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.     

Electrical properties of MgAl2-2xZrxMxO4 (M = Co, Ni and x = 0.00-0.20) synthesized by coprecipitation technique using urea

This paper presents the results of a study concerning the structural and electrical properties of MgAl_2-2xZr_xM_xO₄ (x = 0.00-0.20 and M = Co²⁺ and Ni²⁺) prepared by a coprecipitation technique using urea as a precipitating agent. The X-ray diffraction data for the pure and its doped samples are consistent with the single-phase spinel and their crystallite sizes are in the range 7-20 ± 4 nm. The DC resistivity increases from 3.09 × 10⁹ Ω cm to 6.73 × 10⁹ and 8.06 × 10⁹ Ω cm whereas dielectric constant decreases from 5.80 to 5.11 and 4.95 on doping with Zr-Co and Zr-Ni, respectively. The electrical resistivity variations with increase in the dopant contents indicate two types of conduction mechanisms in operation. Several parameters such as, hopping energy (W), metal-semiconductor transition temperature (T_MS) and Debye temperature (θ_D) have also been determined. The increase in DC resistivity and decrease in dielectric constant suggest that the synthesized materials can be considered for application as an insulating and structural material in fusion reactors.     

Synthesis, structural and magnetic properties of La1−xCdxFeO3 (0.0 ≤ x ≤ 0.3) orthoferrites

Nanocrystalline La_1−xCd_xFeO₃ (0.0 ≤ x ≤ 0.3) solid solutions have been synthesized by a single-step solution combustion method at a relatively low temperature of 400 °C. The combustion-synthesized solid solutions were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), and magnetic measurements. The crystal structure examined by XRD indicates that the samples were single-phase, and crystallize in an orthorhombic (space group, Pbnm no. 62) structure. The parent and doped compounds showed canted antiferromagnetic behavior associated with an increase in magnetic moment with Cd doping. The changes in magnetic properties of the materials are correlated to the changes in structural features resulting from the Rietveld structural refinement of the materials.     

Synthesis of perovskite-type (La1−xCax)FeO3 (0 ≤ x ≤ 0.2) at low temperature

Gel formation was realized by adding citric acid to a solution of La(NO₃)₃·5H₂O, Ca(NO₃)₂·4H₂O, and Fe(NO₃)₂·9H₂O. Perovskite-type (La_1−xCa_x)FeO₃ (0 ≤ x ≤ 0.2) was synthesized by firing the gel at 500 °C in air for 1 h. The crystallite size (D_1 2 1) decreased with increasing x, while the specific surface area was 6.8-9.4 m²/g and independent of x. The XPS measurement of the (La_1−xCa_x)FeO₃ surface indicated that the Ca²⁺ ion content increased with increasing x, while the Fe ion content was independent of x. Catalytic activity for CO oxidation increased with increasing x.     

R3Mn1.5CuV0.5O9 (R = Y, Ho, Er, Tm, Yb and Lu) and Lu3Mn3−3xCu2xVxO9: New noncentrosymmetric oxides related to YMnO3

We report formation of new noncentrosymmetric oxides of the formula, R₃Mn_1.5CuV_0.5O₉ for R = Y, Ho, Er, Tm, Yb and Lu, possessing the hexagonal RMnO₃ (space group P6₃cm) structure. These oxides could be regarded as the x = 0.5 members of a general series R₃Mn_3−3xCu_2xV_xO₉. Investigation of the Lu-Mn-Cu-V-O system reveals the existence of isostructural solid solution series, Lu₃Mn_3−3xCu_2xV_xO₉ for 0 < x ≤ 0.75. Magnetic and dielectric properties of the oxides are consistent with a random distribution of Mn³⁺, Cu²⁺ and V⁵⁺ atoms that preserve the noncentrosymmetric RMnO₃ structure.     

Oxide-ion conductivity in CuxCe1−xO2−δ (0 ≤ x ≤ 0.10)

Up to 10 at.% of copper readily substitutes for cerium in ceria. It is found that at oxygen partial pressures between 0.21 atm and 10⁻⁵ atm, Cu_xCe_1−xO_2−δ (0 ≤ x ≤ 0.10) solid solution behave as an oxide-ion electrolyte. Interestingly, Cu_0.10Ce_0.90O_2−δ exhibits the oxide-ion conductivity of ca. 10⁻⁴ Ω⁻¹ cm⁻¹ at 600 °C at an oxygen partial pressure of 10⁻⁵ atm.     

