Thermoelectric properties of the brownmillerite oxide Ca_2-yLa_yCo_2-xAl_xO₅

We prepared the brownmillerite oxide Ca_2-yLa_yCo_2-xAl_xO₅ and found that it was an n-type conductor. The thermopower and the resistivity of the single crystal are respectively -90 μ V/K and 68 mΩcm along the ab direction at 440 K, which suggest relatively good thermoelectrical properties, compared with other transition-metal oxides. Their temperature dependences are of activation type, and the activation energies are 0.2 eV for the resistivity and 0.04 eV for the thermopower. These energies differ by one order in magnitude, which implies that a polaron dominates the charge transport. The sign of the thermopower of the polycrystals changes from negative to positive at 500 K, indicating that holes are excited thermally to decrease the magnitude of thermopower.