Homogeneity regions of neodymium,samarium, and europium manganites in air

XRD phase analysis of homogeneous phases and heterogeneous compositions of general formula Ln_2?x Mn_xO_3±δ (Ln = Nd, Sm, Eu; 0.90 ≤ x ≤ 1.20; Δx = 0.22) prepared by ceramic synthesis from oxides in air at 900–1400°C was used to determine the solubility boundaries for Ln₂O₃ oxides and maganese oxides in LnMnO_3±δ. The results were represented as fragments of the phase diagrams for the Ln-Mn-O systems in air. It was assumed that the solubility of Ln₂O₃ oxides in LnMnO_3±δ is determined by lattice defects, while that of manganese oxides, in addition to above mechanism, by the disproportionation reaction 2Mn³⁺ = Mn²⁺ + Mn⁴⁺ followed by the partial substitution of divalent magnesium for Ln³⁺ at cuboctahedral positions of the perovskitelike crystal lattice.     

Synthesis,structure, and conduction of solid solutions BIMEVOX (Me = Cu,Ti)

Conditions for synthesizing single-phase solid solutions Bi₄V_{2 ? x} Cu _x/2Ti _x/2O_{11 ? x} and Bi_{4 ? x/2}V_{2 ? x/2}Cu _x/2Ti _x/2O_{11 ? x/2} are studied. The sequence of phase transformations is determined. Possible domains of stability of polymorphic modifications and the overall conductivity as a function of temperature and composition are studied. The structure of the γ-modification is refined using Rietveld’s full-profile analysis.     

Crystal structure and conduction of BICUTIVOX

Existence boundaries, structure, and transport parameters were studied for Bi₄V_{2 ? x} Cu _x/2Ti _x/2O_{11 ? x} solid solutions. Doping levels within x = 0.025–0.15 distort the C2/m crystal lattice (this lattice is characteristic of individual the Bi₄V₂O₁₁ phase) and lowers its symmetry to triclinic. The solid solutions with 0.25 ≤ x ≤ 0.30 crystallize in tetragonal space group I4/mmm. High-temperature X-ray diffraction and dilatometry measurements for Bi₄V_{2 ? x} Cu _x/2Ti _x/2O_{11 ? x} (x ≤ 0.35) solid solutions verified the existence of three structural varieties within 298–1023 K. Electrical conductivity of BICUTIVOX was studied by impedance spectroscopy as a function of temperature, composition, and oxygen partial pressure. Equivalent circuits of cells were analyzed. Features of electrical conductivity versus temperature for the structural varieties are noted. Above 873 K, the solid solutions samples with x = 0.05 have the highest conductivity. At lower temperatures, higher conductivities are in the solid solutions that retain the γ phase in the low-temperature region. The dominant oxygen-ion conduction mechanism was discovered in the solid solutions.     

The diffusion of oxygen and ion transport in lanthanum cobaltite LaCoO<Subscript>3-δ</Subscript>

The chemical diffusion coefficient of oxygen vacancies and oxygen ion conductivity in lanthanum cobaltite LaCoO₃ were determined by the polarization method as functions of oxygen partial pressure \(p_{O_2 } \) (atm) and temperature T(K) over the ranges ?4 ≤ log \(p_{O_2 } \) ≤ 0 and 1173 K ≤ T ≤ 1323 K. The mobilities (cm²/(V s)) of oxygen vacancies calculated over the temperature range studied satisfy the inequalities 1.8 × 10^?5 ≤ \(v_{v_0 } \) ≤ 3.4 × 10^?5. The transfer numbers of oxygen vacancies were calculated. These numbers change depending on oxygen partial pressure over the range 5 × 10^?7 ≤ t ₀ ≤ 1 × 10^?5. The activation energy of self-diffusion of oxygen vacancies was found to be E _a= 104 ± 10 kJ/mol (1.1 ± 0.1 eV).     

