weak ferromagnetic behavior. The variation in μ_eff indicated that the spin state of the Mn³⁺ ion changes from low to high at x = 0.1, then reverts to low in the range of 0.2 ≤ x ≤ 0.7. The variations in ρ and E_a are explained by the number of electrons according to the change in the spin state of the Mn³⁺ ion.     

Dielectric and pyroelectric anisotropy in melt-quenched BaBi2(Nb1 − xVx)2O9 ceramics

The (0 0 l) textured BaBi₂(Nb_1 − xV_x)₂O₉ (where x = 0, 0.03, 0.07, 0.1 and 0.13) ceramics were fabricated via the conventional melt-quenching technique followed by high temperature heat-treatment (800-1000 °C range). The influence of vanadium content and sintering temperature on the texture development and relative density were investigated. The samples corresponding to the composition x = 0.1 sintered at 1000 °C for 10 h exhibited the maximum orientation of about 67%. The Scanning electron microscopic studies revealed the presence of platy grains having the a-b planes perpendicular the pressing axis. The dielectric constant and the pyroelectric co-efficient values in the direction perpendicular to the pressing axis were higher. The anisotropy in the dielectric constant is about 100 (at 100 kHz) at the dielectric maximum temperature and anisotropy in the pyroelectric co-efficient is about 50 μC cm⁻² °C⁻¹ in the vicinity of pyroelectric anomaly for the sample corresponding to the composition x = 0.1 sintered at 1000 °C. Higher values of the dielectric loss and electrical conductivity were observed in the direction perpendicular to the pressing axis which is attributed to the high oxygen ion conduction in the a-b planes.     

Itinerant ferromagnetism to insulating spin glass in SrRu1−xCuxO3 (0 ≤ x ≤ 0.3)

The ferromagnetic metallic oxide, SrRuO₃ (T_C ∼ 165 K) undergoes structural, magnetic and metal-insulator transitions upon substitution of Cu at the Ru-site. For x = 0.2 in SrRu_1−xCu_xO₃, the structure becomes a tetragonal with the space group I4/mcm and there is a signature of both ferromagnetic (T_C = 65 K) and antiferromagnetic (T_N = 32 K) ordering due to possible magnetic phase separation. The antiferromagnetism arises due to short range ordering of Cu- and Ru-moments. Jahn-Teller distortion of (Ru,Cu)-O₆ octahedra indicates that the copper ions are in 2+ oxidation state with ⁶t2g³eg electronic configuration. For x ≥ 0.1, narrowing of Ru-4d bandwidth by the substitution of Cu ions results in semiconducting behavior. For x = 0.3, the ac and dc susceptibility measurements indicate a spin glass behavior. The origin of spin glass behavior has been attributed to competing ferromagnetic and antiferromagnetic interactions.     

Electrochemical removal of NOx with porous cell stacks

In this study porous cell stacks were investigated for their ability to remove NO_x electrochemically. The cell stacks were made from laminated tapes of porous electrolyte Ce_0.9Gd_0.1O_1.95 and composite electrodes of La_1−xSr_xMnO₃ (x = 0.15, and 0.5) and ceria doped with Gd or Pr. The cell stacks were infiltrated with nano-particles of pure ceria, Ce_0.9Gd_0.1O_1.95 and Ce_0.8Pr_0.2O_2−δ after sintering. A gas stream containing NO were sent through the cell stack. When the cell stacks were polarised with 0.75 V per cell then it was possible to remove some of the NO_x in the temperature interval of 250-400 °C. The cell stacks infiltrated with ceria showed the highest activity, while the ones infiltrated with Ce_0.9Gd_0.1O_1.95 had the highest selectivity towards NO compared to O₂. When the cell stack was polarised with 1.5 V for each cell it was possible to remove up to 35% of NO present.     