Single crystal growth and structure refinements of CsMxTe2−xO6 (M = Al, Ga, Ge, In) pyrochlores

Single crystals of CsM_xTe_2−xO₆ pyrochlores with M = Al, Ga, Ge, and In have been grown from a TeO₂ flux. Structure refinements from single crystal X-ray diffraction data are reported. These results are used to discuss deviations from ideal stoichiometry that result in electronic conductivity presumably related to mixed valency of tellurium.     

Template synthesis of magnetic one-dimensional nanostructured spinel MFe2O4 (M = Ni, Mg, Co)

The iron nitrate and suitable salt of nickel, cobalt, or magnesium with molar ratio of 2:1 were introduced into the pores of mesoporous silica SBA-15 via two-solvent method. The thermal decomposition of the precursors and the formation of one-dimensional nanostructured spinel ferrite in SBA-15 pores were monitored by XRD, TG-DTA, N₂ adsorption-desorption, and TEM. The pure spinel MFe₂O₄ nanowires obtained through complete removal of the silica template with aqueous NaOH solution were confirmed by TEM and HRTEM. The spinel MFe₂O₄ nanowires showed increasing normal configuration and exhibited superparamagnetism in comparison with the bulk ones.     

Crystal and magnetic structure and cation distribution of Mn2−xV1+xO4 spinels (x = 0, 1/3 and 1)

We synthesized the spinel-type compounds belonging to the Mn_2−xV_1+xO₄ series with x = 0, 1/3 and 1 as polycrystalline powders. Crystal and magnetic structures were refined using synchrotron X-ray and neutron powder diffraction. At 300 K all members crystallize in the cubic system, space group , and show a structural transition at low temperature, changing to a tetragonal symmetry (space group I4₁/amd). Cations distributions between octahedral and tetrahedral sites were refined from neutrons diffraction (ND) data and explained based on crystal field stabilization energies (CFSE) and ionic radii. The magnetic unit cell is the same as the crystallographic one, having identical symmetry relations. The magnetic structure was refined as an arrangement of collinear spins, antiferromagnetically ordered, parallel to the c-axis of the unit cell. The refined site magnetic moments are smaller than those obtained from hysteresis cycles of the M vs. H measurements, indicating that some non-collinear disordered component coexists with the ordered component along the c-axis.     

Ba3ZnTa2−xNbxO9 and Ba3MgTa2−xNbxO9 (0≤x≤1): synthesis, structure and dielectric properties

Oxides belonging to the families Ba₃ZnTa_2−xNb_xO₉ and Ba₃MgTa_2−xNb_xO₉ were synthesized by the solid state reaction route. Sintering temperatures of 1300°C led to oxides with disordered (cubic) perovskite structure. However, on sintering at 1425°C hexagonally ordered structures were obtained for Ba₃MgTa_2−xNb_xO₉ over the entire range (0≤x≤1) of composition, while for Ba₃ZnTa_2−xNb_xO₉ the ordered structure exists in a limited range (0≤x≤0.5). The dielectric constant is close to 30 for the Ba₃ZnTa_2−xNb_xO₉ family of oxides while the Mg analogues have lower dielectric constant of ∼18 in the range 50 Hz to 500 kHz. At microwave frequencies (5-7 GHz) dielectric constant increases with increase in niobium concentration (22-26) for Ba₃ZnTa_2−xNb_xO₉; for Ba₃MgTa_2−xNb_xO₉ it varies between 12 and 14. The “Zn” compounds have much higher quality factors and lower temperature coefficient of resonant frequency compared to the “Mg” analogues.     

Order-disorder phase transitions and high-temperature oxide ion conductivity of Er2+xTi2−xO7−δ (x = 0, 0.096)

The Er_2+xTi_2−xO_7−δ (x = 0.096; 35.5 mol% Er₂O₃) solid solution and the stoichiometric pyrochlore-structured compound Er₂Ti₂O₇ (x = 0; 33.3 mol% Er₂O₃) are characterized by X-ray diffraction (phase analysis and Rietveld method), thermal analysis and optical spectroscopy. Both oxides were synthesized by thermal sintering of co-precipitated powders. The synthesis study was performed in the temperature range 650-1690 °C. The amorphous phase exists below 700 °C. The crystallization of the ordered pyrochlore phase (P) in the range 800-1000 °C is accompanied by oxygen release. The ordered pyrochlore phase (P) exists in the range 1000−1200 °C. Heat-treatment at T ≥ 1600 °C leads to the formation of an oxide ion-conducting phase with a distorted pyrochlore structure (P2) and an ionic conductivity of about 10⁻³ S/cm at 740 °C. Complex impedance spectra are used to separately assess the bulk and grain-boundary conductivity of the samples. At 700 °C and oxygen pressures above 10⁻¹⁰ Pa, the Er_2+xTi_2−xO_7−δ (x = 0, 0.096) samples are purely ionic conductors.     