Solid solutions (Ru,Nb)Sr<Subscript>2</Subscript>(Sm<Subscript>1.4</Subscript>Ce<Subscript>0.6</Subscript>)Cu<Subscript>2</Subscript>O<Subscript>10−δ</Subscript>: Synthesis,structure, and properties

Solid solutions of as-batch composition (Ru_1?x Nb _x )Sr₂(Sm_1.4Ce_0.6)Cu₂O_10?δ (the Ru,Nb)-1222 phase), where x = 0.0, 0.25, 0.50, 0.75, or 1.00, have been synthesized and characterized by X-ray diffraction. A correlation is proposed between the refined composition of the Ru-1222 and Nb-1222 phases and their structural features. With increasing oxygen concentration in the Ru-1222 phase, the superconducting transition temperature increases from T _c = 28 to T _c = 34 K. The composition and magnetic properties of the Ru-1222 phase are affected by the batch composition: unlike in Ru + RuO₂ mixtures, the presence of ruthenium in the batch decreases the oxygen proportion and increases the magnetic ordering temperature T _m; the phase of as-batch composition NbSr₂(Sm_1.4Ce_0.6)Cu₂O_10?δ is paramagnetic.     

Promising membrane materials based on strontium titanate

Electroconductivity of SrTi_1?x Fe _x O_3?δ(x = 0–0.5) specimens was studied by four-probe method. An opportunity was studied to produce hydrogen by high-temperature electrochemical conversion using SrTi_0.5Fe_0.5O_3?δ as the membrane material of the best conductivity. The effective ambipolar conductivity values calculated for SrTi_0.5Fe_0.5O_3?δ from the leakage experiment were found different from the ambipolar conductivity values calculated by the four-probe data processing because ambipolar conductivity corresponds to bulk transfer of complex oxide particles, taking no surface phenomena into account.     

Preparation of mesoporous Ce<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Fe<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>2</Subscript> mixed oxides and their catalytic properties in methane combustion

Mesoporous Ce_{1 ? x} Fe _x O₂ mixed oxide catalysts of different molar ratios (x = 0.1–0.5) were prepared by the citric acid sol-gel method and the microwave technique. The activities of Ce_{1 ? x} Fe _x O₂ mixed oxides on methane combustion were investigated, and the structure and reductive properties were characterized by XRD, BET, DRS, and TPR. The data showed that Ce_{1 ? x} Fe _x O₂ mixed oxides prepared were mesoporous material. When x ≤ 0.2, the transition metal Fe incorporated into the lattice of CeO₂ to form cubic Ce_{1 ? x} Fe _x O₂ solid solutions, and mixed phases of cubic Ce_{1 ? x} Fe _x O₂ solid solutions and α-Fe₂O₃ existed when x > 0.2. Ce_{1 ? x} Fe _x O₂ solid solutions show higher activity for methane combustion than pure CeO₂, especially for Ce_0.9Fe_0.1O₂.     

Phase equilibria involving solid solutions in the Li–Mn–O system

Phase equilibria involving LiMn₂O_4-, Li₂MnO_3-, LiMnO_2-, Mn₃O_4-, and MnO-base solid solutions were studied with varied temperature and partial oxygen pressure. The \({P_{{o_2}}}\)–T and x–y projections of the P–T–x–y phase diagram of the Li–Mn?O system were constructed, as well as the key x–y isotherms of the Li₂O–MnO–MnO₂ quasi-ternary system. In some experiments, the authors’ hydride lithiation method was employed to prepare lithium-rich homogeneous three-component nonstoichiometric phases.     

Regularities of oxygen and water thermal desorption from barium cerate doped by neodymium,samarium, and gadolinium

Processes of thermal desorption of oxygen molecules and water from BaCe_1–x M _x O_3–δ, where M= Nd, Sm, and Gd, presintered in air at the temperature of 650°C are studied. It is found that oxygen is desorbed only from neodymium–doped barium cerate and is almost not evolved from barium cerate doped by samarium and gadolinium. The amount of desorbed oxygen features a square dependence on cationic doping by neodymium. At similar degrees of cationic doping, the amount of water desorbed from neodymium–doped barium cerate is always lower than that from the cerate doped by samarium and gadolinium. The obtained experimental data on thermal desorption and analysis of literature data served as a basis for the conclusion as to the mixed valency of neodymium Nd(III)–Nd(IV) in BaCe_1–x Nd _x O_3–δ. In this case, at similar doping degrees x, the hydration degree of BaCe_1–x Nd _x O_3–δ is lower and the oxygen index is higher than in BaCe1_–x (Sm,Gd) _x O_3–δ. The differences become more pronounced at high degrees of cationic doping and must decrease at an increase in temperature.     

