Compensating Defects in Highly Donor-Doped BaTiO3

Nb‐doped BaTiO3 has been prepared with various Ba/(Ti + Nb) ratios such that single‐phase products will be obtained if the charged donor center is assumed to be compensated in turn by Ba vacancies, Ti vacancies, equal concentrations of the two cation vacancies, oxygen interstitials, or electrons. For air‐Fired samples, examination by transmission electron microscopy showed that only the composition adjusted for compensation by titanium vacancies was single phase. The other compositions contained a Ti‐rich second phase in order to achieve a matrix with the appropriate concentration of titanium vacancies. When sintered in a reducing atmosphere, compensation was by electrons, and a Ba‐rich second phase was present hi the composition adjusted to give compensation by titanium vacancies. The results indicate that for donor concentrations greater than ∼0.5 mol% in BaTiO3, charge compensation is achieved by Ti vacancies under oxidizing conditions, and by electrons (as is well‐known) under reducing conditions.' The effect of compensating defects on grain growth is also discussed.