Defect structure and Fermi-level pinning of BaTiO3 co-doped with a variable-valence acceptor (Mn) and a fixed-valence donor (Y)
Defect structures of BaTiO3 and the like co-doped with variable-valence acceptors and donors are not clear particularly in transition from acceptor domination to donor domination with increasing oxygen activity. We have, thus, examined the electrical conductivity and thermoelectric power of BaTiO3 co-doped with a variable-valence acceptor and a fixed-valence donor in different co-doping ratios (md/ma) as functions of oxygen activity in the range of −20 < log aO2 ≤ 0 at elevated temperatures of 900–1100 °C. Their systematic variations with md/ma and log aO2 are reported, and thereby defect structures of the co-doped BaTiO3 depending on md/ma are determined. It is found that for the co-doping ratio 1 < md/ma < 2, the Fermi level is pinned at a few kT's around the deep level of across the otherwise p-type semiconducting log aO2-region of Mn-singly doped BaTiO3, and attributed to deep acceptor–shallow donor mutual compensation , thus turning otherwise p-type semiconducting BaTiO3 semi-insulating.