Effect of thermal history on stability of the relaxor state in (Bi1/2K1/2)TiO3 ceramics

Abstract

Bismuth potassium titanate (Bi1/2K1/2)TiO3 (BKT) with disordered A-site Bi3+ and K+ ions is known to undergo a spontaneous phase transition between a high-temperature relaxor (R) state and a low-temperature ferroelectric (FE) state at around 300 °C. Here, we demonstrate that the thermal history of BKT ceramics has a great effect on the stability of the R and FE states. To modulate the distribution of Bi3+ and K+ ions, as-sintered BKT ceramics were postannealed at 900 °C and then cooled to 500 °C at a very low cooling rate of −0.1 °C/min. The slowly cooled samples showed a normal-relaxor-like temperature dependence of dielectric permittivity without any anomaly attributed to the spontaneous R-FE transition. The slow cooling treatment changed the room-temperature crystal system of the BKT ceramics from tetragonal to cubic, although it did not induce the cationic ordering of Bi3+ and K+. It was also observed that the macroscopic domain structure disappeared after the slow cooling treatment. These observations demonstrated that the slow cooling treatment stabilized the R state in BKT to inhibit the spontaneous phase transition into the FE state. The possible reason for the stabilization of the R state in the slowly cooled BKT is discussed on the basis of the changes in cationic distribution and local compositional heterogeneity.