Defect structure of quenched γ-BICOVOX by combined X-ray and neutron powder diffraction

A detailed investigation of the defect structure of the Co doped BIMEVOX solid electrolyte, Bi₂V_{1 − x}Co_xO_{5.5 − 3x/2} (x = 0.1 and x = 0.2), quenched from high temperature, has been carried out using X-ray and neutron powder diffraction data measured at room temperature. The structure is built up from alternating layers of [Bi₂O₂]_n²ⁿ⁺ and [V_{1 − x}Co_xO_{3.5 − 3x/2}]_n²ⁿ⁻ with disorder limited to the vanadate layer. The ideal V/Co co-ordination is octahedral with corner sharing of equatorial oxygens. The refinements show that the true structure is distorted, with disorder in both apical and equatorial oxygens and oxygen vacancies concentrated in the equatorial positions. Detailed analysis of the oxygen site occupancies reveals two main types of V/Co co-ordination viz. distorted octahedral and distorted tetrahedral. The majority of the sites in both compositions are tetrahedral.     

Highconducting oxide ceramics bimevox: Synthesis, structure, and properties

Materials on the basis of BIMEVOX compounds which are stable within wide concentration and temperature regions have been obtained. Refinement of crystal structure of different BIMEVOX modifications obtained at different temperatures and partial oxygen pressures has been carried out. The most promising solid solutions have been identified according to total conductivity value as well as to linearity of conductivity dependence on temperature and independence of that on partial oxygen pressure.     

Correlation between phase structure and electrical conduction in BISNVOX system for intermediate temperature solid oxide fuel cells (IT-SOFC)

Samples of Sn⁴⁺-substituted bismuth vanadate, formulated as Bi₄Sn _x V_{2? x} O_{11?( x /2)? δ} in the composition range 0.07 ≤ x ≤ 0.30, were prepared by standard solid-state reactions. Sample characterization and the principal phase transitions (α ? β, β ? γ and γ′ ? γ) were investigated by FT-IR spectroscopy, X-ray powder diffraction, differential thermal analysis (DTA) and AC impedance spectroscopy. For composition x = 0.07, the α ? β and β ? γ phase transitions were observed at temperatures of 451 and 536°C, respectively. DTA thermograms and Arrhenius plots of conductivities revealed the γ′ ? γ phase transition at 411 and 423°C for x = 0.20 and 0.30, respectively. AC impedance plots showed that conductivity is mainly due to the grain contribution, which is evident in the enhanced short-range diffusion of oxide ion vacancy in the grains with increasing temperature. The highest ionic conductivity (5.03 × 10^?5 S cm^?1 at 300°C) was observed for the x = 0.17 solid solution with less pronounced thermal hysteresis.     

Neutron powder diffraction as a characterization tool of solid oxide fuel cell materials

Neutron diffraction is a powerful tool for the characterization of materials and, particularly, oxides. Oxide materials find applications in solid oxide fuel cells (SOFCs) as solid electrolytes as well as anode and cathode materials. As a structural probe, neutrons are specially suitable for the crystallographic study of oxides, given the comparable scattering factors of O and other heavier elements, allowing its precise localization in the crystal structure. Many problems can be addressed by neutrons, related to the octahedral tilting in perovskites, phase transitions, order–disorder phenomena, presence of anionic vacancies, etc. Neutrons make possible an accurate determination of the thermal factors and provide a visualization of the diffusion paths in ionic conductors. Neutrons allow the localization of light atoms such as hydrogen, and make possible the distinction between neighbouring elements, typically Fe and Mn. In this work we will describe some recent applications of this technique in the field of solid electrolytes and electrode materials, including some examples from our group.     

