Influence of holmium doping and oxygen nonstoichiometry on the transport properties of perovskite-type Ca0.6-xSr0.4HoxMnO3-δ

The Ca0.6-xSr0.4HoxMnO3-delta (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 (sigma), Seebeck coefficient (S), thermal conductivity (kappa), and thermal expansion. The increase in the absolute values of S with temperature and the decline of sigma 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 sigma(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-delta.