The conductivity of Ca-doped BiFeO 3 ceramics varies by many orders of magnitude, depending on the oxygen partial pressure during processing. Bi 1−x Ca x FeO 3−(x/2)+δ ceramics are mixed oxide ion/electron conductors at 800 °C, but the electron conduction can be suppressed; when sintered and cooled in N 2 from 800 °C, they are oxide ion conductors with activation energy ∼0.82−1.04 eV and conductivity ∼1 × 10 −5 S cm −1 at 300 °C, comparable to that of 8 mol % yttria-stabilized zirconia. When heated in O 2 at 125 bar, however, they are mainly p-type semiconductors with conductivity ∼1 × 10 −3 −4 × 10 −5 S cm −1 at room temperature and activation energy ∼0.27−0.40 eV. The oxygen stoichiometry varies over the range 0 < δ < ∼0.016, depending on processing conditions. The semiconductivity is attributed to mixed valence Fe with < ∼3.2% Fe 4+ .

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Yttria-stabilized zirconia, YSZ, is the oxide ion conducting electrolyte of choice in many solid oxide fuel cells, because of its resistance to reduction at the fuel electrode. Here, we demonstrate that electronic conduction is introduced into YSZ under the action of a small dc bias at high temperatures in air. A reversible decrease in sample grain and grain boundary resistances occurs with increasing bias in the range 1–15 V and the Warburg impedance at the sample–electrode–air interface collapses. Similar conductivity changes are seen with acceptor-doped dielectrics and are attributed to the ionization of underbonded oxide ions in the crystal structure, with O– ions acting as the location of the holes and the source of p-type conduction; the ionized electrons are trapped by changes to the equilibria among oxygen species at the sample surface. A similar mechanism may explain the onset of electronic conductivity in YSZ.

The electrical properties of Ca-doped BaTiO3 are very different when Ca substitutes onto Ba or Ti sites. The p-type semiconductivity of Ti-substituted ceramics increases reversibly by one to two orders of magnitude under a dc-bias voltage of ≤100 V cm−1, whereas Ba-substituted ceramics show little sensitivity to a dc bias. This increase in BaTi1−xCaxO3−x, studied over the temperature range 150–600 °C, is independent of electrode material and atmosphere and is attributed to ionization of underbonded O2− ions adjacent to acceptor-doped Ca2+ ions.