Keynote lecture
Strain phase/domain equilibria and mesoscale topological transformations in ferroelectric heterostructures

It has been well established that strains or stresses can have profound effects on the thermodynamic stability of phase and domain states of solids, and they can be judiciously utilized to tune solid state phase and domain microstructures and thus physical properties. In this presentation, I will discuss the formation of thermodynamically stable mesoscale phase/domain structures in ferroelectric thin films and heterostructures, strain phase equilibria and diagrams, and polar topological structural transformations under external stimuli using classical thermodynamics and the phase-field method of ferroelectric crystals. In particular, I will show that one can efficiently construct multidomain and multiphase diagrams of arbitrarily strained or stressed solids, in analogy to the well-known temperature-composition phase diagrams by ignoring the coherency strain energy contribution. Such strain/stress phase diagrams can be refined through phase-field simulations by incorporating the coherency strain energy contributions as well as electrostatic interactions and domain wall energy contributions. One can also employ the phase-field method to model, understand, and predict the mesoscale topological transformations among the different ferroelectric polar structures under mechanical, electric, and thermal stimuli as well as light. It will be shown that a combination of thermodynamic stability analysis and phase-field method offers a powerful theoretical tool for understanding the thermodynamic driving forces for the formation of thermodynamically stable mesoscale polar structures and for guiding the tuning of their properties