Oxygen nonstoichiometry, thermo-chemical stability and lattice expansion of La0.6Sr0.4FeO3 − δ

The oxygen nonstoichiometry of La_0.6Sr_0.4FeO_3 − δ was measured at intermediate temperatures (773 to 1173 K) between 1 bar and the decomposition oxygen partial pressure by thermogravimetry and coulometric titration. The decomposition of the ABO₃ perovskite phase was found to occur at low oxygen partial pressures (below 10^− 20 bar). Using an atmosphere-controlled high-temperature XRD setup, the rhombohedral lattice parameters were obtained between 10^− 4 and 1 bar at 773 to 1173 K. A phase transition from rhombohedral to cubic might be expected to occur at high temperatures and for δ near the plateau at δ = [Sr] / 2. The lattice expansion was separated into “pure” thermal and chemically induced expansion by combining the lattice parameters with the oxygen nonstoichiometry data. The linear thermal expansion was formulated with a “pure” thermal expansion coefficient of α_th = 11.052 · 10^− 6 K^− 1 and a chemical expansion coefficient of α_chem = 1.994 · 10^− 2.The results were compared with previous data obtained for La_0.6Sr_0.4Co_1 − yFe_yO_3 − δ with y = 0.2-0.8. La_0.6Sr_0.4FeO_3 − δ was confirmed to show the highest thermo-chemical stability. While the chemical expansion of La_0.6Sr_0.4Co_1 − yFe_yO_3 − δ seems little affected by the iron content, the thermal expansion coefficient was the lowest for La_0.6Sr_0.4FeO_3 − δ.     

Defect chemical and statistical thermodynamic studies on oxygen nonstoichiometric Nd2 − xSrxNiO4 + δ

In order to elucidate how oxygen content changes in Nd_2 − xSr_xNiO_4 + δ (x = 0, 0.2, 0.4), defect chemical and statistical thermodynamic analyses were carried out. The relationship among δ, P(O₂), and T were analyzed by a defect equilibrium model. Since Nd_2 − xSr_xNiO_4 + δ shows metal like band conduction at high temperatures, chemical potential of hole is expressed by the integration of the Fermi-Dirac distribution function and the density of state. The nonstoichiometric variation of oxygen content in Nd_2 − xSr_xNiO_4 + δ can be explained by the defect equilibrium model with a regular solution approximation. Partial molar entropy and partial molar enthalpy of oxygen are calculated from the nonstoichiometric data and Gibbs–Helmholtz equation. The relationship among defect structure, defect equilibrium, and thermodynamic quantities is elucidated by the statistical thermodynamic model. Thermodynamic quantities are calculated by the statistical thermodynamic model with the results of defect chemical analysis and compared with those obtained from experimental results. Thermodynamic quantities calculated by the statistical thermodynamic model can explain rough tendency of those obtained from the δ–T–P(O₂) relationship.     

Modeling of complex point defects in transition metal compounds

The nature and the concentration of point defects in transition metal compounds is usually inferred from nonstoichiometry and electrical conductivity data in conjunction with the assumption of an ideal defect solution and constant electronic mobility. The extent to which this approach can be used to unambiguously confirm the existence of minority defects is investigated here. Acceptability criteria are suggested and an analytical treatment is specifically given. Examples are provided for p-type CoO and n-type TiO₂.     

Synthesis,crystal structures and properties of the new compounds K7–xAg1+x(XO4)4 (X = Mo,W)

Two new isostructural compounds, namely heptapotassium silver tetrakis(tetraoxomolybdate), K_7–x Ag_1+x (MoO₄)₄ (0 ≤ x ≤ 0.4), and heptapotassium silver tetrakis(tetraoxotungstate), K_7–x Ag_1+x (WO₄)₄ (0 ≤ x ≤ 0.4), have been synthesized and found to crystallize in the polar space group P 6₃mc (Z = 2) with the unit‐cell dimensions a = 12.4188 (2) and c = 7.4338 (2) Å for K_6.68Ag_1.32(MoO₄)₄ (single‐crystal data), and a = 12.4912 (5) and c = 7.4526 (3) Å for K₇Ag(WO₄)₄ (Rietveld analysis data). Both structures represent a new structure type, with characteristic [K1(X O₄)₆] `pinwheels' of K1O₆ octahedra and six X O₄ tetrahedra (X = Mo, W) connected by common opposite faces into columns along the c axes. The octahedral columns are linked to each other through Ag1O₄ tetrahedra along with the K2 and K3/Ag2 polyhedra, forming the polar rods (…Ag1O₄–X 1O₄–empty octahedron–Ag1O₄…). Ag1 is located almost at the centre of the largest face of its coordination tetrahedron and seems to have some mobility. The new structure type is related to the Ba₆Nd₂Al₄O₁₅ and CaBaSiO₄ types, and to other structures of the α‐K₂SO₄–glaserite family. The differential scanning calorimetry (DSC) and second harmonic generation (SHG) results show that both compounds undergo first‐order phase transformations to high‐temperature centrosymmetric phases.     

Transport properties and defect analysis of La1.9Sr0.1NiO4 + δ

The oxygen flux through La_1.9Sr_0.1NiO_4 + δ has been measured as a function of oxygen activity gradient and temperature (750–1000 °C). The oxygen nonstoichiometry was determined by thermogravimetry in the temperature range of 400–1000 °C and oxygen partial pressures of 0.0002–1 atm. The total conductivity was measured over a similar range of conditions. The oxide ion partial conductivity derived from the oxygen flux data is approximately 4 orders of magnitude lower than the total, mainly p-type electronic conductivity. The defect structure was derived based on the data. Combining the oxygen flux and oxygen nonstoichiometry, the self diffusion coefficient of oxygen interstitials was evaluated.     

