The solubilities of acceptor impurities are strongly suppressed when BaTiO, is densified in highly reducing atmospheres. This is evidenced by a shift in the minimum in the equilibrium electrical conductivity to higher oxygen activities, a decrease in the ionic contribution to the conductivity, and a decrease in the leakage current and the rate of leakage current degradation under temperature-voltage stress. The normal solubility is restored by subsequent anneals in air that result in substantial grain growth, and the properties then revert to those of BaTiO, sintered in air. The solubility suppression is attributed to a mass-action interaction between the processes that generate oxygen vacancies, those that result from the compensation of acceptor centers, and those that result from reduction.
The substitution of up to 5% Ca2+ for Ba2+ in BaTiO3 results in a shift in the oxygen pressure dependence of the equilibrium electrical conductivity that is in the same direction as that caused by addition of acceptor impurities such as Al3+ or Ca2+ substituted for Ti4+. In contrast to the latter effect, however, the shape of the conductivity plot is not changed, the conductivity value at the conductivity minimum is not affected, and the amount of the shift increases with decreasing temperature of measurement. It is shown that the shift is primarily due to an increase in the enthalpy of reduction and a decrease in the enthalpy of oxidation as increasing amounts of Ba2+ are replaced by Ca2+.
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