Grain-size-dependent spontaneous relaxor-to-ferroelectric phase transition in (Bi<inf>1/2</inf>K<inf>1/2</inf>)TiO<inf>3</inf> ceramics

Dense and phase-pure (Bi<inf>1/2</inf>K<inf>1/2</inf>)TiO<inf>3</inf> (BKT) ceramics with various average grain sizes from 0.18 to 1.01μm were prepared from a hydrothermally synthesized powder and their phase transition behaviors were studied by means of dielectric measurements. A drastic increase of the maximum dielectric permittivity (ε<inf>m</inf>) with increasing the grain size was found in the temperature dependence of permittivity. The sample with the largest grain size clearly showed both a frequency dependence of dielectric maximum temperature (T<inf>m</inf>) and a dielectric anomaly with a strong thermal hysteresis at a temperature below T<inf>m</inf>, demonstrating that the BKT ceramic is intrinsically a material exhibiting a spontaneous relaxor to normal ferroelectric (R-nFE) phase transition. On the other hand, the suppression of the R-nFE transition was observed in the sample with the smallest grain size, which was explained as an effect of avoiding the internal stress development caused by the volume increase occurring with the phase transition.