We performed visualization of the oxygen diffusion path in oxidized and reduced BaTiO3 ceramics by utilizing an ion-imaging technique. The oxygen tracer diffused quickly from the surface up to the grain boundary, and then behaved as discontinuous steps at the grain boundary. The grain boundary acted as a blocking layer against oxygen diffusion. The reduction process enhanced the blocking effect. The blocking of oxygen diffusion at the grain boundary originated in the formation and distribution of complex defects between the charged oxygen vacancy and the reduced Ti near the grain boundary.
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