Phase diagrams, superdomains, and superdomain walls in K Na1-NbO3 epitaxial thin films

Abstract Domains and domain walls play a critical role in determining the functional properties of ferroelectric thin films for various applications. Understanding the formation and structures of the domains and domain walls is of vital importance to manipulating their configurations and exploring their dynamics. When domains assemble into hierarchical structures, the superdomains and superdomain walls emerge, which exhibits exotic morphology and may lead to distinct new behaviors. Here, taking the K0.5Na0.5NbO3 thin films as a model system, we establish the misfit strain-misfit strain phase diagram at different temperatures and study the formation condition, morphological features, and polarization configurations of two types of superdomain structures by using phase-field simulations, which compare well with prior experiments. We show how the superdomain structures can be viewed as superposition of two sets of polytwin structures along well-defined planes. Several types of superdomain walls are identified, which possess different energies and finite thickness in analogy to conventional domain walls. We also show that some superdomain walls can be anomalously wide and electrically conductive. This work can generate new interests in exploring the structures and functionalities of superdomain and superdomain walls in ferroelectric thin films, which can potentially be utilized for next-generation electronic devices.