Ordered structure of 0.94(Na_1/2Bi_1/2) TiO₃-0.06BaTiO₃ investigated by the high resolution transmission electron microscopy

The microstructures of A-site complex perovskite structure 0.94(Na_1/2Bi_1/2) TiO₃(BNT)-0.06BaTiO₃ (BNBT6) ceramics were extensively studied by the high resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) techniques. The SAED patterns of BNBT6 specimens from the [001], [011] and [111] directions show that there are 1/2{110} and 1/2{111} superstructure reflections caused by Bi³⁺ and Na⁺ at the A-site. According to the analytical results of the SAED patterns, the 1/2{110} and 1/2{111} ordered structural models of BNT were established. The 1/2(110} ordered structural model was formed by the alternate arrangement of Bi³⁺ and Na⁺ layers along the [110] direction, and the 1/2{111} ordered structural model was formed by the alternate arrangement of Bi³⁺ and Na⁺ layers along the [111] direction. The multi-layer technique was employed for computer-aided simulation of HRTEM images of BNT 1/2{111} ordered structure along the [001], [011] and [111] directions. Compared with the experimental HRTEM images, the computer simulation confirmed the validity of the established 1/2(111} ordered structural model. At the same time, a one-dimensional lattice image was obtained from 1/2(330) and 1/2(3̄3̄0) super lattice reflections using the fast Fourier transformation (FFT) technique. The image shows the dislocations and anti-phase boundaries (APB) caused by displacement of local Bi³⁺, Na⁺ layers along the [110] direction. The existence of APB indicates that there is an alternate arrangement of Bi³⁺ and Na⁺ layers along the [110] direction, which confirms the existence of a 1/2(110} ordered structure. The results of the ordered structure of BNBT6 by HRTEM indicate that the chemical ordering of Bi³⁺ and Na⁺ at the A-site is the main reason for the super lattice reflections.