Phase diagrams and electrical properties of (Bi1/2Na1/2)TiO3-based solid solutions

Abstract

In this study, we demonstrated the relationship between the phase diagrams and the electrical properties of (Bi1/2Na1/2)TiO3 (BNT)-based solid solutions. In this study, (1−x)(Bi1/2Na1/2)TiO3–xNaNbO3 and (1−x)(Bi1/2Na1/2)TiO3–xKNbO3 (abbreviated to BNT-NN100x and BNT-KN100x) ceramics were prepared by a conventional ceramic fabrication process, and (1−x)(Bi1/2Na1/2)TiO3–x(Bi1/2K1/2)TiO3 (abbreviated to BNKT100x) ceramic was prepared for comparison. We revealed the phase transition temperatures, such as the depolarization temperature Td, rhombohedral-tetragonal phase transition temperature TR-T, and the temperature Tm of the maximum dielectric constant, from the temperature dependence of dielectric properties using poled and unpoled specimens. As a result, it was shown that the BNT-based solid solutions form three types of phase diagrams. In addition, we clarified the relationship between the phase diagrams and the electrical properties of BNT-NN100x, BNT-KN100x, and BNKT100x. The piezoelectric properties were markedly enhanced when TR-T shifted to a lower temperature, and a large piezoelectric constant d33 of 168 pC/N was obtained at the morphotropic phase boundary (MPB) between the ferroelectric rhombohedral and ferroelectric tetragonal phases for BNKT100x. Although the piezoelectric properties almost disappeared when Td shifted to room temperature, the field-induced strain S and the normalized strain d33∗ (=Smax/Emax) abruptly increased to 0.22% and 259 pm/V, respectively, for BNT-NN100x. In particular, a very large reversible strain of S=0.40 with d33∗=498 pm/V was obtained at the MPB between the ferroelectric rhombohedral and ferroelectric relaxor with pseudocubic (tetragonal) phases for BNT-KN100x. This very large reversible strain was clarified to be due to non-180° (71° and 109°) domain switching of the field-induced ferroelectric rhombohedral phase.