Dielectric anomalies in solid solutions of tetragonal tungsten bronze barium sodium niobate‒potassium bismuth niobate

The crystal structure as well as the dielectric and ferroelectric properties of the tetragonal tungsten bronze (TTB) system (1 ‒ x)Ba₄Na₂Nb₁₀O₃₀‒xK₄Bi₂Nb₁₀O₃₀ (xKBiN) remain not fully understood due to the several phase transitions, and the observed transition from ferroelectric to diffuse dielectric behavior with increasing KBiN content. Here, we report an extended study of the dielectric response at temperatures from ‒150°C to 700°C under various atmospheric conditions for xKBiN (x = 0, 0.20, 0.30, 0.35, and 0.40), which revealed a decrease in the peak permittivity with increasing KBiN content, consistent with previous reports. Permittivity maxima were observed at 580°C, 360°C, 270°C, and 190°C for BNN, 0.20KBiN, 0.30KBiN, and 0.35KBiN, respectively. Dielectric measurements showed that the peak permittivity for 0.40KBiN was either observed at 130°C or as a diffuse response closer to room temperature. Moreover, dielectric phenomenon related to the presence of moisture at the surface was detected. Dielectric permittivity measurements under dry conditions were conducted, revealing another dielectric phenomenon previously reported for other TTBs. This dielectric phenomenon is discussed in terms of a recent study on the origin of a dielectric anomaly in Sr₄Na₂Nb₁₀O₃₀. A signature of the ferroelastic transition exhibited in BNN and 0.20KBiN was also detected in dry conditions.