Influence of the synthesis conditions on the dielectric properties in the Bi0.5Na0.5TiO3-KTaO3 system

A novel (1 − x)Bi_0.5Na_0.5TiO₃-xKTaO₃ system was characterized using X-ray powder diffraction, scanning electron microscopy, as well as dielectric and ferroelectric measurements. The results showed the formation of solid solutions across the whole concentration range; however, using a solid-state reaction method it was not possible to obtain single-phase ceramics. The secondary phases formed in the system were alkali-hexatitanate and -tetratantalate. The formation of the solid solutions initially starts with the formation of the Bi_0.5Na_0.5TiO₃- and KTaO₃-rich phases, which then react towards the nominal composition at higher temperatures. We observed that the structural and dielectric properties are strongly influenced by the heat-treatment conditions. Typical relaxor properties with a frequency dispersion of the dielectric maximum were obtained only after annealing at a higher temperature, which considerably improved the homogeneity of the perovskite phase. In accordance with the decreasing temperature of the permittivity maximum, ferroelectric measurements showed a changing of the properties from ferroelectric through relaxor to paraelectric with an increasing content of KTaO₃.