Synthesis and electrochemical behavior of nanosized LiNi1−xCaxO2 cathode materials for high voltage secondary lithium-ion cells

A new class of LiNi_1−xCa_xO₂ (x = 0.0, 0.1, 0.2, 0.3 and 0.5) layered oxide materials has been synthesized by a simple low temperature solid-state route with mixed nitrates/urea with glycerol as the starting materials. First we have taken TG/DTA for observing the phase transformations of LiNi_0.9Ca_0.1O₂. The structure of the synthesized oxides was analyzed using X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) to identify the crystal structure and cation environment, respectively. The synthesized ceramic oxide battery materials were examined by using transmission electron microscope (TEM), scanning electron microscope (SEM) analysis to determine the particle size, nature and morphological structure. SEM with energy dispersive X-ray spectroscopic analysis (EDAX) analysis was carried out to explore the composition of the prepared materials. The electrochemical performance of LiNi_1−xCa_xO₂ electrodes was analyzed using cyclic voltammetry (CV) and galvanostatic charge-discharge cycling studies in the voltage range 3.0-4.5 V. Electrode made with cathode active material, acetylene black and poly(vinylidene difluoride) yield a discharge capacity of 178.1 mAh g⁻¹ (x = 0.2) with good specific capacity over several charge-discharge cycles. These results have been also supported by cyclic voltammograms.     

Crystal structure and superconductivity of rubidium tungsten bronzes RbxWO3 prepared by a hybrid microwave method

The rubidium tungsten bronzes Rb_xWO₃ have been prepared from Rb₂CO₃, WO₃ and W powders using hybrid microwave method. The single hexagonal phase samples can be obtained as actual rubidium content x in the range of 0.21-0.33, and their lattice parameters a and c linearly drop and rise with the increase of rubidium content respectively. For samples with x = 0.14, 0.16, 0.18, the superconducting transition temperature T_c from resistivity measurements does not change with the rubidium content, while T_c from susceptibility measurements shows a decrease from 5.3 K for x = 0.14 to 4.8 K for x = 0.18. The charge density wave (CDW) transition appears in Rb_0.21WO₃, Rb_0.23WO₃ and Rb_0.25WO₃ at about 200-260 K. The CDW transition is most obvious in Rb_0.23WO₃ which shows the lowest degree of crystallization among the samples.     

Synthesis and electrical properties of cubic NaxWO3 thin films across the metal-insulator transition

Highly oriented (1 0 0) Na_xWO₃ thin films were fabricated in the composition range 0.1 ≤ x ≤ 0.46 by pulsed laser deposition technique. The films showed transition from metallic to insulating behaviour at a critical composition between x = 0.15 and 0.2. The pseudo-cubic symmetry of Na_xWO₃ thin films across the transition region is desirable for understanding the composition controlled metal-insulator transition in the absence of any structural phase transformation. The electrical transport properties exhibited by these films across the transition regime were investigated. While the resistivity varied as T² at low temperatures in the metallic regime, a variable range hopping conduction was observed for the insulating samples. For metallic compositions, a non-linear dependence of resistivity in temperature was also observed from 300 to 7 K, whose exponent varied with the composition of the film.     

Synthesis and characterizations of A-site deficient perovskite Sr0.9Ti0.8−xGaxNb0.2O3

The effects of reduction and Ga-doping on the physicochemical properties of A-site deficient perovskites Sr_0.9Ti_0.8−xGa_xNb_0.2O₃ (x = 0, 0.05, 0.1, 0.15 and 0.2) are reported. With 10% Ga doping, the sample sintered in air and treated at 1400 °C in H₂ atmosphere exhibits the highest electrical conductivity. It is found that the Ga-doping lowers the sinterability but promotes the reduction of Sr_0.9Ti_0.8−xGa_xNb_0.2O₃. The XRD analysis on the reduced samples suggests that some cations are reduced during the treatment. However, without high temperature pre-reduction, the improvement of Ga-doping is limited and the overall cell performance using Sr_0.9Ti_0.8−xGa_xNb_0.2O₃ as an anode without catalysts is still relatively low.     

