Grain-Size Effect on the Dielectric Properties of Bi_0.5Na_0.5TiO₃

We prepared bismuth sodium titanate (Bi_0.5Na_0.5TiO₃) ultrafine powders by the sol-gel method. The dielectric properties of the pressed pellets and fired ceramics with different grain sizes as a function of temperature at various frequencies were studied. With the decrease of grain size, the dielectric anomaly around 200°C increases, while the dielectric thermal hysteresis decreases. All the samples with grain sizes larger than 100 nm show dielectric peaks at temperature of about 350°C. The very little change in T_m observed down to the critical size indicates that Bi_0.5Na_0.5TiO₃ is an order-disorder system above 200°C. In addition, the dielectric peak becomes lower with the decrease of grain size and the ferroelectric critical size of Bi_0.5Na_0.5TiO₃ was eventually determined to be about 100 nm according to the disappearance of dielectric peak.