BaTiO3 and SrTiO3 perovskites of the A2+B4+O3 type form complete solid solution, Sr1−x BaxTiO3, which can accommodate a substantial amount of donor dopants, for example, La. At high oxygen partial pressure, La dopants in SrTiO3 are compensated by A-site vacancies, whereas in BaTiO3 they are compensated by B-site vacancies. Therefore, donor compensation in the Sr1−xBaxTiO3 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, xc, the free energy of the Sr1−xBaxTiO3 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 Sr1−xBaxTiO3 system equilibrated at 1400°C indeed becomes invariant to the vacancy-type compensation at xc≈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 BaTiO3 and SrTiO3. 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.
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Document Type: Research Article
National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan
Department of Materials Science and Engineering, McMaster University, Hamilton, Ontario, Canada L8S 4L7
National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
Publication date: 2010-09-01