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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 Ca2.5Mg2V3O12 vanadate with cubic structure (space group Ia 3 ¯ d) has been prepared by solid-state reaction in air. The crystal structure has been determined by Rietveld refinement of powder X-ray diffraction data. In the garnet-type structure, the Ca2+ ions and cationic vacancies occupy the 24c sites. Conductivity has been measured using AC impedance spectroscopy at 400–950 °С in air and in the range of oxygen partial pressure (p O 2 ) between 10−10 and 0.5 atm. Ca2.5Mg2V3O12 exhibits mixed ionic and electronic conductivity. The n-type electronic conductivity is proportional to p O 2 − 1 / 4 . The ionic and electronic components of conductivity are characterized by the activation energy of about 1.2 and 2.0 eV, respectively. Ion transference number in the air has been shown to increase from 0.6 to almost 1.0 upon temperature decrease from 950 to 600 °С. Analysis of the garnet-like crystal structure allows proposing possible mechanisms of ionic conductivity in which a Ca-ion migration pathway from occupied 24c sites to vacant 24c′ position is carried out through empty 16b sites. [ABSTRACT FROM AUTHOR]

The specific electric conductivity ( σ) and Seebeck coefficient ( S) are measured in electron-doped manganites CaSrLaMnO ( х = 0.4 and y = 0-0.1) in air within temperature ( Т) range 350-1220 K. The new approach is suggested to explain the metallic-like increase of S with T based on combination of a small polaron theory with charge disproportionation and spin direction of manganese cations. The temperature-dependent variations of the charge disproportionation equilibrium constant, concentration, and mobility ( μ) of charge carriers are derived from the experimental data for S( T) and σ( T). The obtained mobility values vary within 0.1-0.55 cm V s. [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]

In this paper, peculiarities are discussed of the temperature control of the methane partial oxidation (POM) in the laboratory reactor with oxygen separating the tubular membrane of lanthanum-strontium ferrite. It is shown that reactions at the catalyst may result in the local decrease of the temperature. Therefore, correct monitoring of the heat regime of the process is possible only when direct contact is avoided of the catalyst with controlling thermocouple. The long-term testing showed stable performance characteristics of the entire POM system during more than 7000 h with the methane conversion level and selectivity of no less than 98.8% and 90%, respectively. [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]

The Ca2.5Mg2V3O12 vanadate with cubic structure (space group Ia 3 ¯ d) has been prepared by solid-state reaction in air. The crystal structure has been determined by Rietveld refinement of powder X-ray diffraction data. In the garnet-type structure, the Ca2+ ions and cationic vacancies occupy the 24c sites. Conductivity has been measured using AC impedance spectroscopy at 400–950 °С in air and in the range of oxygen partial pressure (p O 2 ) between 10−10 and 0.5 atm. Ca2.5Mg2V3O12 exhibits mixed ionic and electronic conductivity. The n-type electronic conductivity is proportional to p O 2 − 1 / 4 . The ionic and electronic components of conductivity are characterized by the activation energy of about 1.2 and 2.0 eV, respectively. Ion transference number in the air has been shown to increase from 0.6 to almost 1.0 upon temperature decrease from 950 to 600 °С. Analysis of the garnet-like crystal structure allows proposing possible mechanisms of ionic conductivity in which a Ca-ion migration pathway from occupied 24c sites to vacant 24c′ position is carried out through empty 16b sites. [ABSTRACT FROM AUTHOR]