Effect of B-site doping on Sm0.5Sr0.5MxCo1−xO3−δ properties for IT-SOFC cathode material (M = Fe, Mn)

The crystal structure, thermal expansion rate, electrical conductivity and electrochemical performance of Sm_0.5Sr_0.5M_xCo_1−xO_3−δ (M = Fe, Mn) have been investigated. Two crystal structures have been observed in the specimens of Sm_0.5Sr_0.5Fe_xCo_1−xO_3−δ (SSFC) at room temperature, the perovskite structure of SSFC has an orthorhombic symmetry for 0 ≤ x ≤ 0.4 and a cubic symmetry for 0.5 ≤ x ≤ 0.9. The specimens of Sm_0.5Sr_0.5Mn_xCo_1−xO_3−δ (SSMC) crystallize in an orthorhombic structure. The adjustment of thermal expansion rate to electrolyte, which is one of the main problems of SSC, can be achieved to lower TEC values with more Fe and Mn substitution. Especially, Sm_0.5Sr_0.5Mn_0.8Co_0.2O_3−δ exhibits good thermal compatibility with La_0.8Sr_0.2Ga_0.8Mg_0.2O₃. High electrical conductivities are obtained for all the specimens and they demonstrate above 100 S/cm at 800 °C in SSFC system. The polarization resistance increases with increasing Mn content, Nevertheless, the polarization resistance of SSFC increases with increasing Fe content, but when the amount of Fe reaches to 0.4, the maximum is obtained while the resistance will decrease when the amount of Fe reaches above 0.4. Sm_0.5Sr_0.5Fe_0.8Co_0.2O_3−δ electrode exhibits high catalytic activity for oxygen reduction operating at temperature from 700 to 800 °C.     

Physicochemical properties of solid-solid interactions in nanosized NiO-substituted Fe2O3/TiO2 system at 1200 °C

The solid-solid interactions between nanosized pure and NiO-substituted ferric and titanium(IV) oxides have been investigated using XRD technique and microstructure studies, also magnetic properties were studied using vibrating samples magnetometer (VSM). The amounts of substituting Ni²⁺ were x = 0, 0.2, 0.4, 0.6, 0.8 and 1 mole. A mixture equimolar proportions of finely powdered Fe₂O₃ and TiO₂ were mixed with NiO, ball milled, compressed at 250 kg/cm² and fired at 1200 °C for 4 h.The obtained results showed that with substituting Ni²⁺ concentration x = 0 only Fe₂TiO₅ phase is present (∼80 nm) which showed a very small saturation magnetic flux density (B_s), remnant magnetic flux density (B_r) and the maximum energy product (BH)_max. By the addition of x = 0.2 NiO, new phases were observed NiTiO₃ and NiFe₂O₄ of crystallite sizes 160 and 110 nm, respectively. By the increase of substituting Ni²⁺ concentration the NiTiO₃ and NiFe₂O₄ phases increased on the expense of Fe₂TiO₅ up to x = 0.4, then the increase in substituting Ni²⁺ concentration led to a decrease in Fe₂TiO₅ and NiTiO₃ while NiFe₂O₄ increases which results in a great improvement of magnetic properties.All samples exhibit a catalytic activity towards H₂O₂ decomposition and the values of rate constant increase with increasing amount of Ni²⁺ substituting. The most acidic active sites are shown by specimens substituted with x = 0 this concludes that H₂O₂ decomposition is not favored on acidic active sites.     

GexNbSe2 and GexNbS2 intercalation compounds

The structures of two intercalation compounds, Ge_∼0.2NbSe₂ and Ge_∼0.3NbS₂ were investigated by single crystal X-ray diffraction and electron microscopy (selected area electron diffraction (SAED), high resolution electron microscopy (HRTEM) and X-ray microanalysis by energy dispersive spectroscopy (XEDS)). Crystal structure determinations of the average structure of the intercalation compounds 2H-Ge_0.217NbSe₂ and 4H-Ge_0.288NbS₂ are reported: the selenide compound crystallizes in the space group P6₃/mmc with a = 3.4560(9) Å and c = 12.966(3) Å and adopts the 2H-NbSe₂ structure-type, while the sulfide compound crystallizes in the P6₃mc space group, with a = 3.3392(9) Å and c = 25.404(7) Å with a structure-type 4H_c-NbS₂ which it is known for TaSe₂. In both structures the germanium atoms are located in the empty octahedral positions of the van der Waals gap between the NbX₂ (X = S, Se) layers. Electron diffraction patterns from several Ge_xNbSe₂ crystal flakes show different superstructures and exhibit diffracted diffuse intensity: weak satellites corresponding to and 2a₀ × 2a₀ superstructures were observed for x ∼ 0.15 (a₀ is the basal lattice parameter of the host structure). For x ∼ 0.25-0.33, the same type of satellite is observed with a stronger intensity. For x ∼ 0.5 only satellites corresponding to the superstructure were present. In the case of Ge_xNbS₂, with 0.10 < x < 0.25, the germanium atoms are ordered in domains with an superstructure. In some crystals disorder along the c-axis has been observed.     

