Fermi level pinning in Co-doped BaTiO3: Part I. DC and AC electrical conductivities and degradation behavior

We explore the synergistic effects of co-doping BaTiO3 with a judicious combination of acceptors and donors to control the point defect chemistry and electrical properties, with the goal of simultaneously limiting the electronic and ionic conductivities over broad temperature and oxygen partial pressure (pO(2)) ranges. Specifically, we compare the temperature- and pO(2)-dependent electrical properties of BaTiO3 ceramics acceptor-doped with either Mn or Mg and co-doped with a Y donor. This study, which is the first of a two-part series, presents the electrical properties as a function of pO(2), temperature, and time, focusing on the grain-interior electrical response. The DC and AC electrical conductivity measurements reveal that co-doping with Mn and Y can result in (1) increased electrical resistivity over a broad temperature range, (2) pO(2)-independent electrical conductivity in oxidizing conditions, and (3) improved time-dependent dielectric degradation resistance. These behaviors are attributed to a Fermi level pinning effect, as is explained in the companion paper, which presents complementary density functional theory (DFT)-based grand-canonical defect chemistry models. The collective experimental and computational studies demonstrate that the pO(2)-independent electrical conductivity in the Mn and Y co-doped BaTiO3 is attributed to a Fermi level pinning mechanism arising from the multivalent Mn dopant, and the background reservoir of positive charge provided by the predominant substitution of Y on the Ba sites. The enhanced degradation resistance is attributed to a reduced oxygen vacancy concentration relative to the other doping chemistries.