The Role of Anion Impurities in Barium Titanate

It has been reported that fluorine ions (F^-) can substitute for oxygen ions (O^2-) in small amounts in barium titanate and act as electron donors. While fluorine is not usually an impurity in barium titanate, this work suggests that ions such as Cl^- or (OH)^-, that can be impurities, might behave similarly to F^-. Cl^- ions can be present in barium titanate made using barium chloride and/or titanium chloride, as in the oxalate process. Hydroxyl ions are common in barium titanate made by the hydrothermal or alkoxide processes. Other common impurities in barium titanate, such as Mg²⁺, Al³⁺, Fe³⁺, or Na⁺, are believed to behave as electron acceptors and induce "extrinsic" oxygen vacancies in the Ba²⁺Ti⁴⁺O₃ crystal structure for charge balance. Acceptor impurities are minimized in the manufacture of barium titanate because the presence of oxygen vacancies can degrade the reliability of multilayer ceramic capacitors (MLC\´s) when the barium titanate is used in thin dielectric layers with Ni electrodes. In this paper we consider the possibility that anion donors, particularly (OH)^-, might counteract the effect of minor levels of acceptor impurities and have a beneficial effect on the performance of MLC\´s with Ni electrodes and thin dielectric layers.