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The Ca0.6−xSr0.4HoxMnO3−δ (x = 0.05, 0.10, and 0.15) manganites with an orthorhombic crystal structure (space group Pbnm) have been produced via citrate–nitrate route in air. The electron donor holmium (Ho3+) and oxygen nonstoichiometry strongly affect the electrical conductivity (σ), Seebeck coefficient (S), thermal conductivity (κ), and thermal expansion. The increase in the absolute values of S with temperature and the decline of σ above 500 K are explained by the decrease in the concentration of Mn3+ ions due to their disproportionation into Mn2+ and Mn4+ ions. The equilibrium constant of disproportionation, the concentration and mobility of charge carriers depend on temperature and can be derived from the data for S(T) and σ(T). The formation of Mn3+ ions as a result of partial removal of oxygen from the manganites has a strong influence on their properties above 1000 K. Temperature-activated character of mobility and the values of S are consistent with the adiabatic small polaron transport mechanism in Ca0.6−xSr0.4HoxMnO3−δ. [ABSTRACT FROM AUTHOR]

The perovskite-like electron doped manganites Ca 0.6− у Sr 0.4 La у MnO 3− δ (0 ⩽ y ⩽ 0.15) were synthesized in order to study oxygen content variations with composition, temperature and oxygen partial pressure. The samples are characterized with orthorhombic Pnma structure at 0 < у < 0.1 and tetragonal I 4/ mcm structure at у ⩾ 0.13 according to room temperature powder X-ray diffraction (XRD). The high-temperature data of XRD in air show transition to cubic Pm 3 ¯ m structure at heating above 550 °С. The variations of oxygen content in Ca 0.6− у Sr 0.4 La у MnO 3− δ with temperature and oxygen pressure were measured using thermogravimetry and coulometric titration at temperatures between 650 and 950 °С in the oxygen pressure range 10 −4 –0.7 atm. The oxygen nonstoichiometry δ changes in Ca 0.6− у Sr 0.4 La у MnO 3− δ are observed to considerably decrease with the increase in lanthanum content. In order to explain the nonstoichiometry data, a thermodynamic model is developed based on defect formation reactions 2Mn 3+ + V O + ½O 2 = 2Mn 4+ + O 2− and 2Mn 4+ = Mn 3+ + Mn 5+ . The respective equilibrium constants, defect formation enthalpies and entropies are obtained. The concentrations of different defects are calculated as functions of temperature, oxygen pressure and lanthanum content. The high-temperature conductivity measurements are carried out for independent verification of the suggested model. [ABSTRACT FROM AUTHOR]

This article gives a brief overview of the modern methods for in situ studying of oxygen non-stoichiometry in oxides. The emphasis is on the techniques that are based on the use of transport properties of solid electrolytes with unipolar oxygen ion conductivity, as well as on the experimental implementation of coulometric titration. The possible sources of experimental errors are considered, and methods are discussed of their accounting and minimization. Several examples are given of the using of the data on non-stoichiometry for the determination of thermodynamic properties and defect structure in oxides. [ABSTRACT FROM AUTHOR]

The Ca0.6−xSr0.4HoxMnO3−δ (x = 0.05, 0.10, and 0.15) manganites with an orthorhombic crystal structure (space group Pbnm) have been produced via citrate–nitrate route in air. The electron donor holmium (Ho3+) and oxygen nonstoichiometry strongly affect the electrical conductivity (σ), Seebeck coefficient (S), thermal conductivity (κ), and thermal expansion. The increase in the absolute values of S with temperature and the decline of σ above 500 K are explained by the decrease in the concentration of Mn3+ ions due to their disproportionation into Mn2+ and Mn4+ ions. The equilibrium constant of disproportionation, the concentration and mobility of charge carriers depend on temperature and can be derived from the data for S(T) and σ(T). The formation of Mn3+ ions as a result of partial removal of oxygen from the manganites has a strong influence on their properties above 1000 K. Temperature-activated character of mobility and the values of S are consistent with the adiabatic small polaron transport mechanism in Ca0.6−xSr0.4HoxMnO3−δ. [ABSTRACT FROM AUTHOR]

Abstract: The conductivity is measured in the series of solid solutions SrFe1− x Nb x O3− δ , where x =0.05, 0.1, 0.2, 0.3, 0.4, within the oxygen partial pressure limits 10−18–0.5atm and temperature range 650–950°C. The contributions to the total conductivity from oxygen ions, electrons and electron holes are obtained based on their different pressure dependences. The doped derivative with x =0.1 is found to be a singular composition where ion conductivity attains a maximal value while activation energy for ion transport is minimal. This peculiar behavior is attributed to formation of favorable microstructure in the oxide. The deeper doping results in deterioration of ion transport, which is explained by oxygen vacancy filling. It is shown that replacement of iron for niobium favors enhanced thermodynamic stability towards reduction. The oxygen permeability is evaluated from the conductivity data, and it achieves rather high values in the doped derivatives. These oxides can, therefore, be recommended for further evaluation as oxygen separating membrane materials for partial oxidation of natural gas. [Copyright &y& Elsevier]