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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 electron conductors Sr0.5Ca0.5Mn1−xVxO3−δ (x = 0.025, 0.05, and 0.1) synthesized via the citrate–nitrate route have been characterized by powder X-ray diffraction at room temperature and at high temperatures in air. At room temperature, the symmetry of unit cells of corresponding morphotropes change from orthorhombic (space group Pbnm) to tetragonal (space group I4/mcm) due to the cooperative Jahn–Teller effect. In samples with x = 0.05 and x = 0.10, the temperature of phase transition from the orthorhombic (space group Pbnm) to tetragonal (space group I4/mcm) polymorph is found to decrease with the increasing vanadium content. The phase transition from tetragonal to cubic (I4/mcm ↔ Pm3¯m) in all samples occur around 673 K. The electrical conductivity (σ) and Seebeck coefficient (S) have been measured in air within 330–1220 K. The increase in absolute values of S with rising temperature is explained employing the model of motion of small polarons, considering the disproportionation of Mn3+ ions to Mn2+ and Mn4+ and the spin direction of t2g-electrons of manganese. The obtained data allow the calculation for the concentration and mobility of charge carriers. The decrease in enthalpy of disproportionation reaction and mobility activation energy is associated with the declining trend of Jahn–Teller distortions following polymorphic transitions with increasing temperature or morphotropic transitions I4/mcm → Pbnm at low temperatures. [ABSTRACT FROM AUTHOR]

The electrical conductivity of SrFe1−xMoxO3−δ (0.07, 0.15, 0.25) was measured in the range of oxygen partial pressure of 10−16–0.5 atm and at temperatures 800–950 °C by a four-probe dc technique. Experimental results were satisfactorily simulated with a model suggesting that oxygen ions and electronic charge carriers of n- and p-types were involved in conduction. The mobility of charge carriers was calculated using partial conductivities and earlier published oxygen nonstoichiometry data. The mobility of p-type charge carriers was found to decrease in response to a decreasing oxygen content or an increasing molybdenum content in the oxide. The mobility of n-type carriers was found to be unaffected by the oxygen content, but exhibited an accelerating increase upon increasing the molybdenum content. Such behavior of the electron mobility was interpreted in view of the tendency of iron and molybdenum cations to undergo ordering based on the supposition that two different mechanisms of electron transport were involved in these oxides. It was assumed that nanoscale ordered areas with fast electron transport dispersed in the disordered perovskite matrix played the role of a high-conductivity filler in a composite consisting of two components with different conductivities. The behavior of the effective electron mobility was approximated well using the percolation theory. The molybdenum content x = 0.327 was calculated to be the percolation threshold in SrFe1−xMoxO3−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 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 electrical conductivity of SrFe1−xMoxO3−δ (0.07, 0.15, 0.25) was measured in the range of oxygen partial pressure of 10−16–0.5 atm and at temperatures 800–950 °C by a four-probe dc technique. Experimental results were satisfactorily simulated with a model suggesting that oxygen ions and electronic charge carriers of n- and p-types were involved in conduction. The mobility of charge carriers was calculated using partial conductivities and earlier published oxygen nonstoichiometry data. The mobility of p-type charge carriers was found to decrease in response to a decreasing oxygen content or an increasing molybdenum content in the oxide. The mobility of n-type carriers was found to be unaffected by the oxygen content, but exhibited an accelerating increase upon increasing the molybdenum content. Such behavior of the electron mobility was interpreted in view of the tendency of iron and molybdenum cations to undergo ordering based on the supposition that two different mechanisms of electron transport were involved in these oxides. It was assumed that nanoscale ordered areas with fast electron transport dispersed in the disordered perovskite matrix played the role of a high-conductivity filler in a composite consisting of two components with different conductivities. The behavior of the effective electron mobility was approximated well using the percolation theory. The molybdenum content x = 0.327 was calculated to be the percolation threshold in SrFe1−xMoxO3−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]