Luminescence characterization of (Ca1−xZnx)Ga2S4:Eu phosphors for a white light-emitting diode

We investigated the luminescence properties of (Ca_1−xZn_x)Ga₂S₄:Eu²⁺ phosphor as a function of Zn²⁺ and Eu²⁺ concentrations. The luminescence intensity was markedly enhanced by increasing the mole fraction of Zn²⁺ at Ca²⁺ sites. Lacking any Zn²⁺ ions, CaGa₂S₄:0.01Eu²⁺ converted only 18.1% of the absorbed blue light into luminescence. As the Zn²⁺ concentration increased, the quantum yield increased and reached a maximum of 24.4% at x = 0.1. Furthermore, to fabricate the device, the optimized green-yellow (Ca_0.9Zn_0.1)Ga₂S₄:Eu²⁺ phosphor was coated with MgO. White light was generated by combining the MgO-coated phosphor and the blue emission from a GaN chip.     

Synthesis and optical properties of (Gd1−x,Eux)2O2SO4 nano-phosphors by a novel co-precipitation method

(Gd_1−x,Eu_x)₂O₂SO₄ nano-phosphors were synthesized by a novel co-precipitation method from commercially available Gd₂O₃, Eu₂O₃, H₂SO₄ and NaOH starting materials. Composition of the precursor is greatly influenced by the molar ratio of NaOH to (Gd_1−x,Eu_x)₂(SO₄)₃ (the m value), and the optimal m value was found to be 4. Fourier transform infrared spectrum (FT-IR) and thermal analysis show that the precursor (m = 4) can be transformed into pure (Gd_1−x,Eu_x)₂O₂SO₄ nano-phosphor by calcining at 900 °C for 2 h in air. Transmission electron microscope (TEM) observation shows that the Gd₂O₂SO₄ phosphor particles (m = 4) are quasi-spherical in shape and well dispersed, with a mean particle size of about 30-50 nm. Photoluminescence (PL) spectroscopy reveals that the strongest emission peak is located at 617 nm under 271 nm light excitation, which corresponds to the ⁵D₀ → ⁷F₂ transition of Eu³⁺ ions. The quenching concentration of Eu³⁺ ions is 10 mol% and the concentration quenching mechanism is exchange interaction among the Eu³⁺ ions. Decay study reveals that the ⁵D₀ → ⁷F₂ transition of Eu³⁺ ions has a single exponential decay behavior.     

Effect of Bi and Ti substitutions for Na and Nb on ferroelectric properties of oriented (Bi4.5+xNa0.5−x)(Ti2xNb2−2x)WO15 compounds

(Bi_4.5+xNa_0.5−x)(Ti_2xNb_2−2x)WO₁₅ (BNTNW) compounds were synthesized and their ferroelectric properties were characterized. The X-ray powder diffraction patterns of the compounds revealed that they have a single phase over the whole composition range. The linear variations of the lattice parameters with composition indicate the formation of solid solutions, resulting in a reduction in the orthorhombicity of the compounds. The remnant polarization of the BNTNW decreased from 8.5 to 5.1 μC/cm² with increasing x, which may be related to the orthorhombicity of the compounds. By using hot forging, an oriented BNTNW compound at x = 0 was obtained. Strong reflections from (0 0 l) were observed for sample // in which the measurement direction is parallel and the orientation factor of such sample was approximately 0.72. A remarkable increase in the remnant polarization (P_r) of the compound was observed for the sample ⊥ in which the direction of applied pressure is perpendicular to the measurement direction; the highest P_r value was 18 μC/cm².     

