Electrical conductivity and defect chemistry of Ba_xSr_1 - xCo_yFe_1 - yO_3 - δ perovskites

Bulk Ba_xSr_1 - xCo_yFe_1 - yO_3 - δ compositions (BSCF) were synthesized by the solid-state reaction method. The electrical conductivity of ceramic bars was measured using a dc four-probe method as a function of temperature in air up to 970 °C. All compositions showed thermally activated p -type semi-conductivity up to 450 °C and then a transition to metal-like conductivity. The small-polaron hopping p -type semi-conductivity depends on the oxygen nonstoichiometry, which increases with increasing temperature. Metal-like conductivity is attributed to the overlap of the transition metal d -electron orbitals with the oxygen p -orbitals. Strontium-rich compositions show higher conductivity. The Co/Fe ratio does not influence much the p- type semi-conduction. Iron-rich compositions revealed more metal-like conduction behavior. The degree of overlap between transition metal d -orbitals and oxygen p -orbitals depends on the Ba/Sr as well as on the Co/Fe ratios.