Defect structure and Fermi-level pinning of BaTiO3 co-doped with a variable-valence acceptor (Mn) and a fixed-valence donor (Y).

ABSTRACT

Defect structures of BaTiO(3) 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 BaTiO(3) co-doped with a variable-valence acceptor Mn(Mn(Ti)'', Mn(Ti)') and a fixed-valence donor Y(Y(ba)·) in different co-doping ratios (m(d)/m(a)) as functions of oxygen activity in the range of -20 < log a(O(2))≤ 0 at elevated temperatures of 900-1100 °C. Their systematic variations with m(d)/m(a) and log a(O(2)) are reported, and thereby defect structures of the co-doped BaTiO(3) depending on m(d)/m(a) are determined. It is found that for the co-doping ratio 1 < m(d)/m(a) < 2, the Fermi level is pinned at a few kT's around the deep level of Mn(Ti)'' across the otherwise p-type semiconducting log a(O(2))-region of Mn-singly doped BaTiO(3), and attributed to deep acceptor-shallow donor mutual compensation 2[Mn(Ti)''] + [Mn(Ti)'] ≈ [Y(ba)·], thus turning otherwise p-type semiconducting BaTiO(3) semi-insulating.