BaTiO 3 and SrTiO 3 perovskites of the A 2+ B 4+ O 3 type form complete solid solution, Sr 1− x Ba x TiO 3 , which can accommodate a substantial amount of donor dopants, for example, La. At high oxygen partial pressure, La dopants in SrTiO 3 are compensated by A‐site vacancies, whereas in BaTiO 3 they are compensated by B‐site vacancies. Therefore, donor compensation in the Sr 1− x Ba x TiO 3 solid solution should demonstrate a crossover from the A‐site vacancies at x =0 to the B‐site vacancies at x =1. One may expect, therefore, that at some critical concentration, x c , the free energy of the Sr 1− x Ba x TiO 3 system can become invariant to the vacancy compensation regime. In other words, the system will adopt either A‐ or B‐site vacancies depending on the target chemical composition. Based on the Rietveld refinement of X‐ray diffraction patterns and their phase composition analysis as well as scanning electron microscopic and transmission electron microscopic data, we demonstrate that the 28% La‐doped Sr 1− x Ba x TiO 3 system equilibrated at 1400°C indeed becomes invariant to the vacancy‐type compensation at x c ≈0.25 and can accommodate A‐ and B‐site vacancies at any given ratio. Finally, we propose a microscopic model based on the off‐center Ti displacement and the partial covalency of Ti–O bond to explain the distinct difference in the vacancy compensation mechanisms in BaTiO 3 and SrTiO 3 . These findings are important for a further understanding of the thermodynamics of the intrinsic point defects in perovskites as well as for the improvement of electrical performance of the solid oxide fuel cells, ferroelectric, and voltage‐tunable ceramics.