Grain size-dependent electrical transport properties in La0.75Sr0.25Mn1−xMgxO3±δ ceramics

The role of GB in Mg-substituted lanthanum-strontium manganite ceramics is studied with microstructural details. At higher concentrations of Mg (x>0.05), where the average grain size is ∼1 μm, the M-I transition is shifted from 348 to 110 K. Annealing in lower p_O2 (10⁻⁶ atm) at 1375 K for 1 h obliterates the M-I transition and brings in insulating behaviour throughout the temperature of measurement. Re-annealing in oxygen atmosphere for 10-25 min reintroduces the M-I transition, indicating that the electrical transport properties depend on the chemical inhomogeneity introduced by the in- or out-diffusion of oxygen through the GB regions. Samples with larger grain size (∼35 μm) do not exhibit major modifications in electrical resistivity on annealing in different p_O2. The insulating manganites display non-linear J-E characteristics below the magnetic transition temperature at electric field strengths <50 V/cm. The non-linear behaviour is explained on the basis of the inelastic tunnelling through the multiple localised states in the insulating GB regions. The external magnetic field lowers the voltage at which the non-linearity sets in. The tunnelling therefore may be not only through independent defect centres of oxygen vacancies (V_O), but possibly from defect complexes such as Mn³⁺-V_O or Mn²⁺-V_O, where spin-dependent tunnelling can take place.     

Synthesis, structure and magnetic properties of Sr2Fe1−xGaxMoO6 (0 ≤ x ≤ 0.6) double perovskites

Polycrystalline Sr₂Fe_1−xGa_xMoO₆ (0 ≤ x ≤ 0.6) materials have been synthesized by solid state reaction method and studied by neutron powder diffraction (NPD) and magnetization measurements. Rietveld analysis of the temperature dependent NPD data shows that the compounds crystallize in the tetragonal symmetry in the space group I4/m. The anti-site (AS) defects concentration increases with Ga doping, giving rise to highly B-site disordered materials. Ga doping at the Fe-site decreases the cell volume. The evolution of bond lengths and the cation oxidation states was determined from the Rietveld refinement data. The saturation magnetization and Curie temperature decreased with the increasing Ga content in the samples. Low temperature neutron diffraction data analysis and magnetization measurements confirm the magnetic interaction as ferrimagnetic in the sample.     

Crystal structures and properties of BiMn1−xAlxO3 with x = 0.03 and 0.1

Crystal structures of BiMn_0.97Al_0.03O₃ (I) at 300 and 470 K and BiMn_0.9Al_0.1O₃ (II) at 90 and 300 K were studied with synchrotron X-ray powder diffraction. The strong Jahn-Teller distortion, observed at 300 K in I and associated with orbital order, disappeared at 470 K completely for one site and partially for the second site. The Mn/Al-O distances were very close to each other in I at 470 K and in II at 90 and 300 K indicating that orbital order did not appear in II even at 90 K. Magnetic properties of I and II were investigated with specific heat, high-temperature dc magnetic susceptibility, and ac magnetic susceptibility using different driving ac and applied dc magnetic fields and different ac magnetic field frequencies. The anomaly on the specific heat associated with a magnetic transition was strongly suppressed in II compared with that of I and BiMnO₃.     

Structural, magnetic and electrochemical characterization of La0.83A0.17Fe0.5Cr0.5O3−δ (A = Ba, Ca) perovskites

The structural, magnetic and electrical properties of the perovskite-type compound La_0.83A_0.17Fe_0.5Cr_0.5O_3−δ (A = Ba, Ca) have been investigated by neutron diffraction, magnetization measurements and conductivity measurements. Rietveld refinement of X-ray and neutron diffraction data shows that the compound adopts an orthorhombic crystal structure with Pbnm symmetry with a random positioning of the iron and chromium cations on the B sublattice. The magnetic structures at 10 K are collinear antiferromagnetic with the magnetic moment per site being equal to 2.91(2)μ_B (for Ba) and 3.05(2)μ_B (for Ca). Magnetization measurements confirm the overall antiferromagnetic behavior. The magnetic structure is based on a unit cell related to that of the nuclear structure and the magnetic cell can be considered the same as nuclear cell. Barium doped samples show lower oxygen deficiency and higher conductivity than calcium doped samples. At low oxygen pressure, both compounds show p-type electronic conduction.     

