Electron transport and thermoelectric properties in (Pb,Ho)-modified Ca0.5Sr0.5MnO3−δ manganites with perovskite-type structure

Abstract

Doped calcium manganite perovskites CaMnO3−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 Ca0.5Sr0.45−xPb0.05HoxMnO3−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 (S2σ) and dimensionless thermoelectric figure-of-merit (ZT = S2σT/κ) have been found at the substitution level x = 0.15.