What We Have Learned So Far from Phase-Field Modeling of Epitaxial Thin Films of Orthorhombic K_xNa_1-xNbO₃

Epitaxial ferroelectric thin films often exhibit unique polymorphic phases, domain structures, and physical properties dissimilar from their bulk counterparts due to the modification of mechanical and electrical boundary conditions. While extensive knowledge has been acquired for epitaxial thin films of bulk tetragonal (such as BaTiO₃ and PbTiO₃) and rhombohedral ferroelectric materials (such as BiFeO₃), much less is known for the thin films of bulk orthorhombic ferroelectrics such as K<i>_x</i>Na_1-<i>x</i>NbO₃. In this talk, I will review our recent progress in the development and employment of thermodynamic theory and phase-field modeling for understanding the phase stability and transformation, domain and domain wall structures, and dielectric and piezoelectric properties of KNN thin films. I will highlight the computational establishment of anisotropic misfit strain-temperature phase diagrams, theoretical prediction and experimental validation of the formation condition and configuration of two types of superdomains, and the strong domain size-dependent piezoelectricity. I will also outlook potential topics unique for the epitaxial thin films of the orthorhombic ferroelectrics.<br/>Part of this work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.