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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]

• Crystal structure features of the solid solution Sr 0.5 Ca 0.5 Mn 1– x V x O 3– δ (0 ≤ x ≤ 0.125). • Electrical conductivity and Seebeck coefficient of Sr 0.5 Ca 0.5 Mn 1– x V x O 3– δ. • Impact of vanadium doping on thermoelectric properties of Sr 0.5 Ca 0.5 Mn 1– x V x O 3– δ. The solid solution Sr 0.5 Ca 0.5 Mn 1– x V x O 3– δ (0 ≤ x ≤ 0.125) has been obtained via a citrate-nitrate synthesis route. Powder X-ray diffraction (XRD) data for x = 0, 0.025 and 0.05 reveal the formation of perovskite-type orthorhombic structure with Pbnm space group (SG). Further increase of the vanadium content results in the appearance of the tetragonal crystal structure (SG I 4/ mcm) so that the composition of x = 0.075 is formed by the orthorhombic and tetragonal phase constituents in the 1:3 vol ratio. The single-phase sample at x = 0.1 is characterized by tetragonal symmetry of its unit cell. The comparison between the calculated (XRD) and pycnometric density values is indicative of 6-fold oxygen coordination of vanadium dopants. High-temperature electrical conductivity (σ) and Seebeck coefficient (S) measurements give evidence to the presence of electronic charge carriers that can be associated with Mn3+ cations appearing in response to vanadium doping. The largest thermoelectric power factor S 2 σ = 1.5 µW·K−2·cm−1 is attained in the two-phase composition x = 0.075 at 450 °C. [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]

Doped calcium manganite perovskites CaMnO 3− δ which constitute some of the most promising materials for all-oxide thermoelectric generators for conversion of heat to electricity operating in air over a wide temperature range have been modified by Sr–Pb–Ho co-doping. The Ca 0.5 Sr 0.45− x Pb 0.05 Ho x MnO 3− δ samples (x = 0.05, 0.10, 0.15 and 0.20) synthesized by the citrate-nitrate method of precursor preparation are shown to have an orthorhombic crystal structure (space group Pbnm) at room temperature. The relationship between temperature, concentration of donor ions, Jahn-Teller distortions and energy parameters of carrier formation and mobility that affect the thermoelectric properties of electron-doped manganites has been studied. The temperature-activated nature of the electrical conductivity (σ) was revealed to be consistent with the adiabatic mechanism of small-radius polaron transport. The increase in the absolute values of the Seebeck coefficient (S) with rising temperature and the decrease in σ above 500 K are attributed to the decrease in the concentration of Mn3+ ions due to their disproportionation into Mn2+ and Mn4+ ions. The equilibrium constants of the disproportionation reaction, carrier concentration and mobility have been derived from the temperature dependencies of S (T) and σ (T) in the conditions when δ ≈ 0. The best values of power factor (S 2 σ) and dimensionless thermoelectric figure-of-merit (ZT = S 2 σT / κ) have been found at the substitution level x = 0.15. [ABSTRACT FROM AUTHOR]

The Sr 0.75 Ca 0.25 Mn 1− x V x O 3− δ (x = 0.05 and 0.10) oxides with a cubic perovskite-type structure (space group P m 3 ‾ m) have been synthesized via citrate-nitrate route in air. The electron donor vanadium (V5+) strongly affects on electrical conductivity (σ), Seebeck coefficient (S), thermal conductivity (κ) and thermoelectrical characteristics in the range of 340–770 K, in which the oxygen content does not change with temperature (T). Temperature activated character of conductivity is consistent with the adiabatic small polaron transport mechanism. The higher n-type electrical conductivity in Sr 0.75 Ca 0.25 Mn 1−x V x O 3− δ than in other cubic SrMnO 3 - based oxides is explained by the decrease in the distortion of MnO 6 octahedra. The metallic-like increase of the absolute S values with T is explained by the decrease in the concentration of Mn3+ ions due to their disproportionation to Mn2+ and Mn4+ ions. The temperature dependent variations of the charge disproportionation equilibrium constant, concentration and mobility of charge carries are derived from the experimental data for S (T) and σ (T). The formation of Mn3+ ions as a result of the partial removal of the oxygen from the samples influences on the thermoelectrical characteristics at high temperatures. The highest values of dimensionless figure-of-merit are achieved in the composition Sr 0.75 Ca 0.25 Mn 0.95 V 0.05 O 3− δ. [Display omitted] [ABSTRACT FROM AUTHOR]

• Vanadium stabilizes the six-layer hexagonal structure of SrMnO 3− δ. • Electrical conductivity and Seebeck coefficient of 6 H -SrMn 0.98 V 0.02 O 3− δ are studied. • Electron transport is carried out by small polarons. • Disproportionation of Mn3+ ions reduces the concentration of charge carriers. In contrast to 6 H -SrMnO 3 which is usually synthesized using a high-pressure and high-temperature technique, SrMn 1− x V x O 3 oxides can be prepared via a solution combustion synthesis route with final annealing in air. Doping of SrMnO 3 with vanadium allows the formation of the SrMn 1− x V x O 3 oxide (x = 0.02) which crystallizes in a six-layer hexagonal structure (space group P 6 3 / mmc , Z = 6). The specific electrical conductivity (σ) and Seebeck coefficient (S) are measured in the temperature (Т) range 350–1220 K in air. The metallic-like change in S with T is explained employing the model of the motion of small polarons considering the disproportionation of Mn3+ ions to Mn2+ and Mn4+ and the spin direction of t 2g -electrons manganese. The enthalpy of disproportionation reaction (130 meV) in 6 H -SrMn 0.98 V 0.02 O 3– δ is higher than in other perovskite-type manganites. [ABSTRACT FROM AUTHOR]

• Phase diagram of the system Sr 0.5 Ca 0.5 Mn 1− x V x O 3− δ (0 ≤ x ≤ 0.1) • Temperature-driven phase transitions in Sr 0.5 Ca 0.5 Mn 1− x V x O 3− δ. • Isosymmetric I 4/ mcm -to- I 4/ mcm phase transition in Sr 0.5 Ca 0.5 Mn 0.9 V 0.1 O 3. • Jahn–Teller effect on the phase transitions in Sr 0.5 Ca 0.5 Mn 1− x V x O 3. • Thermal expansion of Sr 0.5 Ca 0.5 Mn 1− x V x O 3− δ. High-temperature X-ray diffraction and dilatometry have been employed to study the impact of temperature and composition on evolution of the crystal structure and thermal expansion of Sr 0.5 Ca 0.5 Mn 1− x V x O 3− δ (0.025 ≤ x ≤ 0.1). The samples with x ≤ 0.05 undergo a change in the symmetry of their unit cells from orthorhombic (space group (SG) Pbnm) to tetragonal (SG I 4/ mcm) which is related to the first-order transition through the two-phase region. Large tetragonal distortions of the lattice of Sr 0.5 Ca 0.5 Mn 0.9 V 0.1 O 3− δ near room temperature are explained by the d (z 2)-type orbital ordering. The continuous transition from tetragonal (SG I 4/ mcm) to cubic crystal structure (SG Pm 3 ¯ m) occurs near 673 K in all the samples. The coefficients of linear and volumetric thermal expansion are found almost independent of temperature and x in the range of 298 K < T < 770 K. [ABSTRACT FROM AUTHOR]