High-temperature Thermoelectrical Properties of Cubic SrMnO3−-based Manganites

The Sr0.75Ca0.25Mn1-xVxO3-delta (x = 0.05 and 0.10) oxides with a cubic perovskite-type structure (space group Pm3m) have been synthesized via citrate-nitrate route in air. The electron donor vanadium (V5+) strongly affects on electrical conductivity (sigma), Seebeck coefficient (S), thermal conductivity (kappa) 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 Sr0.75Ca0.25Mn1-xVxO3- delta than in other cubic SrMnO3-based oxides is explained by the decrease in the distortion of MnO6 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 sigma (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 Sr0.75Ca0.25Mn0.95V0.05O3-delta.