Multi-Site and multi-ionization of Sn in the doping of BaTiO ₃

This article considers the diverse substitutional effects of the Sn cations in the BaTiO₃ lattice and its impact on the electrical conduction as a function of A/B stoichiometry, oxygen partial pressure, and temperature. High-density specimens were fabricated in the different oxygen partial pressures to control the valence state of Sn ion. Specifically, the nonstoichiometric materials were sintered in a low pO₂ atmosphere (10^-14 atm at 1320°C) and in a high pO₂ atmosphere (10^-0.21 atm at 1320°C), respectively. It is found that Sn occupying the Ti-site acts as an acceptor dopant, and the electronic conductivity varies from a n-type to p-type transition, with increasing oxygen activity as mostly expected. However, there is an unusual case noted with Sn doping the A-site where the conductivity, σ, is invariant at high pO₂'s, i.e., σ ∼ pO2m with m ≈ 0 in the high pO₂ regime. The variation of the conductivity is explained by a valence changing of Sn ion from +2 to +3 to +4 with increasing oxygen partial pressure, and we model this data across all conditions within a self-consistent defect chemistry model.