Oxygen Nonstoichiometry and Transport Properties of Mixed-Conducting Ce<Subscript>0.6–<Emphasis Type="Italic">x</Emphasis></Subscript>La<Subscript>0.4</Subscript>Pr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>2–δ</Subscript>

The oxygen nonstoichiometry and electrical conductivity of fluorite-type solid solutions Ce_0.6?xLa_0.4Pr _x O_2–δ (x = 0.1–0.2) were studied in the oxygen partial pressure range 10^–19–0.35 atm at 1023–1223 K. It was confirmed that the Pr^4+/3+ and Ce^4+/3+ redox pairs, which determine the concentration of p- and n-type electron charge carriers, play the dominant roles under oxidizing and reducing conditions, respectively. The conductivity vs. charge carrier concentration dependencies in these conditions are almost linear. Increasing praseodymium content leads to a substantially higher hole conductivity and an expanded range of the oxygen nonstoichiometry variations at high oxygen partial pressures. Under reducing conditions when praseodymium cations become trivalent opposite trends are observed on doping.     

The thermodynamic parameters of oxygen nonstoichiometry defects in lanthanum cobaltite doped with acceptor impurities (Sr and Ni)

The oxygen nonstoichiometry δ of lanthanum cobaltite doped with acceptor impurities (Sr and Ni), La_{1 ? x} Sr_xCo_0.9Ni_0.1O_{3 ? δ} (x = 0.1, 0.3), was studied by high-temperature thermogravimetry over the temperature and pressure ranges 723 K ≤ T ≤ 1373 K and 10^?3 atm ≤ $p_{O_2 } $ ≤ 1 atm. The partial replacement of cobalt with nickel and lanthanum with strontium increased the oxygen nonstoichiometry δ. The partial molar enthalpies $\Delta \bar H^\circ _O $ and entropies $\Delta \bar S^\circ _O $ of solution of oxygen in the solid phase were calculated. Models of point defect formation were suggested and analyzed. The equilibrium constants of formation and concentrations of predominant point defects, ionized oxygen vacancies V _o ^.. , holes Me _Co ^. (Co _Co ^. and Ni _Co ^. ), and electrons Me _Co ^′ (Co _Co ^′ and Ni _Co ^′ ) localized on 3d transition metals, were determined by nonlinear regression from the experimental and theoretical logp $p_{O_2 } $ ?δ dependences.     

Preparation,structure, and charge transport characteristics of BIFEVOX ultrafine powders

Existence boundaries, structure, and transport parameters of ultrafine powders were studied in Bi₄V_{2 ? x} Fe _x O_{11 ? x} (BIFEVOX) solid solutions. The details of synthesis of the solid solutions via liquid precursors are analyzed comparatively. In general, BIFEVOX formation via liquid precursors is similar to phase formation in solid-phase synthesis. With low iron levels (x = 0.05–0.1), solid solutions are formed in the monoclinic α phase (space group C2/m) The compositions with x = 0.125 and 0.15 are mixtures of α- and β phases. In the range 0.2 < x < 0.7, the Bi₄V_{2 ? x} Fe _x O_{11 ? x} solid solution has the structure of the γ phase of Bi₄V₂O₁₁ (space group I4/mmm). The β phase in the system in question has a very narrow existence range in the vicinity of x = 0.175. The average particle sizes of the powders prepared by various methods are within 0.5–3 μm. In the powders prepared via liquid precursors, however, the distribution peak shifts toward smaller sizes, to 0.3–1 μm. Mechanical activation conserves the structure of the γ phase of BIFEVOX, and unit cell parameters change only insignificantly; however, the crystal lattice is slightly distorted. The electrical conductivity of BIFEVOX was studied as a function of temperature, preparation technology, and composition using impedance spectroscopy. Equivalent circuits of cells were analyzed. The conductivity of samples prepared by solution technology is always higher than for samples prepared by the solid-phase process. Features of electrical conductivity versus temperature for various phases are noted. All transitions on the conductivity curves match the features of linear thermal expansion curves. Compositions with doping levels x= 0.1–0.3 have the highest total conductivities.     

Thermodynamic calculations of phase equilibria in the neodymium-barium-copper-oxygen system at O<Subscript>2</Subscript> pressures of 0.21 and 1 bar

The analytic equations for the Gibbs energy of Nd-Ba-Cu-O system phases obtained earlier were used to calculate the phase diagrams of the Ba_0.5Cu_0.5O-Nd_0.5Cu_0.5O section at oxygen partial pressures of 0.21 and 1 bar. For p(O₂)= 0.21 bar, the coordinates of the nonvariant system points, the projection of the liquidus surface, and the phase diagram of the Nd_{1 + x} Ba_{2 ? x} Cu₃O_{6 + z} compound in the subsolidus region were determined.     

