Influence of holmium doping and oxygen nonstoichiometry on the transport properties of perovskite-type Ca0.6−x Sr0.4HoxMnO3−δ

Abstract The Ca0.6−xSr0.4HoxMnO3−δ (x = 0.05, 0.10 and 0.15) manganites featured 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 on 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, concentration and mobility of charge carriers as a function of temperature are obtained from the dependencies 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 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.