Structure, dielectric and ferroelectric anisotropy of Sr2−xCaxBi4Ti5O18 ceramics

Sr_2−xCa_xBi₄Ti₅O₁₈(x = 0, 0.05) powders synthesized by solid state route were uniaxially pressed and sintered at 1225 °C for 2 h. The obtained dense ceramics exhibited crystallographic anisotropy with a dominant c axis parallel to the uniaxial pressing direction which was quantified in terms of the Lotgering factor. Microstructure anisotropy of both compositions (x = 0, 0.05) consisted of plate like grains exhibiting their larger surfaces mostly perpendicular to the uniaxial pressing direction. Dielectric and ferroelectric properties of Sr_2−xCa_xBi₄Ti₅O₁₈ ceramics were measured under an electric field (E) applied parallel and perpendicularly to uniaxial pressing direction. The obtained dielectric ?_R(T) and polarization (P-E) curves depended strongly on E direction thus denoting a significant effect from microstructure and crystallographic texture. Sr_2−xCa_xBi₄Ti₅O₁₈ properties were also significantly affected by Ca content (x): Curie temperature increased from 280 °C (x = 0) to 310 °C (x = 0.05) while ?_R and remnant polarization decreased for x = 0.05. The present results are discussed within the framework of the processing and crystal structure-properties relationships of Aurivillius oxides ceramics.     

Synthesis and electrical properties of ordered perovskite oxide Cd3−x−yCuxAyTeO6 (A = Li, Na; 0.00 ≤ x, y ≤ 0.15)

Polycrystalline Cd_3−x−yCu_xA_yTeO₆ (A = Li, Na) samples were prepared by solid-state reaction, and their crystal structure and electrical properties were investigated. In Cd_3−xCu_xTeO₆ and Cd_3−yA_yTeO₆ (A = Li, Na), the maxim solubility of x and y was 0.15 and 0.15 for A = Li, 0.05 for A = Na, respectively. For co-substituted samples Cd_2.9−yCu_0.1Li_yTeO₆ and Cd_2.9−yCu_0.1Na_yTeO₆, the maxim solubility of x was the same as single substitution above-mentioned. The alkali-metal substituted samples Cd_3−yA_yTeO₆ (A = Li, Na) showed a negative Seebeck coefficient, which indicates that the major conduction carriers are electron. On the other hand, the co-substituted samples Cd_2.9−yCu_0.1A_yTeO₆ (A = Li, Na) represented a positive Seebeck coefficient, and major conduction carriers were hole through substitution by copper ions.     

Synthesis and characterization of pyrochlore-type solid solutions Y2−xLaxRu2O7

New pyrochlore-type solid solutions Y_2−xLa_xRu₂O₇ have been synthesized for x=0.0, 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6 as single-phase materials. The lattice parameters for these solid solutions are linear with x and range from 1.0140 nm (x=0.0) to 1.0257 nm (x=0.6). All the samples show semiconducting behavior with an activation energy of about 0.1 eV. They show the spin-glass-like magnetic transition at low temperatures. Its transition temperature T_G decreases from 77 K for Y₂Ru₂O₇ with increasing the La³⁺ content.     

Structural study by X-ray diffraction and transmission electron microscopy of the misfit compound (SbS1−xSex)1.16(Nb1.036S2)2

In the Sb-Nb-S-Se system, a new misfit layer compound (MSL) has been synthesized and its structure was determined by combining single crystal X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques. It presents a composite crystal structure formed by (SbS_1−xSe_x) slabs stacking alternately with double NbS₂ layers and both can be treated as separate monoclinic subsystems. The (SbS_1−xSe_x) slabs comprise a distorted, two-atom-thick layer with NaCl-type structure formed by an array of {SbX₅} square pyramids joined by edges (X: S, Se); the NbS₂ layers consist of {NbS₆} trigonal prisms linked through edge-sharing to form sheets, just as in the 2H-NbS₂ structure type. Both sublattices have the same lattice parameters a = 5.7672(19) Å, c = 17.618(6) Å and β = 96.18(3)°, with incommensurability occurring along the b direction: b₁ = 3.3442(13) Å for the NbS₂ subsystem and b₂ = 2.8755(13) Å for the (SbS_1−xSe_x) subsystem. The occurrence of diffuse scattering intensity streaked along c^* indicates that the (SbS_1−xSe_x) subsystem is subjected to extended defects along the stacking direction.