Surface reaction kinetics in oxygen nonstoichiometry re-equilibration of BaTiO3−δ

We report the (bare) surface redox-reaction rate constant that was determined, along with the chemical diffusivity , by a conductivity relaxation technique on Al-doped single crystal and undoped polycrystal BaTiO_3−δ as a function of oxygen activity in its range of −16≤log a_O2≤0 at elevated temperatures of 800–1100 °C. It takes a value in the range of −4<log( /cm s⁻¹)≤−1, which is even larger than that of the oxides that are considered best as oxygen membranes. It has been found that the surface reaction step grows more rate controlling as the electronic transference number gets smaller or the electronic stoichiometric composition (δ≈0) is approached. The oxygen potential drop due to the surface reaction was estimated by an oxygen concentration cell technique. The oxygen potential drop grows larger as the stoichiometric composition is approached, that is in accord with the variation of against oxygen activity.     

Structure and Specific Heat of Disordered and Ordered Titanium Monoxide TiOy

The structure of disordered and ordered titanium monoxide containing structural vacancies in both sublattices was studied by Xray diffraction. The symmetry of the monoclinic Ti₅O₅ superstructure (space group C2/m) was analyzed. The type and channel of the TiO_y–Ti₅O₅ disorder–order transition was determined. The distribution functions of Ti and O atoms in the metal and nonmetal sublattices of titanium monoxide were calculated. The specific heats of TiO_y titanium monoxides (0.8< y< 1.27) were measured by differential scanning calorimetry for the disordered and ordered states in the range from 340 to 600 K. The dependences of the specific heat, enthalpy, and entropy on the composition and structure of TiO_y were found for the first time. A notable effect of the structural state on the specific heat was detected. In the indicated temperature range, the C _p (T) dependence is adequately described by a function that accounts for the Debye effect and the electronic specific heat.     

Growth and Structure of Lanthanum Polysulfide Crystals

Single crystals of lanthanum polysulfide were grown under nearly equilibrium conditions according to the P _S ndash;T–x diagram of the LaS_1.5–LaS₂ system described in the literature, and quenched from a temperature of 820°C. The structure of these crystals was determined. Their compositions are LaS_1.96(2) and LaS_2.00(2). The crystals of both compositions belong to the rhombic Pnma space group with a slight variation in lattice parameters in the ranges a = 8.133–8.124, b = 16.345–16.334, and c = 4.128–4.131. The nonstoichiometric polysulfide LaS_1.96 is treated as a spatially averaged, disordered individual phase. Arguments are given that these polysulfide phases have compositions intermediate between LaS_1.5 and LaS₂.     

Mixed Conduction in Tb2O3 Doped BaCeO3

BaxCe0.8Tb0.2O3-a （x=0.98-1.03） solid electrolytes were synthesized and characterized by using X-ray diffraction （XRD）. By using AC impedance spectroscopy and gas concentration cell electromotive force （EMF） measurements, the electrical conduction behavior of the specimens was investigated in different gases during 500-1000 ℃ The influence of nonstoichiometry in the specimens with x ≠ 1 on conduction properties was studied and compared with that in the specimen with x = 1. The results show that the specimens are all of perovskite-type orthorhombic structure. In 500-1000 ℃, electronic hole conduction is dominant in dry and wet oxygen, air or nitrogen. Protonic conduction is dominant in wet hydrogen and it is about two orders of magnitude higher than that in hydrogen-free atmospheres （oxygen, air and nitrogen）. The electrical conductivity of the same specimen in water vapor-saturated oxygen, air or nitrogen is slightly higher than that in corresponding gas without water vapor. The electrical conductivities of the nonstoichiometric specimens are higher than those of the stoichiometric one.     

Oxygen Nonstoichiometry, Conductivity, and Seebeck Coefficient of La0.3Sr0.7Fe1−xGaxO2.65+δ Perovskites

The total electrical conductivity and the Seebeck coefficient of perovskite phases La_0.3Sr_0.7Fe_1−xGa_xO_2.65+δ (x=0-0.4) were determined as functions of oxygen nonstoichiometry in the temperature range 650-950°C at oxygen partial pressures varying from 10⁻⁴ to 0.5 atm. Doping with gallium was found to decrease oxygen content, p-type electronic conduction and mobility of electron holes. The results on the oxygen nonstoichiometry and electrical properties clearly show that the role of gallium cations in the lattice is not passive, as it could be expected from the constant oxidation state of Ga³⁺. The nonstoichiometry dependencies of the partial molar enthalpy and entropy of oxygen in La_0.3Sr_0.7(Fe,Ga)O_2.65+δ are indicative of local inhomogeneities, such as local lattice distortions or defect clusters, induced by gallium incorporation. Due to B-site cation disorder, this effect may be responsible for suppressing long-range ordering of oxygen vacancies and for enhanced stability of the perovskite phases at low oxygen pressures, confirmed by high-temperature X-ray diffraction and Seebeck coefficient data. The values of the electron-hole mobility in La_0.3Sr_0.7(Fe,Ga)O_2.65+δ, which increases with temperature, suggest a small-polaron conduction mechanism.