Electrical Properties of Ca-Doped BiFeO3 Ceramics: From p-Type Semiconduction to Oxide-Ion Conduction

The conductivity of Ca-doped BiFeO3 ceramics varies by many orders of magnitude, depending on the oxygen partial pressure during processing. Bi1-xCaxFeO3-(x/2)+delta ceramics are mixed oxide ion/electron conductors at 800 degrees C, but the electron conduction can be suppressed; when sintered and cooled in N-2 from 800 degrees C, they are oxide ion conductors with activation energy similar to 0.82-1.04 eV and conductivity similar to 1 X 10(-5) S cm(-1) at 300 degrees 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 similar to 1 X 10(-3)-4 X 10(-5) S cm(-1) at room temperature and activation energy similar to 0.27-0.40 eV. The oxygen stoichiometry varies over the range 0 < delta < similar to 0.016, depending on processing conditions. The semiconductivity is attributed to mixed valence Fe with < similar to 3.2% Fe4+.