Preparation, characterization and oxide ion conductivity in U-substituted Bi4V2O11

A Bi₂V_{1 − x − y}U_xBi_yO_{5.5 + 0.5x − y} solid solution derived from Bi₄V₂O₁₁ has been prepared and characterized with x up to 0.125 for y = 0. Partial substitution of U⁶⁺ for V⁵⁺ in Bi₄V₂O₁₁ leads to the stabilization at room temperature of the high-oxide ion conducting γ-phase, in contrast with other M⁶⁺ dopants which stabilize the β-phase. The lower conductivity in U substituted system compared with BICUVOX.10 is attributed to its higher activation energy. Conductivity values and activation energies of the U substituted phases compare well with Bi₂UO₆.     

Effect of Niobium doping on structural, thermal, sintering and electrical properties of Bi4V1.8Cu0.2O10.7

Doping Bi₄V_1.8Cu_0.2O_10.7 with niobium has led to the formation of the Bi₄V_1.8Cu_0.2−xNb_xO_10.7+3x/2 solid solution. X-ray diffraction and thermal analysis have shown that only the compound with x=0.05 presents a tetragonal symmetry with a γ^′ polymorph while the other compositions are of β polymorph. The influence of sintering temperature on the microstructure of the samples was investigated by the scanning electron microscopy (SEM). The ceramics sintered at temperatures higher than 820 °C present micro-craks. The evolution of the electrical conductivity with temperature and the degree of substitution has been investigated by impedance spectroscopy. Among all compositions studied the sample with x=0.05 presents the highest value of the conductivity.     

Thermal analysis and structural characterization of Bi<Subscript>4</Subscript>V<Subscript>2–x</Subscript>Ba<Subscript>x</Subscript>O<Subscript>11–1.5x</Subscript> (0.02≤<Emphasis Type="Italic">x</Emphasis>≤0.50)

This work reports the study of Bi₄V_2–xBa_xO_11–1.5x (0.02≤x≤0.50) series, which is a potential source of solid electrolytes to apply in oxygen sensors. X-ray powder diffraction was used to point out the formation of major ionic conductive phases and minor ones. The modifications of vanadate substructure were probed, at short range, by Fourier-transform infrared spectroscopy. Differential scanning calorimetry evidenced the formation of tetragonal γ phase, which can be ionic conductive, for x=0.14.     

Synthesis,phase stability and oxide ion conductivity of Ce(IV)–Cd(II) double substituted bismuth vanadate

Bi₄V₂O_11-δ has been doped with Ce and Cd to study double substitution. The system with various dopant concentrations (0.07 ≤ x ≤ 0.30) was prepared by the standard solid-state reaction method. The correlation between the polymorphism and oxide ion performance was well investigated as a function of temperature and composition with the help of thermal analysis, X-ray diffraction (XRD) and AC impedance spectroscopy. From XRD results it is seen that the high oxide ion conducting tetragonal γ-phase is stabilized for x = 0.17. For the compositions x ≤ 0.10, monoclinic α-phase is retained at room temperature with clear evidence for two successive phase transitions α ? β and β ? γ. For x = 0.13, β ? γ phase transition is seen. However, the existence of order–disorder, γ' ? γ transition was confirmed for x = 0.17. It is seen that the highest low-temperature ionic conductivity at 320 °C is 3.19 × 10^?4 S cm^?1 which was observed for x = 0.17.     

Specific features in the synthesis, crystal structure and electrical conductivity of BICUTIVOX

The formation of solid solutions Bi₄V_2 − xCu_x/2Ti_x/2O_11 − x (0.025 ≤ x ≤ 0.5) known as BICUTIVOX, synthesized by three different methods (a conventional solid-state synthesis, solid-state synthesis enhanced by mechanical activation, and through liquid precursors), has been studied. Based on crystal structure investigations carried out at different temperatures, ranges of stability and temperatures of phase transitions for different polymorphous modifications have been defined. The morphology and the local chemical composition of the ceramic samples obtained have been studied. Thermal expansion coefficients have been measured. The electrical conductivity of ceramic samples has been investigated in a wide range of temperatures and partial oxygen pressures.