Hierarchical domain structure of lead-free piezoelectric (Na1/2 Bi1/2)TiO3-(K1/2 Bi1/2)TiO3 single crystals

We report a unique hierarchical domain structure in single crystals of (Na1/2Bi1/2)TiO3-xat. %(K1/2Bi1/2)TiO3 for x = 5 and 8 by transmission electron microscopy (TEM). A high density of polar nano-domains with a lamellar morphology was found, which were self-assembled into a quadrant-like configuration, which then assembled into conventional ferroelectric macro-domains. Studies by high resolution TEM revealed that the polar lamellar regions contained a coexistence of in-phase and anti-phase oxygen octahedral tilt regions of a few nanometers in size. Domain frustration over multiple length scales may play an important role in the stabilization of the hierarchy, and in reducing the piezoelectric response of this Pb-free piezoelectric solid solution.