Effect of oxygen nonstoichiometry on electrical parameters and thermoelectric performance of electron-doped Ca0.5Sr0.45−xPb0.05LuxMnO3−δ

Abstract The present paper is focused on the analysis of the electrical conductivity (σ), Seebeck coefficient (S), and oxygen content of perovskite-type Ca0.5Sr0.45−xPb0.05LuxMnO3−δ (x = 0.05, 0.10, 0.15 and 0.20) over the temperature (T) range from 300 K to 1220 K in air and over the wide range of oxygen partial pressure in the gas phase (Po2 = ~10−4–0.8 atm) at T = 1023–1223 K. The samples have been synthesized via the citrate-nitrate method of precursor preparation. These manganites have been demonstrated to exhibit the orthorhombic structure (space group Pbnm) at room temperature. The value of δ has been determined by thermogravimetry and coulometric titration. At δ ≈ 0 near room temperature the increase in the activation energy of n-type conductivity with growing concentrations of lutetium and Mn3+ ions has been explained by the influence of the Jahn-Teller effect. A new model have been proposed to explain the dependencies of σ and S on the oxygen content and temperature. It takes into account the removal of oxygen binding the neighbouring Mn4+O6 and Mn3+O6 octahedra in addition to the reduction reaction of Mn4+ to Mn3+. It has been definitively shown that the primary charge carriers in stoichiometric and oxygen-deficient compositions are electrons localised on Mn3+ ions in Mn3+O6 octahedra. Moreover, the values of σ and S in oxygen-deficient manganites are also influenced by charge transfer between two Mn3+ ions with sixfold and fivefold oxygen coordination, resulting in the formation of hole Mn4+O6 and electron Mn2+O5 centres. The composition with x = 0.05 exhibits the highest average values of figure-of-merit in the Ca0.5Sr0.45−xPb0.05LuxMnO3−δ series within the temperature interval from 320 K to 1000 K in air.