Insulator-metal transition and anomalous sign reversal of the dominant charge carriers in perovskite BaTiO3-δ

Dc resistivity, magnetic susceptibility, and Hall and Seebeck coefficients were measured in the 2-400 K range on perovskite BaTiO(3-delta) single crystals with electron concentration of 9.8 x 10(17)-3.5 x 10(20) cm(-3). The insulator-metal transition in BaTiO(3-delta) was found at critical electron concentration n(c) approximate to 1.6 x 10(20) cm(-3). In contrast to SrTiO(3-delta), both Hall and Seebeck coefficients of metallic BaTiO(3-delta) show strong temperature dependence below 290 K which culminates by a sign reversal from negative to positive. The temperature of the sign reversal anomaly is concentration dependent and increases with n. Magnetic susceptibility of metallic samples shows anomalous decrease upon cooling below 300 K. It is proposed that the low-temperature p-type conductivity in BaTiO(3-delta) may be attributed to the charge-carrier contributions from the two electronic bands separated by a narrow (3-7 meV) energy gap.