Composition and microstructure of Na0.5Bi0.5TiO3 ceramics with excess Bi

In the present work, Na0.5Bi0.5TiO3 (NBT) ceramics with the addition of excess Bi in two different ways - before calcination and before sintering - are considered, revealing how the excess Bi affects their microstructure and chemical content. Average grain size is seen to decrease, with the grain size distribution becoming less diffused at higher excess Bi concentrations. The reason for such a feature is the shift of the sintering temperature region where the abnormal grain growth starts to contribute towards higher temperatures. The influence of excess Bi is more pronounced in the case if it is added before calcination. It was discovered that a small amount of excess Bi helps to prevent the formation of Bi-deficient inclusions. While, high concentrations of excess Bi induce the formation of Bi-rich inclusions - most probably Na0.5Bi4.5Ti4O15. Possible mechanisms of formation of both types of inclusions are discussed in detail. Instead of Bi over-stoichiometry, elevated Na content and slightly lower O content were detected in the matrix grains of the sintered NBT ceramics prepared with excess Bi. These deviations increase upon increasing the added excess Bi concentration. The presence of another, Na-rich phase, is assumed, which could not be detected by X-ray diffraction or by energy-dispersive X-ray analysis.