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 Mg
²⁺
, Al
³⁺
, Fe
³⁺
, or Na
⁺
, are believed to behave as electron acceptors and induce "extrinsic" oxygen vacancies in the Ba
²⁺
Ti
⁴⁺
O
₃
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.