Structural and electrical properties of La2−xBaxMo2O9−x/2 (x = 0-0.18)

La_2−xBa_xMo₂O_9−x/2 (x ≤ 0.18) have been prepared by solid state reaction method. The lattice parameter of La_2−xBa_xMo₂O_9−x/2 (x ≤ 0.18) determined by XRD data refinement shows a linear dependence on the dopant Ba content x. For the specimen with a La/Ba molar ratio of 0.18-0.2, additional reflection of secondary phase exists in the XRD pattern, so the value of solubility limit for Ba in La₂Mo₂O₉ is defined in range of 0.18 < x < 0.2. As the replacement degree of La³⁺ by Ba²⁺ increases, the bulk conductivity of La_2−xBa_xMo₂O_9−x/2 (x ≤ 0.18) decreases initially and then increases, a minimum value at La_1.9Ba_0.1Mo₂O_8.95 exists. Hebb-Wagner studies in argon atmosphere, which use an oxide-ion blocking electrode, show that La_2−xBa_xMo₂O_9−x/2 (x ≤ 0.18) are predominantly oxide-ion conducting in the temperature ranging from 773 to 1173 K. The average thermal expansion coefficient of La_1.84Ba_0.16Mo₂O_8.92 determined by high-temperature XRD was deduced as great as 17.5 × 10⁻⁶ K⁻¹ between 298 and 1173 K.     

Crystal structure and electrical properties of new tungsten bronzes: BxWO3 (0.01 ≤ x ≤ 0.08)

Boron tungsten bronzes B_xWO₃ (0.01 ≤ x ≤ 0.08) were synthesized by hybrid microwave method from mixtures of WO₃ and amorphous boron powder. With the increase of boron content, the crystal structure of B_xWO₃ transforms from orthorhombic (x = 0.01) to tetragonal α (x = 0.048) and then to tetragonal β (0.07 ≤ x ≤ 0.08). The average size of crystallites is in the range of 1-10 μm. All samples show semiconducting behaviour in their temperature dependence of resistivity. The conduction behaviour above 80 K for samples with x = 0.01 and 0.08 can be explained using the variable-range hopping and thermally activated mechanism, respectively. Comparative experiments showed that boron bronze phases cannot be obtained by the microwave heating of pure WO₃ powder or a mixture of B₂O₃ and WO₃ under the same conditions.     

Magnetic and transport properties of La0.3Ca0.7Mn0.9T0.1O3 (T = Cr, Fe, and W)

We have carried out structural, magnetic and magneto transport measurements of the electron-doped manganite La_0.3Ca_0.7MnO₃ substituted with 10% of Cr, Fe and W on the Mn site. The substitution by Cr, Fe and W suppresses the charge order transition present at 260 K in the parent compound. All the samples show a semiconducting behavior. Whereas the parent compound does not show any magneto resistance (MR) even in a field of 14 T, a maximum MR of 6% in 5 T at 25 K is observed for the Cr substituted sample that is attributed to a spin-cluster glass like states induced by Cr. The Fe and W substituted samples showed a MR of 1.5 and 3%, respectively which may be attributed to a smaller number of FM domains/spin-clusters and to an increase in anti-ferromagnetic interaction.     

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.     

Catalytic behavior of AMoOx (A = Ba, Sr) in oxidation of 2-propanol

Perovskite-type oxides, BaMoO₃ and SrMoO₃, were prepared by reduction of scheelite-type oxides, BaMoO₄ and SrMoO₄, in H₂ flow at 873 K and characterized by XRD, TG, SEM, TPR, NH₃-TPD, UV-vis DRS and BET measurement. The catalytic activity of these alkaline-earth molybdenum oxide catalysts was tested for oxidation of 2-propanol with gaseous oxygen under atmospheric pressure. Dehydration to propylene was mainly promoted over the scheelite-type with Mo⁶⁺, while oxidative dehydrogenation to acetone was mainly promoted over the perovskite-type with Mo⁴⁺, and selectivity to acetone was much higher over BaMoO₃ than over SrMoO₃. Both perovskite-type oxide catalysts underwent oxidation to some degree during the catalytic reaction, so that they also contained some Mo⁶⁺. We concluded that the high selectivity to acetone resulting from oxidative dehydrogenation during 2-propanol conversion is related to the constantly changing oxidation state of the catalyst, resulting in coexistence of Mo⁶⁺ octahedra and Mo⁴⁺ octahedra on the AMoO₃ oxides.