Phase-transformation-induced microstructure in lead-free ferroelectric ceramics based on (Bi0.5Na0.5)TiO3-BaTiO3-(Bi0.5K0.5)TiO3

Lead-free ferroelectric ceramics with a morphotropic phase boundary (MPB) composition 85.4% (Bi0.5Na0.5)TiO3-2.6%BaTiO3-12.0% (Bi0.5K0.5)TiO3 (BNT-BT-BKT at a molar ratio of 85.4: 2.6: 12.0) doped with 0.8 mol% Nb2O5 were studied for their crystalline phases and microstructure. The crystalline phases were identified using X-ray diffractometry (XRD) with the contents determined using the Rietveld refinement technique. The phase-transformation-induced microstructure was analyzed using transmission electron microscopy (TEM) and the crystal symmetries were determined using the convergent-beam electron diffraction (CBED) technique. Samples sintered at 1200 degrees C contain a mixture of cubic (C-), tetragonal (T-) and rhombohedral (R-) phases at a ratio of C/T/R = 56.6: 28.4: 15.0 wt%. Two types of grains are produced: one characterized by a featureless contrast consisting of nano-scale T-domains dispersed in a C-phase matrix; the other a core-shell structure with a shell containing twin and anti-phase-boundary (APB) domains coexisting with a (C+T)-phase mixture core. The T-and R-twin boundaries are determined to {111}(T) and {110}(R), respectively, and the fault vector for T-APB to R = 1/2 < 110](T). The characteristic microstructure is discussed in terms of the reduction in the point group symmetry and changes in the unit cell volume or the Bravais lattice upon phase transformation among the C-, T-and R-phases. The twin and the APB domains are induced and explained.