Two perovskite cobaltites, LaCoO3 and DyCoO3, which are border compounds with respect to the Ln size, were investigated by the electric resistivity and thermopower measurements up to 800-1000 K. Special attention was given to effects of extra holes or electrons, introduced by light doping of Co sites by Mg2+ or Ti4+ ions. The experiments on the La-based compounds were complemented by magnetic measurements. The study shows that both kinds of charge carriers induce magnetic states on surrounding Co3+ sites and form thus thermally stable polarons of large total spin. Their itinerancy is characterized by low-temperature resistivity, which is of Arrhenius type rho similar to exp(E-A/kT) for the hole (Co4+) -doped samples, while an unusual dependence rho similar to 1/T-nu (n=8-10) is observed for the electron (Co2+) -doped samples. At higher temperatures, additional hole carriers are massively populated in the Co3+ background, leading to a resistivity drop. This transition become evident at similar to 300 K and 450 K and culminates at TI-M=540 and 780 K for the La- and Dy-based samples, respectively. The electronic behaviors of the cobaltites in dependence on temperature are explained considering local excitations from the diamagnetic low-spin (LS) Co3+ to close-lying paramagnetic high-spin (HS) Co3+ states and subsequent formation of a metallic phase of the IS Co3+ character through a charge transfer mechanism between LS/HS pairs. The magnetic polarons associated with doped carriers are interpreted as droplets of such intermediate (IS) phase.
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