Correlative analysis of local structure in (Bi1/2Na1/2)TiO3-(Bi1/2K1/2)TiO3-BaTiO3 ceramics compositions through MPB
Due to the environmental and health urgency in replacing Pb-based piezo-materials e.g. lead zirconate titanates, which are used in several technological applications a rising interest in studying relaxor Pb-free Bi-based Perovskite ceramics, such as (Bi1/2Na1/2)TiO3, BNT has been seen since the last decade. However, BNT-based ceramics develop a disordered crystal structure leading to relaxation mechanisms which are still unclear for some binary and ternary systems combining BNT with (Bi1/2K1/2)TiO3 (BKT), BaTiO3 (BT), and (K1/2Na1/2)NbO3 (KNN). Depending on cations involved in the formation of the Perovskite and the susceptibility of complex defects formation, the relaxation mechanisms are therefore complex since they deal with an extremely sensitive dynamic nano-polar regions, which can change over several orders of magnitude in time and space. Therefore, these ordering/ionic conditions impact electro-mechanical and optical properties and ferro/piezoelectric performance. For instance, contradictory results of the existence of the morphotropic phase boundary (MPB) region of BNT-based materials are consistently reported based on different processing routes. The present work proposes an MPB zone in the BNT-BKT-BT quasi-ternary system based on direct evidence of strong correlative structure-properties relationships. The quantification of local structure was performed by XRD, NMR uRaman and PFM. The results clearly showed non-cubic diffraction profiles for all ferroelectric/relaxor compositions due to an enhanced lattice distortion when the material purity was improved. This behavior is observed when the material experiences a field-induced relaxor-ferroelectric transition. The mechanisms behind the thermal-evolution of the structure due to local order/disorder transition were elucidated by correlative uRaman/PFM studies. In addition, a modified analysis of the intensity ratio of specific reflections for quantitative analysis of XRD diffraction patterns is proposed revealing compositional-driven changes in the structural disorder (non-polar cubic phase percent). These results showed a good match with those obtained by 23Na NMR analysis. On the other hand, a gradual structural phase transition from rhombohedral to tetragonal symmetry through the MPB zone in the BNT-BT, BNT-BKT and BNT-BKT-BT systems was observed with increasing BT, BKT contents. Moreover, structural behavior showed a good correlation with dielectric and piezo/ferroelectric properties.