Effects of Bi substitution on the structures, electrical properties, and transparency of Sr2Na1 - 3xBixNb5O15 ceramics

Sr2Na1-3xBixNb5O15 ceramics (SNBN; x = 0.00, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, and 0.08) were synthesized by a conventional solid-state reaction method. To eliminate the impurity phase in the Sr2NaNb5O15 ceramics, we introduced high-charge Bi3+ into the SNN ceramics to reduce the filling degree of A1 and A2 sites. X-ray diffraction results confirmed this compositional design, because all SNBN ceramics showed a pure tetragonal tungsten-bronze phase along with shrinkage of the unit cell when x > 0.01. Meanwhile, Bi3+ introduction improved their dielectric, ferroelectric, and transparent properties. As suggested by Raman and SEM, both the higher densification with relatively uniform grain size and more distorting degree of the NbO6 polar unit contributed to the enhanced electrical properties and transparency at x = 0.04.