High-temperature proton conductors with structure-disordered oxygen sublattice

Data on the thermogravimetry, spectroscopy, and electrical charge transfer as functions of T, aH₂O, and aO₂ for niobates and tantalates of alkali-earth metals with structure disordering of the oxygen sublattice, which can show high-temperature proton conduction, are summarized. It is shown that in the solid solution series with decreasing x (that is, with the increasing of the oxygen vacancies concentration) the proton conductivity increase, which is caused by the increasing of both the concentration of proton defects formed in the structure (in compliance with the formula Sr_{6 ? 2x} M _{2 + 2x} ⁺⁵ O₁₀(OH)_2?6x and their mobility. The proton transfer dominates for the compositions with x < 0.15 at temperatures below 550°C. In the solid solutions (Ba_1?y Ca _y )₆Nb₂O₁₁ (0.23 ≤ y ≤ 0.47) characterized by equal concentration of oxygen vacancies, with the increasing of barium content (correspondingly, with the increasing of the lattice parameter) the oxygen-ion conductivity (at aH₂O = 3 × 10^?5) grows monotonically, which is caused by the decreasing of the oxygen atom migration energy and increasing of their mobility. In this series, the proton conductivity (at aH₂O = 2 × 10^?2) increased. It was shown, by using IR-spectroscopy and the ¹H NMR method, that the protons exist in the complex oxide structure mainly as energy-wise nonequivalent OH^? groups: isolated, closely set, and paired, whose quantitative ratios are determined by the coordination preference of the B-sublattice elements.     

Alkali (alkaline-earth) metal,aluminum, and titanium complex orthophosphates: Synthesis and characterization

Phase formation in the A_{1 + x} Al _x Ti_{2 ? x} P₃O₁₂ (A = Li, Na, K, Rb, or Cs; 0 ≤ x ≤ 2.0) and B_{0.5(l + x)}Al _x Ti_{2 ? x} P₃O₁₂ (B = Mg, Ca, Sr, or Ba; 0 ≤ x ≤ 2.0) systems was studied using X-ray powder diffraction, electron probe microanalysis, and IR spectroscopy. The following double and triple orthophosphates were found to exist: A_{1 + x} Al _x Ti_{2 ? x} (PO₄)₃ with A = Li (0 ≤ x ≤ 0.3), Na (0 ≤ x ≤ 1.0), K (x = 0, 1.0, or 2.0), Rb (x = 0, 1.0, or 2.0), or Cs (0 ≤ x ≤ 1.0) and B_{0.5(l + x)}Al _x Ti_{2 ? x} (PO₄)₃ with B = Mg and Ba (x = 0), Ca and Sr (0 ≤ x ≤ 0.2). These orthophosphates crystallize in the structure types of kosnarite, langbeinite, cesium titanium arsenate, potassium aluminum phosphate, or rubidium aluminum phosphate. Their crystal parameters were calculated. For CsTi₂(PO₄)₃ (x = 0), Rietveld refinement was carried out: space group Ia \(\bar 3\) d, Z = 32, a = 19.909(5) Å, V = 7892(1) ų. This compound has a framework structure. The framework is built of TiO₆ octahedra and PO₄ tetrahedra; eight- and 12-coordinated Cs⁺ cations populate interstices.     

Phase equilibria in the system LaMnO<Subscript>3</Subscript>-SrMnO<Subscript>3</Subscript>-SrCrO<Subscript>4</Subscript>-LaCrO<Subscript>3</Subscript>

A continuous solid solution LaMn_1?y Cr _y O₃ with an orthorhombic structure is found to exist in the range of 0.0 ≤ y ≤ 1.0. An orthorhombic solid solution La_1?x Sr _x CrO₃ exists in the range of 0.0 ≤ x ≤ 0.1. The stability boundaries are determined for the perovskite phase La_1?x Sr _x Mn_1?y Cr _y O₃. An isobaric-isothermal section LaMnO₃-SrMnO₃-SrCrO₄-LaCrO₃ of the system La₂O₃-SrO-Mn₃O₄-Cr₂O₃ in air at 1100°C is designed.     

