Microstructure evolution and dielectric properties of Ba _5-xNa_2xNb₁₀O₃₀ ceramics with different Ba-Na Ratios

The tetragonal tungsten bronzes of Ba5-xNa_2x Nb ₁₀O₃₀ (BNN, 0.5 ≤ x ≤ 1.3) ceramics were synthesized using the solid state reaction method. The sintering behavior and dielectric characteristics of the BNN ceramics, as a function of the Ba-Na ratio, were examined. Densification of the samples with excess compositions of Ba and Na was higher than that of the stoichiometric BNN sample. The maximum dielectric constant and the Curie temperature showed highest values at the stoichiometric composition and decreased as the composition shifted away from the stoichiometry. in order to obtain a quantitative evaluation of the diffuse phase transition (DPT) behavior of the BNN ceramics, γ and C/κ _max were calculated. The weakest DPT behavior was observed in the stoichiometric composition. An increase in the DPT is in correlation with the increase in the number of ways of cation distribution by the disordered occupation of Ba and Na and the vacancies in the A1 and A2 sites of the tungsten bronze structure.