Electrical Properties and Oxygen Stoichiometry of Ba_1-xSr_xTiO_3-δ Ceramics

Ba_1-xSr_xTiO₃ solid solutions prepared by a solid-state reaction in air at 1200-1400 °C, followed by slow cooling to room temperature at the end of the reaction, were essentially oxygen-stoichiometric and p-type. Their conductivity increased reversibly when either p(O₂) in the surrounding atmosphere was increased or a dc bias as small as 1 V was applied across the samples. The enhanced p-type conductivity is attributed to the creation of mobile holes on underbonded oxide ions. The same samples quenched from >∼1400 °C were increasingly oxygen deficient and n-type. They showed reduced conductivity with either a dc bias or increased p(O₂), attributed to the trapping of mobile electrons. These materials provide a rare example of a switch between n-type and p-type conductivity, in the same material, linked to oxygen stoichiometry variation. In both n- and p-type materials, the samples responded to external stimuli in a way similar to that of a leaky capacitor; polarization processes at sample surfaces led first to charge storage and second to a reversible change in bulk electrical properties.