A New Phase Boundary in (Bi1/2Na1/ 2)TiO3BaTiO3 Revealed via a Novel Method of Electron Diffraction Analysis

A new phase boundary is revealed in (1-x)(Bi1/2Na1/2)TiO3xBaTiO3, the most extensively studied leadfree piezoelectric solid solution. This discovery results from a novel method of electron diffraction analysis, which allows for the precise determination of oxygen octahedra tilting in multi-domain perovskite ferroelectrics. The study using this method supports the recently proposed Cc symmetry for pure (Bi1/2Na1/ 2)TiO3, and, more importantly, indicates the crystal structure evolves into the R3c symmetry with the addition of BaTiO3, forming a Cc/R3c phase boundary at x = 3~4%. In the poling field Epol vs. composition x phase diagram for polycrystalline ceramics, this phase boundary exists with Epol below 5.5 kV/mm; the Cc phase is transformed to the R3c phase during poling at higher fields. The results reported here provide the microstructural origin for the previously unexplained strain behavior and clarify the low-BaTiO3-content phase relationship in this popular lead-free piezoelectric system.