Prediction of transport properties of new functional materials based on lanthanum-strontium cuprates: Molecular mechanics calculations

The method of molecular dynamics is used for prediction of properties of new functional materials based on lanthanum-strontium cuprates La_{2 ? x} Sr _x CuO_{4 ? δ} as new materials of the solid state ionics. The most interesting phases are synthesized to test the obtained calculation data and their electrophysical and thermomechanical characteristics are studied. It is shown that the high values of the oxygen diffusion coefficients are obtained in the La_{2 ? x} Sr _x CuO_{4 ? δ} solid solutions with a high replacement degree of Sr → La (up to x = 1). The calculated values of lattice cell parameters, thermal expansion coefficients and oxygen diffusion coefficients agree with the experimental data. The observed anisotropy of anionic transport for all the studied compositions corresponds to the regularities of crystal structure of complex oxides. Using the molecular dynamics method allows tracing the contribution of separate types of oxygen ions (equatorial and apical) into ionic transport at the microscopic level and also confirming directly that oxygen diffusion occurs according to the usual jump mechanism, mainly in (CuO₂) layers.     

Hydration and proton transport in solid solutions based on Ba<Subscript>2</Subscript>CaWO<Subscript>6</Subscript>

Hydrated alkaline-earth metal tungstates Ba₄Ca_{2 + x} W_{2 ? x} O_{12 ? 2x} with perovskite structure were studied by the thermogravimetry, ¹H NMR, IR, and Raman spectroscopy methods. Electrical conductivity and transfer numbers were measured with varying T, \(p_{O_2 } \) and \(p_{H_2 O} \). The solid solutions are capable of reversibly intercalating water and can exhibit high-temperature proton transport. The localization of protons on oxygen results in the appearance of energetically nonequivalent OH groups; a small fraction of protons are present in the form of H₂O and H₃O⁺.     

Isomorphic substitution of neodymium and gadolinium for calcium in synthetic hydroxovanadate

Isomorphic substitution of neodymium and gadolinium for calcium in synthetic hydroxovanadate Ca_{5 ? x} M _x (VO₄)₃(OH)_{1 ? x} O _x (M = Nd, Gd) is studied in the range 700–1000°C using X-ray powder diffraction, single-crystal X-ray diffraction (Rietveld technique), and IR spectroscopy. Single-phase solid solutions at 800°C are formed with x ≤ 0.35 for M(III) = Nd and x ≤ 0.3 for M(III) = Gd. With high x, the apatite solid solution coexists with Ca₃(VO₄)₂, Nd₂O₃, and X phases. With increasing x in the homogeneous region, the intensity of the bands of stretching vibrations and librations of OH groups decrease. Single-crystal X-ray diffraction shows that neodymium and gadolinium substitute for calcium in solid solutions mostly in Ca(2) positions.     

Synthesis and characterization of Sr<Subscript>0.75</Subscript>Y<Subscript>0.25</Subscript>Co<Subscript>0.9</Subscript>Ru<Subscript>0.1</Subscript>O<Subscript>3−<Emphasis Type="Italic">δ</Emphasis></Subscript> perovskite as a solid oxide fuel cell cathode material

The oxygen-deficient Sr_0.75Y_0.25Co_0.9Ru_0.1O_3?δ (SYCR) cathode is systematically evaluated for the application of solid oxide fuel cells. X-ray diffraction analysis indicates that SYCR presents a tetragonal structure with space group of I4/mmm (139). In the measured high oxygen partial pressure (pO₂) region (0.01–0.21 atm), the conductivity increases with increasing pO₂ because of the oxygen vacancy annihilation and hole creation, relating to a general p-type semiconducting mechanism. To get an insight into the rate-limiting step of SYCR cathode, behaviors of individual polarization resistance (R ₁ and R ₂) are investigated in different pO₂. The obtained fitting results reveal that R ₁ is nearly independent on the pO₂, while R ₂ presents a (pO₂)^?0.5 dependence. At 800 °C, SYCR cathode exhibits an R _p value of 0.14 Ω cm², moreover, when using the wet hydrogen (～ 3% H₂O) as fuel and ambient air as oxidant, the maximum power density of single cell Ni-YSZ (yttria-stabilized zirconia)|YSZ|SYCR reaches 452.9 W cm^?2.